listobject.c

/* List object implementation */

#include "Python.h"
#include "pycore_abstract.h"      // _PyIndex_Check()
#include "pycore_critical_section.h"
#include "pycore_interp.h"        // PyInterpreterState.list
#include "pycore_list.h"          // struct _Py_list_state, _PyListIterObject
#include "pycore_object.h"        // _PyObject_GC_TRACK()
#include "pycore_tuple.h"         // _PyTuple_FromArray()
#include <stddef.h>

#include "mimalloc.h"
#include "mimalloc-internal.h"

/*[clinic input]
class list "PyListObject *" "&PyList_Type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=f9b222678f9f71e0]*/

#include "clinic/listobject.c.h"

_Py_DECLARE_STR(list_err, "list index out of range");

#if PyList_MAXFREELIST > 0
static struct _Py_list_state *
get_list_state(void)
{
    return &((PyThreadStateImpl *)_PyThreadState_GET())->list;
}
#endif

static size_t
list_good_size(Py_ssize_t size)
{
    // 4, 8, 16, 24, 32, 40, 48, 64, 80, ...
    // NOTE: we add one here so that the rounding accounts for the "allocated"
    // field in _PyListArray.
    size_t reqsize = (size_t)size + 1;
    if (reqsize <= 4) {
        reqsize = 4;
    }
    else if (reqsize <= 48) {
        reqsize = (reqsize + 7) & ~7;
    }
    else {
        reqsize = (reqsize + 15) & ~15;
        if (reqsize <= MI_MEDIUM_OBJ_WSIZE_MAX) {
            reqsize = mi_good_size(reqsize * sizeof(PyObject *))/sizeof(PyObject*);
        }
        else {
            // ensure geometric spacing for large arrays
            size_t shift = mi_bsr(reqsize) - 2;
            reqsize = ((reqsize >> shift) + 1) << shift;
        }
    }
    return reqsize - 1;
}

static _PyListArray *
list_allocate_array(size_t capacity)
{
    if (capacity > PY_SSIZE_T_MAX/sizeof(PyObject*) - 1) {
        return NULL;
    }
    _PyListArray *arr = PyMem_Malloc(sizeof(_PyListArray) + (capacity - 1) * sizeof(PyObject *));
    if (arr == NULL) {
        return NULL;
    }
    arr->allocated = capacity;
    return arr;
}

static _PyListArray *
list_array(PyObject **items)
{
    char *mem = (char *)items;
    mem -= offsetof(_PyListArray, ob_item);
    return (_PyListArray *)mem;
}

static void
free_list_array(PyObject **items, bool use_qsbr)
{
    _PyListArray *arr = list_array(items);
    if (use_qsbr) {
        _PyMem_FreeQsbr(arr);
    }
    else {
        PyMem_Free(arr);
    }
}

/* Ensure ob_item has room for at least newsize elements, and set
 * ob_size to newsize.  If newsize > ob_size on entry, the content
 * of the new slots at exit is undefined heap trash; it's the caller's
 * responsibility to overwrite them with sane values.
 * The number of allocated elements may grow, shrink, or stay the same.
 * Note that self->ob_item may change, and even if newsize is less
 * than ob_size on entry.
 */
static int
list_ensure_capacity_slow(PyListObject *self, Py_ssize_t base, Py_ssize_t extra)
{
    if (base > PY_SSIZE_T_MAX/(Py_ssize_t)sizeof(PyObject*) - extra) {
        PyErr_NoMemory();
        return -1;
    }

    Py_ssize_t reqsize = base + extra;
    Py_ssize_t allocated = self->allocated;
    if (allocated >= reqsize) {
        assert(self->ob_item != NULL || reqsize == 0);
        return 0;
    }

    if (!_Py_ThreadLocal((PyObject *)self)) {
        _PyObject_GC_SET_SHARED(self);
    }

    size_t capacity = list_good_size(reqsize);
    _PyListArray *arr = list_allocate_array(capacity);
    if (arr == NULL) {
        PyErr_NoMemory();
        return -1;
    }
    PyObject **old = self->ob_item;
    if (self->ob_item) {
        memcpy(&arr->ob_item, self->ob_item, allocated * sizeof(PyObject*));
    }
    _Py_atomic_store_ptr_release(&self->ob_item, &arr->ob_item);
    self->allocated = capacity;
    if (old) {
        free_list_array(old, _PyObject_GC_IS_SHARED(self));
    }
    return 0;
}

static int
list_ensure_capacity(PyListObject *self, Py_ssize_t base, Py_ssize_t extra)
{
    if (base > self->allocated - extra) {
        return list_ensure_capacity_slow(self, base, extra);
    }
    return 0;
}

static int
list_ensure_capacity_single(PyListObject *self, Py_ssize_t size)
{
    if (self->allocated <= size) {
        return list_ensure_capacity_slow(self, size, 1);
    }
    return 0;
}

/* Ensure ob_item has room for at least newsize elements, and set
 * ob_size to newsize.  If newsize > ob_size on entry, the content
 * of the new slots at exit is undefined heap trash; it's the caller's
 * responsibility to overwrite them with sane values.
 * The number of allocated elements may grow, shrink, or stay the same.
 * Failure is impossible if newsize <= self.allocated on entry, although
 * that partly relies on an assumption that the system realloc() never
 * fails when passed a number of bytes <= the number of bytes last
 * allocated (the C standard doesn't guarantee this, but it's hard to
 * imagine a realloc implementation where it wouldn't be true).
 * Note that self->ob_item may change, and even if newsize is less
 * than ob_size on entry.
 */
static int
list_resize(PyListObject *self, Py_ssize_t newsize)
{
    if (list_ensure_capacity(self, newsize, 0) != 0) {
        return -1;
    }
    Py_SET_SIZE(self, newsize);
    return 0;
}

void
_PyList_ClearFreeList(PyThreadState *tstate)
{
#if PyList_MAXFREELIST > 0
    struct _Py_list_state *state = &((PyThreadStateImpl *)tstate)->list;
    while (state->numfree) {
        PyListObject *op = state->free_list[--state->numfree];
        assert(PyList_CheckExact(op));
        PyObject_GC_Del(op);
    }
#endif
}

void
_PyList_Fini(PyThreadState *tstate)
{
    _PyList_ClearFreeList(tstate);
#if defined(Py_DEBUG) && PyList_MAXFREELIST > 0
    struct _Py_list_state *state = &((PyThreadStateImpl *)tstate)->list;
    state->numfree = -1;
#endif
}

/* Print summary info about the state of the optimized allocator */
void
_PyList_DebugMallocStats(FILE *out)
{
#if PyList_MAXFREELIST > 0
    struct _Py_list_state *state = get_list_state();
    _PyDebugAllocatorStats(out,
                           "free PyListObject",
                           state->numfree, sizeof(PyListObject));
#endif
}

static PyListObject *
list_new(Py_ssize_t size)
{
    PyListObject *op;
    assert(size >= 0);
#if PyList_MAXFREELIST > 0
    struct _Py_list_state *state = get_list_state();
#ifdef Py_DEBUG
    // PyList_New() must not be called after _PyList_Fini()
    assert(state->numfree != -1);
#endif
    if (PyList_MAXFREELIST && state->numfree) {
        state->numfree--;
        op = state->free_list[state->numfree];
        OBJECT_STAT_INC(from_freelist);
        _Py_NewReference((PyObject *)op);
    }
    else
#endif
    {
        op = PyObject_GC_New(PyListObject, &PyList_Type);
        if (!op) {
            return NULL;
        }
    }
    if (size <= 0) {
        op->ob_item = NULL;
        op->allocated = 0;
    }
    else {
        size_t capacity = list_good_size(size);
        _PyListArray *arr = list_allocate_array(capacity);
        if (arr == NULL) {
            op->ob_item = NULL;
            Py_DECREF(op);
            PyErr_NoMemory();
            return NULL;
        }
        op->ob_item = arr->ob_item;
        op->allocated = capacity;
    }
    Py_SET_SIZE(op, size);
    _PyObject_GC_TRACK(op);
    return op;
}

PyObject *
PyList_New(Py_ssize_t size)
{
    if (size < 0) {
        PyErr_BadInternalCall();
        return NULL;
    }
    PyListObject *op = list_new(size);
    if (op && op->ob_item) {
        PyObject **items = op->ob_item;
        for (Py_ssize_t i = 0, n = op->allocated; i < n; i++) {
            _Py_atomic_store_ptr_relaxed(&items[i], NULL);
        }
    }
    return (PyObject *) op;
}

Py_ssize_t
PyList_Size(PyObject *op)
{
    if (!PyList_Check(op)) {
        PyErr_BadInternalCall();
        return -1;
    }
    else
        return Py_SIZE(op);
}

static inline int
valid_index(Py_ssize_t i, Py_ssize_t limit)
{
    /* The cast to size_t lets us use just a single comparison
       to check whether i is in the range: 0 <= i < limit.

       See:  Section 14.2 "Bounds Checking" in the Agner Fog
       optimization manual found at:
       https://www.agner.org/optimize/optimizing_cpp.pdf
    */
    return (size_t) i < (size_t) limit;
}

Py_NO_INLINE static PyObject *
list_item_locked(PyListObject *self, Py_ssize_t idx, PyObject *dead)
{
    Py_XDECREF(dead);

    PyObject *item = NULL;
    Py_BEGIN_CRITICAL_SECTION(self);
    if (!_PyObject_GC_IS_SHARED(self)) {
        _PyObject_GC_SET_SHARED(self);
    }
    Py_ssize_t size = Py_SIZE(self);
    if (!valid_index(idx, size)) {
        goto exit;
    }
    item = Py_NewRef(self->ob_item[idx]);
exit:
    Py_END_CRITICAL_SECTION;
    return item;
}

static int
list_needs_read_lock(PyListObject *self)
{
    return (!_Py_ThreadLocal((PyObject *)self) &&
            !_PyObject_GC_IS_SHARED(self));
}

static inline PyObject *
list_fetch_item(PyListObject *self, Py_ssize_t idx)
{
    if (list_needs_read_lock(self)) {
        return list_item_locked(self, idx, NULL);
    }
    Py_ssize_t size = _Py_atomic_load_ssize_relaxed(&((PyVarObject *)self)->ob_size);
    if (!valid_index(idx, size)) {
        return NULL;
    }
    PyObject **ob_item = _Py_atomic_load_ptr(&self->ob_item);
    if (ob_item == NULL) {
        return NULL;
    }
    if (!valid_index(idx, _PyList_Capacity(ob_item))) {
        return NULL;
    }
    PyObject *item = _Py_atomic_load_ptr(&ob_item[idx]);
    if (_PY_UNLIKELY(!item)) {
        return list_item_locked(self, idx, NULL);
    }
    if (_PY_LIKELY(_Py_TryIncrefFast(item))) {
        goto check_array;
    }
    if (!_Py_TryIncRefShared(item)) {
        return list_item_locked(self, idx, NULL);
    }
    if (_PY_UNLIKELY(item != _Py_atomic_load_ptr(&ob_item[idx]))) {
        return list_item_locked(self, idx, item);
    }
check_array:
    if (_PY_UNLIKELY(ob_item != _Py_atomic_load_ptr(&self->ob_item))) {
        return list_item_locked(self, idx, item);
    }
    return item;
}

static PyObject *
list_item(PyListObject *self, Py_ssize_t idx)
{
    PyObject *item = list_fetch_item(self, idx);
    if (!item) {
        _Py_DECLARE_STR(list_err, "list index out of range");
        PyErr_SetObject(PyExc_IndexError, &_Py_STR(list_err));
        return NULL;
    }
    return item;
}

PyObject *
PyList_GetItem(PyObject *op, Py_ssize_t i)
{
    if (!PyList_Check(op)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    if (!valid_index(i, Py_SIZE(op))) {
        _Py_DECLARE_STR(list_err, "list index out of range");
        PyErr_SetObject(PyExc_IndexError, &_Py_STR(list_err));
        return NULL;
    }
    return ((PyListObject *)op) -> ob_item[i];
}

PyObject *
PyList_FetchItem(PyObject *op, Py_ssize_t i)
{
    if (!PyList_Check(op)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    return list_item(((PyListObject *)op), i);
}

static int
setitem(PyListObject *self, Py_ssize_t i, PyObject *newitem)
{
    if (!valid_index(i, Py_SIZE(self))) {
        Py_XDECREF(newitem);
        PyErr_SetString(PyExc_IndexError,
                        "list assignment index out of range");
        return -1;
    }

    PyObject *tmp = self->ob_item[i];
    self->ob_item[i] = newitem;
    Py_XDECREF(tmp);
    return 0;
}

int
PyList_SetItem(PyObject *op, Py_ssize_t i,
               PyObject *newitem)
{
    if (!PyList_Check(op)) {
        Py_XDECREF(newitem);
        PyErr_BadInternalCall();
        return -1;
    }

    int ret;
    PyListObject *self = ((PyListObject *)op);
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = setitem(self, i, newitem);
    Py_END_CRITICAL_SECTION;
    return ret;
}

static void
PyList_SetSize(PyListObject *self, Py_ssize_t size)
{
    // TODO(sgross): release?
    _Py_atomic_store_ssize_relaxed(&((PyVarObject *)self)->ob_size, size);
}

#define PyList_SET_SIZE PyList_SetSize

static int
ins1(PyListObject *self, Py_ssize_t where, PyObject *v)
{
    Py_ssize_t n = Py_SIZE(self);
    if (list_ensure_capacity_single(self, n) < 0) {
        return -1;
    }
    if (where < 0) {
        where += n;
        if (where < 0)
            where = 0;
    }
    if (where > n)
        where = n;
    PyObject **items = self->ob_item;
    for (Py_ssize_t i = n; --i >= where; ) {
        _Py_atomic_store_ptr_relaxed(&items[i+1], items[i]);
    }
    _Py_atomic_store_ptr_relaxed(&items[where], Py_NewRef(v));
    PyList_SetSize(self, n+1);
    return 0;
}

int
PyList_Insert(PyObject *op, Py_ssize_t where, PyObject *newitem)
{
    if (!PyList_Check(op)) {
        PyErr_BadInternalCall();
        return -1;
    }
    int err;
    PyListObject *self = (PyListObject *)op;
    Py_BEGIN_CRITICAL_SECTION(self);
    err = ins1(self, where, newitem);
    Py_END_CRITICAL_SECTION;
    return err;
}

static int
app1(PyListObject *self, PyObject *v)
{
    Py_ssize_t n = Py_SIZE(self);
    if (list_ensure_capacity_single(self, n) < 0) {
        return -1;
    }
    _Py_atomic_store_ptr_relaxed(&self->ob_item[n], Py_NewRef(v));
    PyList_SetSize(self, n+1);
    return 0;
}

int
_PyList_AppendPrivate(PyObject *op, PyObject *v)
{
    return app1((PyListObject *)op, v);
}

/* internal, used by _PyList_AppendTakeRef */
int
_PyList_AppendTakeRefListResize(PyListObject *self, PyObject *newitem)
{
    Py_ssize_t len = PyList_GET_SIZE(self);
    assert(self->allocated == -1 || self->allocated == len);
    if (list_resize(self, len + 1) < 0) {
        Py_DECREF(newitem);
        return -1;
    }
    PyList_SET_ITEM(self, len, newitem);
    return 0;
}

int
PyList_Append(PyObject *op, PyObject *newitem)
{
    if (!PyList_Check(op) || newitem == NULL) {
        PyErr_BadInternalCall();
        return -1;
    }
    int ret;
    PyListObject *self = (PyListObject *)op;
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = app1(self, newitem);
    Py_END_CRITICAL_SECTION;
    return ret;
}

/* Methods */

static void
list_dealloc(PyListObject *op)
{
    Py_ssize_t i;
    PyObject_GC_UnTrack(op);
    Py_TRASHCAN_BEGIN(op, list_dealloc)
    if (op->ob_item != NULL) {
        /* Do it backwards, for Christian Tismer.
           There's a simple test case where somehow this reduces
           thrashing when a *very* large list is created and
           immediately deleted. */
        i = Py_SIZE(op);
        while (--i >= 0) {
            Py_XDECREF(op->ob_item[i]);
        }
        free_list_array(op->ob_item, false);
    }
#if PyList_MAXFREELIST > 0
    struct _Py_list_state *state = get_list_state();
#ifdef Py_DEBUG
    // list_dealloc() must not be called after _PyList_Fini()
    assert(state->numfree != -1);
#endif
    if (state->numfree < PyList_MAXFREELIST && PyList_CheckExact(op)) {
        state->free_list[state->numfree++] = op;
        OBJECT_STAT_INC(to_freelist);
    }
    else
#endif
    {
        Py_TYPE(op)->tp_free((PyObject *)op);
    }
    Py_TRASHCAN_END
}

static PyObject *
list_repr_locked(PyListObject *v)
{
    Py_ssize_t i;
    PyObject *s;
    _PyUnicodeWriter writer;

    i = Py_ReprEnter((PyObject*)v);
    if (i != 0) {
        return i > 0 ? PyUnicode_FromString("[...]") : NULL;
    }

    _PyUnicodeWriter_Init(&writer);
    writer.overallocate = 1;
    /* "[" + "1" + ", 2" * (len - 1) + "]" */
    writer.min_length = 1 + 1 + (2 + 1) * (Py_SIZE(v) - 1) + 1;

    if (_PyUnicodeWriter_WriteChar(&writer, '[') < 0)
        goto error;

    /* Do repr() on each element.  Note that this may mutate the list,
       so must refetch the list size on each iteration. */
    for (i = 0; i < Py_SIZE(v); ++i) {
        if (i > 0) {
            if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0)
                goto error;
        }

        s = PyObject_Repr(v->ob_item[i]);
        if (s == NULL)
            goto error;

        if (_PyUnicodeWriter_WriteStr(&writer, s) < 0) {
            Py_DECREF(s);
            goto error;
        }
        Py_DECREF(s);
    }

    writer.overallocate = 0;
    if (_PyUnicodeWriter_WriteChar(&writer, ']') < 0)
        goto error;

    Py_ReprLeave((PyObject *)v);
    return _PyUnicodeWriter_Finish(&writer);

error:
    _PyUnicodeWriter_Dealloc(&writer);
    Py_ReprLeave((PyObject *)v);
    return NULL;
}

static PyObject *
list_repr(PyListObject *v)
{
    if (Py_SIZE(v) == 0) {
        return PyUnicode_FromString("[]");
    }
    PyObject *ret;
    Py_BEGIN_CRITICAL_SECTION(v);
    ret = list_repr_locked(v);
    Py_END_CRITICAL_SECTION;
    return ret;
}

static Py_ssize_t
list_length(PyListObject *a)
{
    return _Py_atomic_load_ssize_relaxed(&((PyVarObject *)a)->ob_size);
}

static int
list_contains(PyListObject *a, PyObject *el)
{
    for (Py_ssize_t i = 0; ; i++) {
        PyObject *item = list_fetch_item(a, i);
        if (!item) {
            // out-of-bounds
            return 0;
        }
        int cmp = PyObject_RichCompareBool(item, el, Py_EQ);
        Py_DECREF(item);
        if (cmp != 0) {
            return cmp;
        }
    }
}

static Py_ssize_t
clamp_indices(Py_ssize_t size, Py_ssize_t *start, Py_ssize_t *stop)
{
    if (*start < 0) {
        *start = 0;
    }
    else if (*start > size) {
        *start = size;
    }

    if (*stop < *start) {
        *stop = *start;
    }
    else if (*stop > size) {
        *stop = size;
    }
    return *stop - *start;
}

static PyObject *
list_slice_locked(PyListObject *a, Py_ssize_t start, Py_ssize_t len)
{
    PyListObject *np = list_new(len);
    if (np == NULL) {
        return NULL;
    }
    PyObject **src = a->ob_item + start;
    PyObject **dest = np->ob_item;
    for (Py_ssize_t i = 0; i < len; i++) {
        PyObject *v = src[i];
        _Py_atomic_store_ptr_relaxed(&dest[i], Py_NewRef(v));
    }
    return (PyObject *)np;
}

static PyObject *
list_slice_step_locked(PyListObject *a, Py_ssize_t start, Py_ssize_t step,
                       Py_ssize_t len)
{
    PyListObject *np = list_new(len);
    if (np == NULL) {
        return NULL;
    }
    size_t cur;
    Py_ssize_t i;
    PyObject **src = a->ob_item;
    PyObject **dest = np->ob_item;
    for (cur = start, i = 0; i < len;
            cur += (size_t)step, i++) {
        PyObject *v = src[cur];
        _Py_atomic_store_ptr_relaxed(&dest[i], Py_NewRef(v));
    }
    return (PyObject *)np;
}

static PyObject *
list_slice_clamp(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh)
{
    PyObject *res;
    Py_BEGIN_CRITICAL_SECTION(a);
    Py_ssize_t len = clamp_indices(Py_SIZE(a), &ilow, &ihigh);
    res = list_slice_locked(a, ilow, len);
    Py_END_CRITICAL_SECTION;
    return res;
}

PyObject *
PyList_GetSlice(PyObject *a, Py_ssize_t ilow, Py_ssize_t ihigh)
{
    if (!PyList_Check(a)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    return list_slice_clamp((PyListObject *)a, ilow, ihigh);
}

static PyObject *
list_concat_locked(PyListObject *a, PyListObject *b)
{
    Py_ssize_t size;
    Py_ssize_t i;
    PyObject **src, **dest;
    PyListObject *np;
    assert((size_t)Py_SIZE(a) + (size_t)Py_SIZE(b) < PY_SSIZE_T_MAX);
    size = Py_SIZE(a) + Py_SIZE(b);
    if (size == 0) {
        return PyList_New(0);
    }
    np = (PyListObject *) list_new(size);
    if (np == NULL) {
        return NULL;
    }
    src = a->ob_item;
    dest = np->ob_item;
    for (i = 0; i < Py_SIZE(a); i++) {
        PyObject *v = src[i];
        _Py_atomic_store_ptr_relaxed(&dest[i], Py_NewRef(v));
    }
    src = b->ob_item;
    dest = np->ob_item + Py_SIZE(a);
    for (i = 0; i < Py_SIZE(b); i++) {
        PyObject *v = src[i];
        _Py_atomic_store_ptr_relaxed(&dest[i], Py_NewRef(v));
    }
    Py_SET_SIZE(np, size);
    return (PyObject *)np;
}

static PyObject *
list_concat(PyListObject *a, PyObject *bb)
{
    if (!PyList_Check(bb)) {
        PyErr_Format(PyExc_TypeError,
                  "can only concatenate list (not \"%.200s\") to list",
                  Py_TYPE(bb)->tp_name);
        return NULL;
    }
    PyObject *ret;
    PyListObject *b = (PyListObject *)bb;
    Py_BEGIN_CRITICAL_SECTION2(a, b);
    ret = list_concat_locked(a, b);
    Py_END_CRITICAL_SECTION2;
    return ret;
}

static PyObject *
list_repeat_locked(PyListObject *a, Py_ssize_t n)
{
    const Py_ssize_t input_size = Py_SIZE(a);
    if (input_size == 0 || n <= 0)
        return PyList_New(0);
    assert(n > 0);

    if (input_size > PY_SSIZE_T_MAX / n)
        return PyErr_NoMemory();
    Py_ssize_t output_size = input_size * n;

    PyListObject *np = (PyListObject *) list_new(output_size);
    if (np == NULL)
        return NULL;

    PyObject **dest = np->ob_item;
    if (input_size == 1) {
        PyObject *elem = a->ob_item[0];
        _Py_RefcntAdd(elem, n);
        PyObject **dest_end = dest + output_size;
        while (dest < dest_end) {
            _Py_atomic_store_ptr_relaxed(dest++, elem);
        }
    }
    else {
        PyObject **src = a->ob_item;
        PyObject **src_end = src + input_size;
        while (src < src_end) {
            _Py_RefcntAdd(*src, n);
            _Py_atomic_store_ptr_relaxed(dest++, *src++);
        }

        _Py_memory_repeat((char *)np->ob_item, sizeof(PyObject *)*output_size,
                                        sizeof(PyObject *)*input_size);
    }

    PyList_SetSize(np, output_size);
    return (PyObject *) np;
}

static PyObject *
list_repeat(PyListObject *a, Py_ssize_t n)
{
    PyObject *ret;
    Py_BEGIN_CRITICAL_SECTION(a);
    ret = list_repeat_locked(a, n);
    Py_END_CRITICAL_SECTION;
    return ret;
}

static void
_list_clear_impl(PyListObject *a, bool is_resize)
{
    Py_ssize_t i;
    PyObject **item = a->ob_item;
    if (item != NULL) {
        /* Because XDECREF can recursively invoke operations on
           this list, we make it empty first. */
        i = Py_SIZE(a);
        PyList_SetSize(a, 0);
        _Py_atomic_store_ptr_release(&a->ob_item, NULL);
        a->allocated = 0;
        while (--i >= 0) {
            Py_XDECREF(item[i]);
        }
        free_list_array(item, is_resize && _PyObject_GC_IS_SHARED(a));
    }
    /* Note that there is no guarantee that the list is actually empty
       at this point, because XDECREF may have populated it again! */
}

static void
_list_clear(PyListObject *a)
{
    _list_clear_impl(a, true);
}

static int
list_tp_clear(PyListObject *a)
{
    _list_clear_impl(a, false);
    return 0;
}

enum {
    CLAMP_INDICES,
    WRAP_INDICES
};

static Py_ssize_t
adjust_indices(Py_ssize_t size, Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t step, int wrap_mode)
{
    if (wrap_mode == CLAMP_INDICES) {
        assert(step == 1);
        return clamp_indices(size, start, stop);
    }
    else {
        Py_ssize_t slicelength = PySlice_AdjustIndices(size, start, stop, step);
        if ((step < 0 && *start < *stop) ||
            (step > 0 && *start > *stop)) {
            *stop = *start;
        }
        return slicelength;
    }
}

static int
list_ass_slice_locked(PyListObject *a, Py_ssize_t start, Py_ssize_t stop, Py_ssize_t step, int wrap_mode,
                      PyObject *v, PyObject **seqitems, Py_ssize_t n)
{
    Py_ssize_t size = Py_SIZE(a);
    Py_ssize_t slicelength = adjust_indices(size, &start, &stop, step, wrap_mode);
    if (step != 1 && v && n != slicelength) {
        PyErr_Format(PyExc_ValueError,
            "attempt to assign sequence of "
            "size %zd to extended slice of "
            "size %zd", n, slicelength);
        return -1;
    }

    /* Change in size */
    Py_ssize_t d = n - slicelength;
    if (d > 0 && list_ensure_capacity(a, size, d) < 0) {
        return -1;
    }

    /* recycle the items that we are about to remove */
    PyObject *recycle_on_stack[8];
    PyObject **recycle = recycle_on_stack; /* will allocate more if needed */

    size_t s = slicelength * sizeof(PyObject *);
    if (s > sizeof(recycle_on_stack)) {
        recycle = (PyObject **)PyMem_MALLOC(s);
        if (recycle == NULL) {
            PyErr_NoMemory();
            return -1;
        }
    }

    PyObject **item = a->ob_item;
    if (d == 0) {
        for (Py_ssize_t k = 0, i = start; k < n; k++, i+=step) {
            PyObject *w = seqitems[k];
            Py_XINCREF(w);
            recycle[k] = item[i];
            item[i] = w;        // todo: relaxed store
        }
    }
    else if (step == 1) {
        for (Py_ssize_t k = 0, i = start; k < slicelength; k++, i+=step) {
            recycle[k] = item[i];
        }
        if (d != 0) {
            Py_ssize_t tail = (size - stop) * sizeof(PyObject *);
            memmove(&item[stop+d], &item[stop], tail);
        }
        for (Py_ssize_t k = 0, i = start; k < n; k++, i+=step) {
            PyObject *w = seqitems[k];
            Py_XINCREF(w);
            item[i] = w;
        }
    }
    else {
        assert(d < 0);

        if (step < 0) {
            stop = start + 1;
            start = stop + step*(slicelength - 1) - 1;
            step = -step;
        }

        /* drawing pictures might help understand these for
           loops. Basically, we memmove the parts of the
           list that are *not* part of the slice: step-1
           items for each item that is part of the slice,
           and then tail end of the list that was not
           covered by the slice */
        size_t cur, i;
        for (cur = start, i = 0;
             cur < (size_t)stop;
             cur += step, i++) {
            Py_ssize_t lim = step - 1;

            recycle[i] = item[cur];

            if (cur + step >= (size_t)size) {
                lim = size - cur - 1;
            }

            memmove(item + cur - i,
                item + cur + 1,
                lim * sizeof(PyObject *));
        }
        cur = start + (size_t)slicelength * step;
        if (cur < (size_t)size) {
            memmove(item + cur - slicelength,
                item + cur,
                (size - cur) *
                 sizeof(PyObject *));
        }
    }

    Py_SET_SIZE(a, size + d);

    for (Py_ssize_t k = slicelength - 1; k >= 0; --k) {
        Py_XDECREF(recycle[k]);
    }
    if (recycle != recycle_on_stack) {
        PyMem_FREE(recycle);
    }
    return 0;
}

static int
list_ass_slice(PyListObject *a, Py_ssize_t start, Py_ssize_t stop, Py_ssize_t step, int wrap_mode, PyObject *v)
{
    /* protect against a[::-1] = a */
    int ret;
    if (a == (PyListObject*)v) {
        Py_BEGIN_CRITICAL_SECTION(a);
        Py_ssize_t n = PyList_GET_SIZE(a);
        PyListObject *copy = (PyListObject *)list_slice_locked(a, 0, n);
        if (copy == NULL) {
            ret = -1;
        }
        else {
            ret = list_ass_slice_locked(a, start, stop, step, wrap_mode, v, copy->ob_item, n);
        }
        Py_END_CRITICAL_SECTION;
    }
    else if (v == NULL) {
        Py_BEGIN_CRITICAL_SECTION(a);
        ret = list_ass_slice_locked(a, start, stop, step, wrap_mode, NULL, NULL, 0);
        Py_END_CRITICAL_SECTION;
    }
    else if (PyList_CheckExact(v)) {
        PyListObject *b = (PyListObject *)v;
        Py_BEGIN_CRITICAL_SECTION2(a, b);
        Py_ssize_t n = PyList_GET_SIZE(b);
        ret = list_ass_slice_locked(a, start, stop, step, wrap_mode, v, b->ob_item, n);
        Py_END_CRITICAL_SECTION2;
    }
    else {
        PyObject *seq = PySequence_Fast(v, "can only assign an iterable");
        if (seq == NULL) {
            return -1;
        }

        Py_ssize_t n = PySequence_Fast_GET_SIZE(seq);
        PyObject **seqitems = PySequence_Fast_ITEMS(seq);

        Py_BEGIN_CRITICAL_SECTION(a);
        ret = list_ass_slice_locked(a, start, stop, step, wrap_mode, v, seqitems, n);
        Py_END_CRITICAL_SECTION;
        Py_DECREF(seq);
    }
    return ret;
}

static int
list_del_slice_locked(PyListObject *a, Py_ssize_t start, Py_ssize_t stop)
{
    return list_ass_slice_locked(a, start, stop, 1, CLAMP_INDICES, NULL, NULL, 0);
}

int
PyList_SetSlice(PyObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
{
    if (!PyList_Check(a)) {
        PyErr_BadInternalCall();
        return -1;
    }
    return list_ass_slice((PyListObject *)a, ilow, ihigh, 1, CLAMP_INDICES, v);
}

static PyObject *
list_inplace_repeat_locked(PyListObject *self, Py_ssize_t n)
{
    Py_ssize_t input_size = PyList_GET_SIZE(self);
    if (input_size == 0 || n == 1) {
        return Py_NewRef(self);
    }

    if (n < 1) {
        _list_clear(self);
        return Py_NewRef(self);
    }

    if (input_size > PY_SSIZE_T_MAX / n) {
        return PyErr_NoMemory();
    }
    Py_ssize_t output_size = input_size * n;

    if (list_resize(self, output_size) < 0)
        return NULL;

    PyObject **items = self->ob_item;
    for (Py_ssize_t j = 0; j < input_size; j++) {
        _Py_RefcntAdd(items[j], n-1);
    }
    _Py_memory_repeat((char *)items, sizeof(PyObject *)*output_size,
                      sizeof(PyObject *)*input_size);

    return Py_NewRef(self);
}

static PyObject *
list_inplace_repeat(PyListObject *self, Py_ssize_t n)
{
    PyObject *ret;
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = list_inplace_repeat_locked(self, n);
    Py_END_CRITICAL_SECTION;
    return ret;
}

static int
list_ass_item_locked(PyListObject *a, Py_ssize_t i, PyObject *v)
{
    if (!valid_index(i, Py_SIZE(a))) {
        PyErr_SetString(PyExc_IndexError,
                        "list assignment index out of range");
        return -1;
    }
    PyObject *tmp = a->ob_item[i];
    if (v == NULL) {
        Py_ssize_t size = Py_SIZE(a);
        for (Py_ssize_t idx = i; idx < size - 1; idx++) {
            _Py_atomic_store_ptr_relaxed(&a->ob_item[idx], a->ob_item[idx + 1]);
        }
        PyList_SetSize(a, size - 1);
    }
    else {
        _Py_atomic_store_ptr_release(&a->ob_item[i], Py_NewRef(v));
    }
    Py_DECREF(tmp);
    return 0;
}

static int
list_ass_item(PyListObject *a, Py_ssize_t i, PyObject *v)
{
    int ret;
    Py_BEGIN_CRITICAL_SECTION(a);
    ret = list_ass_item_locked(a, i, v);
    Py_END_CRITICAL_SECTION;
    return ret;
}

/*[clinic input]
list.insert

    index: Py_ssize_t
    object: object
    /

Insert object before index.
[clinic start generated code]*/

static PyObject *
list_insert_impl(PyListObject *self, Py_ssize_t index, PyObject *object)
/*[clinic end generated code: output=7f35e32f60c8cb78 input=858514cf894c7eab]*/
{
    PyObject *ret = Py_None;
    Py_BEGIN_CRITICAL_SECTION(self);
    if (ins1(self, index, object) < 0) {
        ret = NULL;
    }
    Py_END_CRITICAL_SECTION;
    return ret;
}

/*[clinic input]
list.clear

Remove all items from list.
[clinic start generated code]*/

static PyObject *
list_clear_impl(PyListObject *self)
/*[clinic end generated code: output=67a1896c01f74362 input=ca3c1646856742f6]*/
{
    Py_BEGIN_CRITICAL_SECTION(self);
    _list_clear(self);
    Py_END_CRITICAL_SECTION;
    Py_RETURN_NONE;
}

/*[clinic input]
list.copy

Return a shallow copy of the list.
[clinic start generated code]*/

static PyObject *
list_copy_impl(PyListObject *self)
/*[clinic end generated code: output=ec6b72d6209d418e input=6453ab159e84771f]*/
{
    return list_slice_clamp(self, 0, PY_SSIZE_T_MAX);
}

/*[clinic input]
list.append

     object: object
     /

Append object to the end of the list.
[clinic start generated code]*/

static PyObject *
list_append(PyListObject *self, PyObject *object)
/*[clinic end generated code: output=7c096003a29c0eae input=43a3fe48a7066e91]*/
{
    int ret;
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = app1(self, object);
    Py_END_CRITICAL_SECTION;
    return ret == 0 ? Py_None : NULL;
}

/* Extend a list from the array of a tuple or list. */
static PyObject *
list_extend_fast(PyListObject *self, PyObject **src, Py_ssize_t n)
{
    Py_ssize_t m = Py_SIZE(self);
    /* It should not be possible to allocate a list large enough to cause
    an overflow on any relevant platform */
    assert(m < PY_SSIZE_T_MAX - n);
    if (list_ensure_capacity(self, m, n) < 0) {
        return NULL;
    }
    /* populate the end of self with src's items */
    PyObject **dest = self->ob_item + m;
    for (Py_ssize_t i = 0; i < n; i++) {
        PyObject *o = Py_NewRef(src[i]);
        _Py_atomic_store_ptr_release(&dest[i], o);
    }
    PyList_SET_SIZE(self, m + n);
    Py_RETURN_NONE;
}

static PyObject *
list_extend_set(PyListObject *self, PySetObject *other)
{
    Py_ssize_t m = Py_SIZE(self);
    Py_ssize_t n = PySet_GET_SIZE(other);
    if (list_ensure_capacity(self, m, n) < 0) {
        return NULL;
    }
    /* populate the end of self with iterable's items */
    Py_ssize_t setpos = 0;
    Py_hash_t hash;
    PyObject *key;
    PyObject **dest = self->ob_item + m;
    while (_PySet_NextEntry((PyObject *)other, &setpos, &key, &hash)) {
        Py_INCREF(key);
        *dest = key;
        dest++;
    }
    PyList_SET_SIZE(self, m + n);
    Py_RETURN_NONE;
}

static PyObject *
list_extend_dict(PyListObject *self, PyDictObject *dict, int which_item)
{
    // which_item: 0 for keys and 1 for values
    Py_ssize_t m = Py_SIZE(self);
    Py_ssize_t n = PyDict_GET_SIZE(dict);
    if (list_ensure_capacity(self, m, n) < 0) {
        return NULL;
    }

    PyObject **dest = self->ob_item + m;
    Py_ssize_t pos = 0;
    PyObject *keyvalue[2];
    while (PyDict_Next((PyObject *)dict, &pos, &keyvalue[0], &keyvalue[1])) {
        PyObject *obj = keyvalue[which_item];
        Py_INCREF(obj);
        *dest = obj;
        dest++;
    }

    PyList_SET_SIZE(self, m + n);
    Py_RETURN_NONE;
}

static PyObject *
list_extend_dictitems(PyListObject *self, PyDictObject *dict)
{
    Py_ssize_t m = Py_SIZE(self);
    Py_ssize_t n = PyDict_GET_SIZE(dict);
    if (list_ensure_capacity(self, m, n) < 0) {
        return NULL;
    }

    // don't start a garbage collection cycle when creating tuples
    PyThreadState *tstate = PyThreadState_GET();
    tstate->cant_stop_wont_stop++;

    PyObject **dest = self->ob_item + m;
    Py_ssize_t pos = 0;
    Py_ssize_t i = 0;
    PyObject *key, *value;
    while (PyDict_Next((PyObject *)dict, &pos, &key, &value)) {
        PyObject *item = PyTuple_Pack(2, key, value);
        if (item == NULL) {
            tstate->cant_stop_wont_stop--;
            PyList_SET_SIZE(self, m + i);
            return NULL;
        }
        _Py_atomic_store_ptr_release(dest, item);
        dest++;
        i++;
    }

    tstate->cant_stop_wont_stop--;
    PyList_SET_SIZE(self, m + n);
    Py_RETURN_NONE;
}

static PyObject *
list_extend_iterator(PyListObject *self, PyObject *it, Py_ssize_t n)
{
    Py_ssize_t m = Py_SIZE(self);
    if (list_ensure_capacity(self, m, n) < 0) {
        // The lenght hint might be fake, so continue anwyays. If we actually
        // don't have enough memory we'll fail in the app1 step.
        PyErr_Clear();
    }
    /* Run iterator to exhaustion. */
    iternextfunc iternext = *Py_TYPE(it)->tp_iternext;
    for (;;) {
        PyObject *item = iternext(it);
        if (item == NULL) {
            if (PyErr_Occurred()) {
                if (PyErr_ExceptionMatches(PyExc_StopIteration))
                    PyErr_Clear();
                else
                    return NULL;
            }
            break;
        }
        if (_PyList_AppendTakeRef(self, item) < 0) {
            return NULL;
        }
    }

    /* Cut back result list if initial guess was too large. */
    // FIXME(sgross): add this back in a sane way
    // if (Py_SIZE(self) < self->allocated) {
    //     if (list_resize(self, Py_SIZE(self)) < 0)
    //         return NULL;
    // }
    Py_RETURN_NONE;
}

/*[clinic input]
list.extend

     iterable: object
     /

Extend list by appending elements from the iterable.
[clinic start generated code]*/

static PyObject *
list_extend(PyListObject *self, PyObject *iterable)
/*[clinic end generated code: output=630fb3bca0c8e789 input=9ec5ba3a81be3a4d]*/
{
    /* Lots of special cases for performance and atomicity. */
    if ((PyObject *)self == iterable) {
        return list_inplace_repeat(self, 2);
    }
    else if (PyList_CheckExact(iterable)) {
        PyListObject *other = (PyListObject *)iterable;
        PyObject *ret;
        Py_BEGIN_CRITICAL_SECTION2(self, other);
        ret = list_extend_fast(self, other->ob_item, Py_SIZE(other));
        Py_END_CRITICAL_SECTION2;
        return ret;
    }
    else if (PyTuple_CheckExact(iterable)) {
        PyTupleObject *other = (PyTupleObject *)iterable;
        PyObject *ret;
        Py_BEGIN_CRITICAL_SECTION(self);
        ret = list_extend_fast(self, _PyTuple_ITEMS(other), Py_SIZE(other));
        Py_END_CRITICAL_SECTION;
        return ret;
    }
    else if (PyAnySet_CheckExact(iterable)) {
        // special-case set handling to make list(set) and list.extend(set)
        // atomic with respect to other set modifications.
        PySetObject *other = (PySetObject *)iterable;
        PyObject *ret;
        Py_BEGIN_CRITICAL_SECTION2(self, other);
        ret = list_extend_set(self, other);
        Py_END_CRITICAL_SECTION2;
        return ret;
    }
    else if (PyDictKeys_Check(iterable)) {
        PyObject *ret;
        PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
        Py_BEGIN_CRITICAL_SECTION2(self, dict);
        ret = list_extend_dict(self, dict, 0 /*keys*/);
        Py_END_CRITICAL_SECTION2;
        return ret;
    }
    else if (PyDictValues_Check(iterable)) {
        PyObject *ret;
        PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
        Py_BEGIN_CRITICAL_SECTION2(self, dict);
        ret = list_extend_dict(self, dict, 1 /*values*/);
        Py_END_CRITICAL_SECTION2;
        return ret;
    }
    else if (PyDictItems_Check(iterable)) {
        PyObject *ret;
        PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
        Py_BEGIN_CRITICAL_SECTION2(self, dict);
        ret = list_extend_dictitems(self, dict);
        Py_END_CRITICAL_SECTION2;
        return ret;
    }

    PyObject *it = PyObject_GetIter(iterable);
    if (it == NULL) {
        return NULL;
    }

    /* Guess a result list size. */
    Py_ssize_t n = PyObject_LengthHint(iterable, 8);
    if (n < 0) {
        Py_DECREF(it);
        return NULL;
    }

    PyObject *ret;
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = list_extend_iterator(self, it, n);
    Py_END_CRITICAL_SECTION;
    Py_DECREF(it);
    return ret;
}

PyObject *
_PyList_Extend(PyListObject *self, PyObject *iterable)
{
    return list_extend(self, iterable);
}

static PyObject *
list_inplace_concat(PyListObject *self, PyObject *other)
{
    PyObject *result;

    result = list_extend(self, other);
    if (result == NULL)
        return result;
    Py_DECREF(result);
    return Py_NewRef(self);
}

/*[clinic input]
list.pop

    index: Py_ssize_t = -1
    /

Remove and return item at index (default last).

Raises IndexError if list is empty or index is out of range.
[clinic start generated code]*/

static PyObject *
list_pop_impl(PyListObject *self, Py_ssize_t index)
/*[clinic end generated code: output=6bd69dcb3f17eca8 input=b83675976f329e6f]*/
{
    PyObject *v = NULL;
    Py_BEGIN_CRITICAL_SECTION(self);
    Py_ssize_t size = Py_SIZE(self);
    if (index < 0) {
        index += size;
    }
    if (!valid_index(index, size)) {
        if (size == 0) {
            PyErr_SetString(PyExc_IndexError, "pop from empty list");
        }
        else {
            PyErr_SetString(PyExc_IndexError, "pop index out of range");
        }
        goto exit;
    }
    PyList_SET_SIZE(self, size - 1);
    for (Py_ssize_t j = size - 1; j >= index; j--) {
        PyObject *tmp = self->ob_item[j];
        _Py_atomic_store_ptr_relaxed(&self->ob_item[j], v);
        v = tmp;
    }
exit:
    Py_END_CRITICAL_SECTION;
    return v;
}

/* Reverse a slice of a list in place, from lo up to (exclusive) hi. */
static void
reverse_slice(PyObject **lo, PyObject **hi)
{
    assert(lo && hi);

    --hi;
    while (lo < hi) {
        PyObject *t = *lo;
        *lo = *hi;
        *hi = t;
        ++lo;
        --hi;
    }
}

/* Lots of code for an adaptive, stable, natural mergesort.  There are many
 * pieces to this algorithm; read listsort.txt for overviews and details.
 */

/* A sortslice contains a pointer to an array of keys and a pointer to
 * an array of corresponding values.  In other words, keys[i]
 * corresponds with values[i].  If values == NULL, then the keys are
 * also the values.
 *
 * Several convenience routines are provided here, so that keys and
 * values are always moved in sync.
 */

typedef struct {
    PyObject **keys;
    PyObject **values;
} sortslice;

Py_LOCAL_INLINE(void)
sortslice_copy(sortslice *s1, Py_ssize_t i, sortslice *s2, Py_ssize_t j)
{
    s1->keys[i] = s2->keys[j];
    if (s1->values != NULL)
        s1->values[i] = s2->values[j];
}

Py_LOCAL_INLINE(void)
sortslice_copy_incr(sortslice *dst, sortslice *src)
{
    *dst->keys++ = *src->keys++;
    if (dst->values != NULL)
        *dst->values++ = *src->values++;
}

Py_LOCAL_INLINE(void)
sortslice_copy_decr(sortslice *dst, sortslice *src)
{
    *dst->keys-- = *src->keys--;
    if (dst->values != NULL)
        *dst->values-- = *src->values--;
}


Py_LOCAL_INLINE(void)
sortslice_memcpy(sortslice *s1, Py_ssize_t i, sortslice *s2, Py_ssize_t j,
                 Py_ssize_t n)
{
    memcpy(&s1->keys[i], &s2->keys[j], sizeof(PyObject *) * n);
    if (s1->values != NULL)
        memcpy(&s1->values[i], &s2->values[j], sizeof(PyObject *) * n);
}

Py_LOCAL_INLINE(void)
sortslice_memmove(sortslice *s1, Py_ssize_t i, sortslice *s2, Py_ssize_t j,
                  Py_ssize_t n)
{
    memmove(&s1->keys[i], &s2->keys[j], sizeof(PyObject *) * n);
    if (s1->values != NULL)
        memmove(&s1->values[i], &s2->values[j], sizeof(PyObject *) * n);
}

Py_LOCAL_INLINE(void)
sortslice_advance(sortslice *slice, Py_ssize_t n)
{
    slice->keys += n;
    if (slice->values != NULL)
        slice->values += n;
}

/* Comparison function: ms->key_compare, which is set at run-time in
 * listsort_impl to optimize for various special cases.
 * Returns -1 on error, 1 if x < y, 0 if x >= y.
 */

#define ISLT(X, Y) (*(ms->key_compare))(X, Y, ms)

/* Compare X to Y via "<".  Goto "fail" if the comparison raises an
   error.  Else "k" is set to true iff X<Y, and an "if (k)" block is
   started.  It makes more sense in context <wink>.  X and Y are PyObject*s.
*/
#define IFLT(X, Y) if ((k = ISLT(X, Y)) < 0) goto fail;  \
           if (k)

/* The maximum number of entries in a MergeState's pending-runs stack.
 * For a list with n elements, this needs at most floor(log2(n)) + 1 entries
 * even if we didn't force runs to a minimal length.  So the number of bits
 * in a Py_ssize_t is plenty large enough for all cases.
 */
#define MAX_MERGE_PENDING (SIZEOF_SIZE_T * 8)

/* When we get into galloping mode, we stay there until both runs win less
 * often than MIN_GALLOP consecutive times.  See listsort.txt for more info.
 */
#define MIN_GALLOP 7

/* Avoid malloc for small temp arrays. */
#define MERGESTATE_TEMP_SIZE 256

/* One MergeState exists on the stack per invocation of mergesort.  It's just
 * a convenient way to pass state around among the helper functions.
 */
struct s_slice {
    sortslice base;
    Py_ssize_t len;   /* length of run */
    int power; /* node "level" for powersort merge strategy */
};

typedef struct s_MergeState MergeState;
struct s_MergeState {
    /* This controls when we get *into* galloping mode.  It's initialized
     * to MIN_GALLOP.  merge_lo and merge_hi tend to nudge it higher for
     * random data, and lower for highly structured data.
     */
    Py_ssize_t min_gallop;

    Py_ssize_t listlen;     /* len(input_list) - read only */
    PyObject **basekeys;    /* base address of keys array - read only */

    /* 'a' is temp storage to help with merges.  It contains room for
     * alloced entries.
     */
    sortslice a;        /* may point to temparray below */
    Py_ssize_t alloced;

    /* A stack of n pending runs yet to be merged.  Run #i starts at
     * address base[i] and extends for len[i] elements.  It's always
     * true (so long as the indices are in bounds) that
     *
     *     pending[i].base + pending[i].len == pending[i+1].base
     *
     * so we could cut the storage for this, but it's a minor amount,
     * and keeping all the info explicit simplifies the code.
     */
    int n;
    struct s_slice pending[MAX_MERGE_PENDING];

    /* 'a' points to this when possible, rather than muck with malloc. */
    PyObject *temparray[MERGESTATE_TEMP_SIZE];

    /* This is the function we will use to compare two keys,
     * even when none of our special cases apply and we have to use
     * safe_object_compare. */
    int (*key_compare)(PyObject *, PyObject *, MergeState *);

    /* This function is used by unsafe_object_compare to optimize comparisons
     * when we know our list is type-homogeneous but we can't assume anything else.
     * In the pre-sort check it is set equal to Py_TYPE(key)->tp_richcompare */
    PyObject *(*key_richcompare)(PyObject *, PyObject *, int);

    /* This function is used by unsafe_tuple_compare to compare the first elements
     * of tuples. It may be set to safe_object_compare, but the idea is that hopefully
     * we can assume more, and use one of the special-case compares. */
    int (*tuple_elem_compare)(PyObject *, PyObject *, MergeState *);
};

/* binarysort is the best method for sorting small arrays: it does
   few compares, but can do data movement quadratic in the number of
   elements.
   [lo, hi) is a contiguous slice of a list, and is sorted via
   binary insertion.  This sort is stable.
   On entry, must have lo <= start <= hi, and that [lo, start) is already
   sorted (pass start == lo if you don't know!).
   If islt() complains return -1, else 0.
   Even in case of error, the output slice will be some permutation of
   the input (nothing is lost or duplicated).
*/
static int
binarysort(MergeState *ms, sortslice lo, PyObject **hi, PyObject **start)
{
    Py_ssize_t k;
    PyObject **l, **p, **r;
    PyObject *pivot;

    assert(lo.keys <= start && start <= hi);
    /* assert [lo, start) is sorted */
    if (lo.keys == start)
        ++start;
    for (; start < hi; ++start) {
        /* set l to where *start belongs */
        l = lo.keys;
        r = start;
        pivot = *r;
        /* Invariants:
         * pivot >= all in [lo, l).
         * pivot  < all in [r, start).
         * The second is vacuously true at the start.
         */
        assert(l < r);
        do {
            p = l + ((r - l) >> 1);
            IFLT(pivot, *p)
                r = p;
            else
                l = p+1;
        } while (l < r);
        assert(l == r);
        /* The invariants still hold, so pivot >= all in [lo, l) and
           pivot < all in [l, start), so pivot belongs at l.  Note
           that if there are elements equal to pivot, l points to the
           first slot after them -- that's why this sort is stable.
           Slide over to make room.
           Caution: using memmove is much slower under MSVC 5;
           we're not usually moving many slots. */
        for (p = start; p > l; --p)
            *p = *(p-1);
        *l = pivot;
        if (lo.values != NULL) {
            Py_ssize_t offset = lo.values - lo.keys;
            p = start + offset;
            pivot = *p;
            l += offset;
            for (p = start + offset; p > l; --p)
                *p = *(p-1);
            *l = pivot;
        }
    }
    return 0;

 fail:
    return -1;
}

/*
Return the length of the run beginning at lo, in the slice [lo, hi).  lo < hi
is required on entry.  "A run" is the longest ascending sequence, with

    lo[0] <= lo[1] <= lo[2] <= ...

or the longest descending sequence, with

    lo[0] > lo[1] > lo[2] > ...

Boolean *descending is set to 0 in the former case, or to 1 in the latter.
For its intended use in a stable mergesort, the strictness of the defn of
"descending" is needed so that the caller can safely reverse a descending
sequence without violating stability (strict > ensures there are no equal
elements to get out of order).

Returns -1 in case of error.
*/
static Py_ssize_t
count_run(MergeState *ms, PyObject **lo, PyObject **hi, int *descending)
{
    Py_ssize_t k;
    Py_ssize_t n;

    assert(lo < hi);
    *descending = 0;
    ++lo;
    if (lo == hi)
        return 1;

    n = 2;
    IFLT(*lo, *(lo-1)) {
        *descending = 1;
        for (lo = lo+1; lo < hi; ++lo, ++n) {
            IFLT(*lo, *(lo-1))
                ;
            else
                break;
        }
    }
    else {
        for (lo = lo+1; lo < hi; ++lo, ++n) {
            IFLT(*lo, *(lo-1))
                break;
        }
    }

    return n;
fail:
    return -1;
}

/*
Locate the proper position of key in a sorted vector; if the vector contains
an element equal to key, return the position immediately to the left of
the leftmost equal element.  [gallop_right() does the same except returns
the position to the right of the rightmost equal element (if any).]

"a" is a sorted vector with n elements, starting at a[0].  n must be > 0.

"hint" is an index at which to begin the search, 0 <= hint < n.  The closer
hint is to the final result, the faster this runs.

The return value is the int k in 0..n such that

    a[k-1] < key <= a[k]

pretending that *(a-1) is minus infinity and a[n] is plus infinity.  IOW,
key belongs at index k; or, IOW, the first k elements of a should precede
key, and the last n-k should follow key.

Returns -1 on error.  See listsort.txt for info on the method.
*/
static Py_ssize_t
gallop_left(MergeState *ms, PyObject *key, PyObject **a, Py_ssize_t n, Py_ssize_t hint)
{
    Py_ssize_t ofs;
    Py_ssize_t lastofs;
    Py_ssize_t k;

    assert(key && a && n > 0 && hint >= 0 && hint < n);

    a += hint;
    lastofs = 0;
    ofs = 1;
    IFLT(*a, key) {
        /* a[hint] < key -- gallop right, until
         * a[hint + lastofs] < key <= a[hint + ofs]
         */
        const Py_ssize_t maxofs = n - hint;             /* &a[n-1] is highest */
        while (ofs < maxofs) {
            IFLT(a[ofs], key) {
                lastofs = ofs;
                assert(ofs <= (PY_SSIZE_T_MAX - 1) / 2);
                ofs = (ofs << 1) + 1;
            }
            else                /* key <= a[hint + ofs] */
                break;
        }
        if (ofs > maxofs)
            ofs = maxofs;
        /* Translate back to offsets relative to &a[0]. */
        lastofs += hint;
        ofs += hint;
    }
    else {
        /* key <= a[hint] -- gallop left, until
         * a[hint - ofs] < key <= a[hint - lastofs]
         */
        const Py_ssize_t maxofs = hint + 1;             /* &a[0] is lowest */
        while (ofs < maxofs) {
            IFLT(*(a-ofs), key)
                break;
            /* key <= a[hint - ofs] */
            lastofs = ofs;
            assert(ofs <= (PY_SSIZE_T_MAX - 1) / 2);
            ofs = (ofs << 1) + 1;
        }
        if (ofs > maxofs)
            ofs = maxofs;
        /* Translate back to positive offsets relative to &a[0]. */
        k = lastofs;
        lastofs = hint - ofs;
        ofs = hint - k;
    }
    a -= hint;

    assert(-1 <= lastofs && lastofs < ofs && ofs <= n);
    /* Now a[lastofs] < key <= a[ofs], so key belongs somewhere to the
     * right of lastofs but no farther right than ofs.  Do a binary
     * search, with invariant a[lastofs-1] < key <= a[ofs].
     */
    ++lastofs;
    while (lastofs < ofs) {
        Py_ssize_t m = lastofs + ((ofs - lastofs) >> 1);

        IFLT(a[m], key)
            lastofs = m+1;              /* a[m] < key */
        else
            ofs = m;                    /* key <= a[m] */
    }
    assert(lastofs == ofs);             /* so a[ofs-1] < key <= a[ofs] */
    return ofs;

fail:
    return -1;
}

/*
Exactly like gallop_left(), except that if key already exists in a[0:n],
finds the position immediately to the right of the rightmost equal value.

The return value is the int k in 0..n such that

    a[k-1] <= key < a[k]

or -1 if error.

The code duplication is massive, but this is enough different given that
we're sticking to "<" comparisons that it's much harder to follow if
written as one routine with yet another "left or right?" flag.
*/
static Py_ssize_t
gallop_right(MergeState *ms, PyObject *key, PyObject **a, Py_ssize_t n, Py_ssize_t hint)
{
    Py_ssize_t ofs;
    Py_ssize_t lastofs;
    Py_ssize_t k;

    assert(key && a && n > 0 && hint >= 0 && hint < n);

    a += hint;
    lastofs = 0;
    ofs = 1;
    IFLT(key, *a) {
        /* key < a[hint] -- gallop left, until
         * a[hint - ofs] <= key < a[hint - lastofs]
         */
        const Py_ssize_t maxofs = hint + 1;             /* &a[0] is lowest */
        while (ofs < maxofs) {
            IFLT(key, *(a-ofs)) {
                lastofs = ofs;
                assert(ofs <= (PY_SSIZE_T_MAX - 1) / 2);
                ofs = (ofs << 1) + 1;
            }
            else                /* a[hint - ofs] <= key */
                break;
        }
        if (ofs > maxofs)
            ofs = maxofs;
        /* Translate back to positive offsets relative to &a[0]. */
        k = lastofs;
        lastofs = hint - ofs;
        ofs = hint - k;
    }
    else {
        /* a[hint] <= key -- gallop right, until
         * a[hint + lastofs] <= key < a[hint + ofs]
        */
        const Py_ssize_t maxofs = n - hint;             /* &a[n-1] is highest */
        while (ofs < maxofs) {
            IFLT(key, a[ofs])
                break;
            /* a[hint + ofs] <= key */
            lastofs = ofs;
            assert(ofs <= (PY_SSIZE_T_MAX - 1) / 2);
            ofs = (ofs << 1) + 1;
        }
        if (ofs > maxofs)
            ofs = maxofs;
        /* Translate back to offsets relative to &a[0]. */
        lastofs += hint;
        ofs += hint;
    }
    a -= hint;

    assert(-1 <= lastofs && lastofs < ofs && ofs <= n);
    /* Now a[lastofs] <= key < a[ofs], so key belongs somewhere to the
     * right of lastofs but no farther right than ofs.  Do a binary
     * search, with invariant a[lastofs-1] <= key < a[ofs].
     */
    ++lastofs;
    while (lastofs < ofs) {
        Py_ssize_t m = lastofs + ((ofs - lastofs) >> 1);

        IFLT(key, a[m])
            ofs = m;                    /* key < a[m] */
        else
            lastofs = m+1;              /* a[m] <= key */
    }
    assert(lastofs == ofs);             /* so a[ofs-1] <= key < a[ofs] */
    return ofs;

fail:
    return -1;
}

/* Conceptually a MergeState's constructor. */
static void
merge_init(MergeState *ms, Py_ssize_t list_size, int has_keyfunc,
           sortslice *lo)
{
    assert(ms != NULL);
    if (has_keyfunc) {
        /* The temporary space for merging will need at most half the list
         * size rounded up.  Use the minimum possible space so we can use the
         * rest of temparray for other things.  In particular, if there is
         * enough extra space, listsort() will use it to store the keys.
         */
        ms->alloced = (list_size + 1) / 2;

        /* ms->alloced describes how many keys will be stored at
           ms->temparray, but we also need to store the values.  Hence,
           ms->alloced is capped at half of MERGESTATE_TEMP_SIZE. */
        if (MERGESTATE_TEMP_SIZE / 2 < ms->alloced)
            ms->alloced = MERGESTATE_TEMP_SIZE / 2;
        ms->a.values = &ms->temparray[ms->alloced];
    }
    else {
        ms->alloced = MERGESTATE_TEMP_SIZE;
        ms->a.values = NULL;
    }
    ms->a.keys = ms->temparray;
    ms->n = 0;
    ms->min_gallop = MIN_GALLOP;
    ms->listlen = list_size;
    ms->basekeys = lo->keys;
}

/* Free all the temp memory owned by the MergeState.  This must be called
 * when you're done with a MergeState, and may be called before then if
 * you want to free the temp memory early.
 */
static void
merge_freemem(MergeState *ms)
{
    assert(ms != NULL);
    if (ms->a.keys != ms->temparray) {
        PyMem_Free(ms->a.keys);
        ms->a.keys = NULL;
    }
}

/* Ensure enough temp memory for 'need' array slots is available.
 * Returns 0 on success and -1 if the memory can't be gotten.
 */
static int
merge_getmem(MergeState *ms, Py_ssize_t need)
{
    int multiplier;

    assert(ms != NULL);
    if (need <= ms->alloced)
        return 0;

    multiplier = ms->a.values != NULL ? 2 : 1;

    /* Don't realloc!  That can cost cycles to copy the old data, but
     * we don't care what's in the block.
     */
    merge_freemem(ms);
    if ((size_t)need > PY_SSIZE_T_MAX / sizeof(PyObject *) / multiplier) {
        PyErr_NoMemory();
        return -1;
    }
    ms->a.keys = (PyObject **)PyMem_Malloc(multiplier * need
                                          * sizeof(PyObject *));
    if (ms->a.keys != NULL) {
        ms->alloced = need;
        if (ms->a.values != NULL)
            ms->a.values = &ms->a.keys[need];
        return 0;
    }
    PyErr_NoMemory();
    return -1;
}
#define MERGE_GETMEM(MS, NEED) ((NEED) <= (MS)->alloced ? 0 :   \
                                merge_getmem(MS, NEED))

/* Merge the na elements starting at ssa with the nb elements starting at
 * ssb.keys = ssa.keys + na in a stable way, in-place.  na and nb must be > 0.
 * Must also have that ssa.keys[na-1] belongs at the end of the merge, and
 * should have na <= nb.  See listsort.txt for more info.  Return 0 if
 * successful, -1 if error.
 */
static Py_ssize_t
merge_lo(MergeState *ms, sortslice ssa, Py_ssize_t na,
         sortslice ssb, Py_ssize_t nb)
{
    Py_ssize_t k;
    sortslice dest;
    int result = -1;            /* guilty until proved innocent */
    Py_ssize_t min_gallop;

    assert(ms && ssa.keys && ssb.keys && na > 0 && nb > 0);
    assert(ssa.keys + na == ssb.keys);
    if (MERGE_GETMEM(ms, na) < 0)
        return -1;
    sortslice_memcpy(&ms->a, 0, &ssa, 0, na);
    dest = ssa;
    ssa = ms->a;

    sortslice_copy_incr(&dest, &ssb);
    --nb;
    if (nb == 0)
        goto Succeed;
    if (na == 1)
        goto CopyB;

    min_gallop = ms->min_gallop;
    for (;;) {
        Py_ssize_t acount = 0;          /* # of times A won in a row */
        Py_ssize_t bcount = 0;          /* # of times B won in a row */

        /* Do the straightforward thing until (if ever) one run
         * appears to win consistently.
         */
        for (;;) {
            assert(na > 1 && nb > 0);
            k = ISLT(ssb.keys[0], ssa.keys[0]);
            if (k) {
                if (k < 0)
                    goto Fail;
                sortslice_copy_incr(&dest, &ssb);
                ++bcount;
                acount = 0;
                --nb;
                if (nb == 0)
                    goto Succeed;
                if (bcount >= min_gallop)
                    break;
            }
            else {
                sortslice_copy_incr(&dest, &ssa);
                ++acount;
                bcount = 0;
                --na;
                if (na == 1)
                    goto CopyB;
                if (acount >= min_gallop)
                    break;
            }
        }

        /* One run is winning so consistently that galloping may
         * be a huge win.  So try that, and continue galloping until
         * (if ever) neither run appears to be winning consistently
         * anymore.
         */
        ++min_gallop;
        do {
            assert(na > 1 && nb > 0);
            min_gallop -= min_gallop > 1;
            ms->min_gallop = min_gallop;
            k = gallop_right(ms, ssb.keys[0], ssa.keys, na, 0);
            acount = k;
            if (k) {
                if (k < 0)
                    goto Fail;
                sortslice_memcpy(&dest, 0, &ssa, 0, k);
                sortslice_advance(&dest, k);
                sortslice_advance(&ssa, k);
                na -= k;
                if (na == 1)
                    goto CopyB;
                /* na==0 is impossible now if the comparison
                 * function is consistent, but we can't assume
                 * that it is.
                 */
                if (na == 0)
                    goto Succeed;
            }
            sortslice_copy_incr(&dest, &ssb);
            --nb;
            if (nb == 0)
                goto Succeed;

            k = gallop_left(ms, ssa.keys[0], ssb.keys, nb, 0);
            bcount = k;
            if (k) {
                if (k < 0)
                    goto Fail;
                sortslice_memmove(&dest, 0, &ssb, 0, k);
                sortslice_advance(&dest, k);
                sortslice_advance(&ssb, k);
                nb -= k;
                if (nb == 0)
                    goto Succeed;
            }
            sortslice_copy_incr(&dest, &ssa);
            --na;
            if (na == 1)
                goto CopyB;
        } while (acount >= MIN_GALLOP || bcount >= MIN_GALLOP);
        ++min_gallop;           /* penalize it for leaving galloping mode */
        ms->min_gallop = min_gallop;
    }
Succeed:
    result = 0;
Fail:
    if (na)
        sortslice_memcpy(&dest, 0, &ssa, 0, na);
    return result;
CopyB:
    assert(na == 1 && nb > 0);
    /* The last element of ssa belongs at the end of the merge. */
    sortslice_memmove(&dest, 0, &ssb, 0, nb);
    sortslice_copy(&dest, nb, &ssa, 0);
    return 0;
}

/* Merge the na elements starting at pa with the nb elements starting at
 * ssb.keys = ssa.keys + na in a stable way, in-place.  na and nb must be > 0.
 * Must also have that ssa.keys[na-1] belongs at the end of the merge, and
 * should have na >= nb.  See listsort.txt for more info.  Return 0 if
 * successful, -1 if error.
 */
static Py_ssize_t
merge_hi(MergeState *ms, sortslice ssa, Py_ssize_t na,
         sortslice ssb, Py_ssize_t nb)
{
    Py_ssize_t k;
    sortslice dest, basea, baseb;
    int result = -1;            /* guilty until proved innocent */
    Py_ssize_t min_gallop;

    assert(ms && ssa.keys && ssb.keys && na > 0 && nb > 0);
    assert(ssa.keys + na == ssb.keys);
    if (MERGE_GETMEM(ms, nb) < 0)
        return -1;
    dest = ssb;
    sortslice_advance(&dest, nb-1);
    sortslice_memcpy(&ms->a, 0, &ssb, 0, nb);
    basea = ssa;
    baseb = ms->a;
    ssb.keys = ms->a.keys + nb - 1;
    if (ssb.values != NULL)
        ssb.values = ms->a.values + nb - 1;
    sortslice_advance(&ssa, na - 1);

    sortslice_copy_decr(&dest, &ssa);
    --na;
    if (na == 0)
        goto Succeed;
    if (nb == 1)
        goto CopyA;

    min_gallop = ms->min_gallop;
    for (;;) {
        Py_ssize_t acount = 0;          /* # of times A won in a row */
        Py_ssize_t bcount = 0;          /* # of times B won in a row */

        /* Do the straightforward thing until (if ever) one run
         * appears to win consistently.
         */
        for (;;) {
            assert(na > 0 && nb > 1);
            k = ISLT(ssb.keys[0], ssa.keys[0]);
            if (k) {
                if (k < 0)
                    goto Fail;
                sortslice_copy_decr(&dest, &ssa);
                ++acount;
                bcount = 0;
                --na;
                if (na == 0)
                    goto Succeed;
                if (acount >= min_gallop)
                    break;
            }
            else {
                sortslice_copy_decr(&dest, &ssb);
                ++bcount;
                acount = 0;
                --nb;
                if (nb == 1)
                    goto CopyA;
                if (bcount >= min_gallop)
                    break;
            }
        }

        /* One run is winning so consistently that galloping may
         * be a huge win.  So try that, and continue galloping until
         * (if ever) neither run appears to be winning consistently
         * anymore.
         */
        ++min_gallop;
        do {
            assert(na > 0 && nb > 1);
            min_gallop -= min_gallop > 1;
            ms->min_gallop = min_gallop;
            k = gallop_right(ms, ssb.keys[0], basea.keys, na, na-1);
            if (k < 0)
                goto Fail;
            k = na - k;
            acount = k;
            if (k) {
                sortslice_advance(&dest, -k);
                sortslice_advance(&ssa, -k);
                sortslice_memmove(&dest, 1, &ssa, 1, k);
                na -= k;
                if (na == 0)
                    goto Succeed;
            }
            sortslice_copy_decr(&dest, &ssb);
            --nb;
            if (nb == 1)
                goto CopyA;

            k = gallop_left(ms, ssa.keys[0], baseb.keys, nb, nb-1);
            if (k < 0)
                goto Fail;
            k = nb - k;
            bcount = k;
            if (k) {
                sortslice_advance(&dest, -k);
                sortslice_advance(&ssb, -k);
                sortslice_memcpy(&dest, 1, &ssb, 1, k);
                nb -= k;
                if (nb == 1)
                    goto CopyA;
                /* nb==0 is impossible now if the comparison
                 * function is consistent, but we can't assume
                 * that it is.
                 */
                if (nb == 0)
                    goto Succeed;
            }
            sortslice_copy_decr(&dest, &ssa);
            --na;
            if (na == 0)
                goto Succeed;
        } while (acount >= MIN_GALLOP || bcount >= MIN_GALLOP);
        ++min_gallop;           /* penalize it for leaving galloping mode */
        ms->min_gallop = min_gallop;
    }
Succeed:
    result = 0;
Fail:
    if (nb)
        sortslice_memcpy(&dest, -(nb-1), &baseb, 0, nb);
    return result;
CopyA:
    assert(nb == 1 && na > 0);
    /* The first element of ssb belongs at the front of the merge. */
    sortslice_memmove(&dest, 1-na, &ssa, 1-na, na);
    sortslice_advance(&dest, -na);
    sortslice_advance(&ssa, -na);
    sortslice_copy(&dest, 0, &ssb, 0);
    return 0;
}

/* Merge the two runs at stack indices i and i+1.
 * Returns 0 on success, -1 on error.
 */
static Py_ssize_t
merge_at(MergeState *ms, Py_ssize_t i)
{
    sortslice ssa, ssb;
    Py_ssize_t na, nb;
    Py_ssize_t k;

    assert(ms != NULL);
    assert(ms->n >= 2);
    assert(i >= 0);
    assert(i == ms->n - 2 || i == ms->n - 3);

    ssa = ms->pending[i].base;
    na = ms->pending[i].len;
    ssb = ms->pending[i+1].base;
    nb = ms->pending[i+1].len;
    assert(na > 0 && nb > 0);
    assert(ssa.keys + na == ssb.keys);

    /* Record the length of the combined runs; if i is the 3rd-last
     * run now, also slide over the last run (which isn't involved
     * in this merge).  The current run i+1 goes away in any case.
     */
    ms->pending[i].len = na + nb;
    if (i == ms->n - 3)
        ms->pending[i+1] = ms->pending[i+2];
    --ms->n;

    /* Where does b start in a?  Elements in a before that can be
     * ignored (already in place).
     */
    k = gallop_right(ms, *ssb.keys, ssa.keys, na, 0);
    if (k < 0)
        return -1;
    sortslice_advance(&ssa, k);
    na -= k;
    if (na == 0)
        return 0;

    /* Where does a end in b?  Elements in b after that can be
     * ignored (already in place).
     */
    nb = gallop_left(ms, ssa.keys[na-1], ssb.keys, nb, nb-1);
    if (nb <= 0)
        return nb;

    /* Merge what remains of the runs, using a temp array with
     * min(na, nb) elements.
     */
    if (na <= nb)
        return merge_lo(ms, ssa, na, ssb, nb);
    else
        return merge_hi(ms, ssa, na, ssb, nb);
}

/* Two adjacent runs begin at index s1. The first run has length n1, and
 * the second run (starting at index s1+n1) has length n2. The list has total
 * length n.
 * Compute the "power" of the first run. See listsort.txt for details.
 */
static int
powerloop(Py_ssize_t s1, Py_ssize_t n1, Py_ssize_t n2, Py_ssize_t n)
{
    int result = 0;
    assert(s1 >= 0);
    assert(n1 > 0 && n2 > 0);
    assert(s1 + n1 + n2 <= n);
    /* midpoints a and b:
     * a = s1 + n1/2
     * b = s1 + n1 + n2/2 = a + (n1 + n2)/2
     *
     * Those may not be integers, though, because of the "/2". So we work with
     * 2*a and 2*b instead, which are necessarily integers. It makes no
     * difference to the outcome, since the bits in the expansion of (2*i)/n
     * are merely shifted one position from those of i/n.
     */
    Py_ssize_t a = 2 * s1 + n1;  /* 2*a */
    Py_ssize_t b = a + n1 + n2;  /* 2*b */
    /* Emulate a/n and b/n one bit a time, until bits differ. */
    for (;;) {
        ++result;
        if (a >= n) {  /* both quotient bits are 1 */
            assert(b >= a);
            a -= n;
            b -= n;
        }
        else if (b >= n) {  /* a/n bit is 0, b/n bit is 1 */
            break;
        } /* else both quotient bits are 0 */
        assert(a < b && b < n);
        a <<= 1;
        b <<= 1;
    }
    return result;
}

/* The next run has been identified, of length n2.
 * If there's already a run on the stack, apply the "powersort" merge strategy:
 * compute the topmost run's "power" (depth in a conceptual binary merge tree)
 * and merge adjacent runs on the stack with greater power. See listsort.txt
 * for more info.
 *
 * It's the caller's responsibility to push the new run on the stack when this
 * returns.
 *
 * Returns 0 on success, -1 on error.
 */
static int
found_new_run(MergeState *ms, Py_ssize_t n2)
{
    assert(ms);
    if (ms->n) {
        assert(ms->n > 0);
        struct s_slice *p = ms->pending;
        Py_ssize_t s1 = p[ms->n - 1].base.keys - ms->basekeys; /* start index */
        Py_ssize_t n1 = p[ms->n - 1].len;
        int power = powerloop(s1, n1, n2, ms->listlen);
        while (ms->n > 1 && p[ms->n - 2].power > power) {
            if (merge_at(ms, ms->n - 2) < 0)
                return -1;
        }
        assert(ms->n < 2 || p[ms->n - 2].power < power);
        p[ms->n - 1].power = power;
    }
    return 0;
}

/* Regardless of invariants, merge all runs on the stack until only one
 * remains.  This is used at the end of the mergesort.
 *
 * Returns 0 on success, -1 on error.
 */
static int
merge_force_collapse(MergeState *ms)
{
    struct s_slice *p = ms->pending;

    assert(ms);
    while (ms->n > 1) {
        Py_ssize_t n = ms->n - 2;
        if (n > 0 && p[n-1].len < p[n+1].len)
            --n;
        if (merge_at(ms, n) < 0)
            return -1;
    }
    return 0;
}

/* Compute a good value for the minimum run length; natural runs shorter
 * than this are boosted artificially via binary insertion.
 *
 * If n < 64, return n (it's too small to bother with fancy stuff).
 * Else if n is an exact power of 2, return 32.
 * Else return an int k, 32 <= k <= 64, such that n/k is close to, but
 * strictly less than, an exact power of 2.
 *
 * See listsort.txt for more info.
 */
static Py_ssize_t
merge_compute_minrun(Py_ssize_t n)
{
    Py_ssize_t r = 0;           /* becomes 1 if any 1 bits are shifted off */

    assert(n >= 0);
    while (n >= 64) {
        r |= n & 1;
        n >>= 1;
    }
    return n + r;
}

static void
reverse_sortslice(sortslice *s, Py_ssize_t n)
{
    reverse_slice(s->keys, &s->keys[n]);
    if (s->values != NULL)
        reverse_slice(s->values, &s->values[n]);
}

/* Here we define custom comparison functions to optimize for the cases one commonly
 * encounters in practice: homogeneous lists, often of one of the basic types. */

/* This struct holds the comparison function and helper functions
 * selected in the pre-sort check. */

/* These are the special case compare functions.
 * ms->key_compare will always point to one of these: */

/* Heterogeneous compare: default, always safe to fall back on. */
static int
safe_object_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    /* No assumptions necessary! */
    return PyObject_RichCompareBool(v, w, Py_LT);
}

/* Homogeneous compare: safe for any two comparable objects of the same type.
 * (ms->key_richcompare is set to ob_type->tp_richcompare in the
 *  pre-sort check.)
 */
static int
unsafe_object_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    PyObject *res_obj; int res;

    /* No assumptions, because we check first: */
    if (Py_TYPE(v)->tp_richcompare != ms->key_richcompare)
        return PyObject_RichCompareBool(v, w, Py_LT);

    assert(ms->key_richcompare != NULL);
    res_obj = (*(ms->key_richcompare))(v, w, Py_LT);

    if (res_obj == Py_NotImplemented) {
        Py_DECREF(res_obj);
        return PyObject_RichCompareBool(v, w, Py_LT);
    }
    if (res_obj == NULL)
        return -1;

    if (PyBool_Check(res_obj)) {
        res = (res_obj == Py_True);
    }
    else {
        res = PyObject_IsTrue(res_obj);
    }
    Py_DECREF(res_obj);

    /* Note that we can't assert
     *     res == PyObject_RichCompareBool(v, w, Py_LT);
     * because of evil compare functions like this:
     *     lambda a, b:  int(random.random() * 3) - 1)
     * (which is actually in test_sort.py) */
    return res;
}

/* Latin string compare: safe for any two latin (one byte per char) strings. */
static int
unsafe_latin_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    Py_ssize_t len;
    int res;

    /* Modified from Objects/unicodeobject.c:unicode_compare, assuming: */
    assert(Py_IS_TYPE(v, &PyUnicode_Type));
    assert(Py_IS_TYPE(w, &PyUnicode_Type));
    assert(PyUnicode_KIND(v) == PyUnicode_KIND(w));
    assert(PyUnicode_KIND(v) == PyUnicode_1BYTE_KIND);

    len = Py_MIN(PyUnicode_GET_LENGTH(v), PyUnicode_GET_LENGTH(w));
    res = memcmp(PyUnicode_DATA(v), PyUnicode_DATA(w), len);

    res = (res != 0 ?
           res < 0 :
           PyUnicode_GET_LENGTH(v) < PyUnicode_GET_LENGTH(w));

    assert(res == PyObject_RichCompareBool(v, w, Py_LT));;
    return res;
}

/* Bounded int compare: compare any two longs that fit in a single machine word. */
static int
unsafe_long_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    PyLongObject *vl, *wl; sdigit v0, w0; int res;

    /* Modified from Objects/longobject.c:long_compare, assuming: */
    assert(Py_IS_TYPE(v, &PyLong_Type));
    assert(Py_IS_TYPE(w, &PyLong_Type));
    assert(Py_ABS(Py_SIZE(v)) <= 1);
    assert(Py_ABS(Py_SIZE(w)) <= 1);

    vl = (PyLongObject*)v;
    wl = (PyLongObject*)w;

    v0 = Py_SIZE(vl) == 0 ? 0 : (sdigit)vl->ob_digit[0];
    w0 = Py_SIZE(wl) == 0 ? 0 : (sdigit)wl->ob_digit[0];

    if (Py_SIZE(vl) < 0)
        v0 = -v0;
    if (Py_SIZE(wl) < 0)
        w0 = -w0;

    res = v0 < w0;
    assert(res == PyObject_RichCompareBool(v, w, Py_LT));
    return res;
}

/* Float compare: compare any two floats. */
static int
unsafe_float_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    int res;

    /* Modified from Objects/floatobject.c:float_richcompare, assuming: */
    assert(Py_IS_TYPE(v, &PyFloat_Type));
    assert(Py_IS_TYPE(w, &PyFloat_Type));

    res = PyFloat_AS_DOUBLE(v) < PyFloat_AS_DOUBLE(w);
    assert(res == PyObject_RichCompareBool(v, w, Py_LT));
    return res;
}

/* Tuple compare: compare *any* two tuples, using
 * ms->tuple_elem_compare to compare the first elements, which is set
 * using the same pre-sort check as we use for ms->key_compare,
 * but run on the list [x[0] for x in L]. This allows us to optimize compares
 * on two levels (as long as [x[0] for x in L] is type-homogeneous.) The idea is
 * that most tuple compares don't involve x[1:]. */
static int
unsafe_tuple_compare(PyObject *v, PyObject *w, MergeState *ms)
{
    PyTupleObject *vt, *wt;
    Py_ssize_t i, vlen, wlen;
    int k;

    /* Modified from Objects/tupleobject.c:tuplerichcompare, assuming: */
    assert(Py_IS_TYPE(v, &PyTuple_Type));
    assert(Py_IS_TYPE(w, &PyTuple_Type));
    assert(Py_SIZE(v) > 0);
    assert(Py_SIZE(w) > 0);

    vt = (PyTupleObject *)v;
    wt = (PyTupleObject *)w;

    vlen = Py_SIZE(vt);
    wlen = Py_SIZE(wt);

    for (i = 0; i < vlen && i < wlen; i++) {
        k = PyObject_RichCompareBool(vt->ob_item[i], wt->ob_item[i], Py_EQ);
        if (k < 0)
            return -1;
        if (!k)
            break;
    }

    if (i >= vlen || i >= wlen)
        return vlen < wlen;

    if (i == 0)
        return ms->tuple_elem_compare(vt->ob_item[i], wt->ob_item[i], ms);
    else
        return PyObject_RichCompareBool(vt->ob_item[i], wt->ob_item[i], Py_LT);
}

/* An adaptive, stable, natural mergesort.  See listsort.txt.
 * Returns Py_None on success, NULL on error.  Even in case of error, the
 * list will be some permutation of its input state (nothing is lost or
 * duplicated).
 */
/*[clinic input]
list.sort

    *
    key as keyfunc: object = None
    reverse: bool = False

Sort the list in ascending order and return None.

The sort is in-place (i.e. the list itself is modified) and stable (i.e. the
order of two equal elements is maintained).

If a key function is given, apply it once to each list item and sort them,
ascending or descending, according to their function values.

The reverse flag can be set to sort in descending order.
[clinic start generated code]*/

static PyObject *
list_sort_impl(PyListObject *self, PyObject *keyfunc, int reverse)
/*[clinic end generated code: output=57b9f9c5e23fbe42 input=a74c4cd3ec6b5c08]*/
{
    MergeState ms;
    Py_ssize_t nremaining;
    Py_ssize_t minrun;
    sortslice lo;
    Py_ssize_t saved_ob_size, saved_allocated;
    PyObject **saved_ob_item;
    PyObject **final_ob_item;
    PyObject *result = NULL;            /* guilty until proved innocent */
    Py_ssize_t i;
    PyObject **keys;

    assert(self != NULL);
    assert(PyList_Check(self));
    if (keyfunc == Py_None)
        keyfunc = NULL;

    /* The list is temporarily made empty, so that mutations performed
     * by comparison functions can't affect the slice of memory we're
     * sorting (allowing mutations during sorting is a core-dump
     * factory, since ob_item may change).
     */
    saved_ob_size = Py_SIZE(self);
    saved_ob_item = self->ob_item;
    saved_allocated = self->allocated;
    Py_SET_SIZE(self, 0);
    self->ob_item = NULL;
    self->allocated = -1; /* any operation will reset it to >= 0 */

    if (keyfunc == NULL) {
        keys = NULL;
        lo.keys = saved_ob_item;
        lo.values = NULL;
    }
    else {
        if (saved_ob_size < MERGESTATE_TEMP_SIZE/2)
            /* Leverage stack space we allocated but won't otherwise use */
            keys = &ms.temparray[saved_ob_size+1];
        else {
            keys = PyMem_Malloc(sizeof(PyObject *) * saved_ob_size);
            if (keys == NULL) {
                PyErr_NoMemory();
                goto keyfunc_fail;
            }
        }

        for (i = 0; i < saved_ob_size ; i++) {
            keys[i] = PyObject_CallOneArg(keyfunc, saved_ob_item[i]);
            if (keys[i] == NULL) {
                for (i=i-1 ; i>=0 ; i--)
                    Py_DECREF(keys[i]);
                if (saved_ob_size >= MERGESTATE_TEMP_SIZE/2)
                    PyMem_Free(keys);
                goto keyfunc_fail;
            }
        }

        lo.keys = keys;
        lo.values = saved_ob_item;
    }


    /* The pre-sort check: here's where we decide which compare function to use.
     * How much optimization is safe? We test for homogeneity with respect to
     * several properties that are expensive to check at compare-time, and
     * set ms appropriately. */
    if (saved_ob_size > 1) {
        /* Assume the first element is representative of the whole list. */
        int keys_are_in_tuples = (Py_IS_TYPE(lo.keys[0], &PyTuple_Type) &&
                                  Py_SIZE(lo.keys[0]) > 0);

        PyTypeObject* key_type = (keys_are_in_tuples ?
                                  Py_TYPE(PyTuple_GET_ITEM(lo.keys[0], 0)) :
                                  Py_TYPE(lo.keys[0]));

        int keys_are_all_same_type = 1;
        int strings_are_latin = 1;
        int ints_are_bounded = 1;

        /* Prove that assumption by checking every key. */
        for (i=0; i < saved_ob_size; i++) {

            if (keys_are_in_tuples &&
                !(Py_IS_TYPE(lo.keys[i], &PyTuple_Type) && Py_SIZE(lo.keys[i]) != 0)) {
                keys_are_in_tuples = 0;
                keys_are_all_same_type = 0;
                break;
            }

            /* Note: for lists of tuples, key is the first element of the tuple
             * lo.keys[i], not lo.keys[i] itself! We verify type-homogeneity
             * for lists of tuples in the if-statement directly above. */
            PyObject *key = (keys_are_in_tuples ?
                             PyTuple_GET_ITEM(lo.keys[i], 0) :
                             lo.keys[i]);

            if (!Py_IS_TYPE(key, key_type)) {
                keys_are_all_same_type = 0;
                /* If keys are in tuple we must loop over the whole list to make
                   sure all items are tuples */
                if (!keys_are_in_tuples) {
                    break;
                }
            }

            if (keys_are_all_same_type) {
                if (key_type == &PyLong_Type &&
                    ints_are_bounded &&
                    Py_ABS(Py_SIZE(key)) > 1) {

                    ints_are_bounded = 0;
                }
                else if (key_type == &PyUnicode_Type &&
                         strings_are_latin &&
                         PyUnicode_KIND(key) != PyUnicode_1BYTE_KIND) {

                        strings_are_latin = 0;
                    }
                }
            }

        /* Choose the best compare, given what we now know about the keys. */
        if (keys_are_all_same_type) {

            if (key_type == &PyUnicode_Type && strings_are_latin) {
                ms.key_compare = unsafe_latin_compare;
            }
            else if (key_type == &PyLong_Type && ints_are_bounded) {
                ms.key_compare = unsafe_long_compare;
            }
            else if (key_type == &PyFloat_Type) {
                ms.key_compare = unsafe_float_compare;
            }
            else if ((ms.key_richcompare = key_type->tp_richcompare) != NULL) {
                ms.key_compare = unsafe_object_compare;
            }
            else {
                ms.key_compare = safe_object_compare;
            }
        }
        else {
            ms.key_compare = safe_object_compare;
        }

        if (keys_are_in_tuples) {
            /* Make sure we're not dealing with tuples of tuples
             * (remember: here, key_type refers list [key[0] for key in keys]) */
            if (key_type == &PyTuple_Type) {
                ms.tuple_elem_compare = safe_object_compare;
            }
            else {
                ms.tuple_elem_compare = ms.key_compare;
            }

            ms.key_compare = unsafe_tuple_compare;
        }
    }
    /* End of pre-sort check: ms is now set properly! */

    merge_init(&ms, saved_ob_size, keys != NULL, &lo);

    nremaining = saved_ob_size;
    if (nremaining < 2)
        goto succeed;

    /* Reverse sort stability achieved by initially reversing the list,
    applying a stable forward sort, then reversing the final result. */
    if (reverse) {
        if (keys != NULL)
            reverse_slice(&keys[0], &keys[saved_ob_size]);
        reverse_slice(&saved_ob_item[0], &saved_ob_item[saved_ob_size]);
    }

    /* March over the array once, left to right, finding natural runs,
     * and extending short natural runs to minrun elements.
     */
    minrun = merge_compute_minrun(nremaining);
    do {
        int descending;
        Py_ssize_t n;

        /* Identify next run. */
        n = count_run(&ms, lo.keys, lo.keys + nremaining, &descending);
        if (n < 0)
            goto fail;
        if (descending)
            reverse_sortslice(&lo, n);
        /* If short, extend to min(minrun, nremaining). */
        if (n < minrun) {
            const Py_ssize_t force = nremaining <= minrun ?
                              nremaining : minrun;
            if (binarysort(&ms, lo, lo.keys + force, lo.keys + n) < 0)
                goto fail;
            n = force;
        }
        /* Maybe merge pending runs. */
        assert(ms.n == 0 || ms.pending[ms.n -1].base.keys +
                            ms.pending[ms.n-1].len == lo.keys);
        if (found_new_run(&ms, n) < 0)
            goto fail;
        /* Push new run on stack. */
        assert(ms.n < MAX_MERGE_PENDING);
        ms.pending[ms.n].base = lo;
        ms.pending[ms.n].len = n;
        ++ms.n;
        /* Advance to find next run. */
        sortslice_advance(&lo, n);
        nremaining -= n;
    } while (nremaining);

    if (merge_force_collapse(&ms) < 0)
        goto fail;
    assert(ms.n == 1);
    assert(keys == NULL
           ? ms.pending[0].base.keys == saved_ob_item
           : ms.pending[0].base.keys == &keys[0]);
    assert(ms.pending[0].len == saved_ob_size);
    lo = ms.pending[0].base;

succeed:
    result = Py_None;
fail:
    if (keys != NULL) {
        for (i = 0; i < saved_ob_size; i++)
            Py_DECREF(keys[i]);
        if (saved_ob_size >= MERGESTATE_TEMP_SIZE/2)
            PyMem_Free(keys);
    }

    if (self->allocated != -1 && result != NULL) {
        /* The user mucked with the list during the sort,
         * and we don't already have another error to report.
         */
        PyErr_SetString(PyExc_ValueError, "list modified during sort");
        result = NULL;
    }

    if (reverse && saved_ob_size > 1)
        reverse_slice(saved_ob_item, saved_ob_item + saved_ob_size);

    merge_freemem(&ms);

keyfunc_fail:
    final_ob_item = self->ob_item;
    i = Py_SIZE(self);
    Py_SET_SIZE(self, saved_ob_size);
    self->ob_item = saved_ob_item;
    self->allocated = saved_allocated;
    if (final_ob_item != NULL) {
        /* we cannot use _list_clear() for this because it does not
           guarantee that the list is really empty when it returns */
        while (--i >= 0) {
            Py_XDECREF(final_ob_item[i]);
        }
        free_list_array(final_ob_item, _PyObject_GC_IS_SHARED(self));
    }
    return Py_XNewRef(result);
}
#undef IFLT
#undef ISLT

int
PyList_Sort(PyObject *v)
{
    if (v == NULL || !PyList_Check(v)) {
        PyErr_BadInternalCall();
        return -1;
    }
    v = list_sort_impl((PyListObject *)v, NULL, 0);
    if (v == NULL)
        return -1;
    Py_DECREF(v);
    return 0;
}

/*[clinic input]
list.reverse

Reverse *IN PLACE*.
[clinic start generated code]*/

static PyObject *
list_reverse_impl(PyListObject *self)
/*[clinic end generated code: output=482544fc451abea9 input=eefd4c3ae1bc9887]*/
{
    Py_BEGIN_CRITICAL_SECTION(self);
    if (Py_SIZE(self) > 1)
        reverse_slice(self->ob_item, self->ob_item + Py_SIZE(self));
    Py_END_CRITICAL_SECTION;
    Py_RETURN_NONE;
}

int
PyList_Reverse(PyObject *v)
{
    PyListObject *self = (PyListObject *)v;

    if (v == NULL || !PyList_Check(v)) {
        PyErr_BadInternalCall();
        return -1;
    }
    Py_BEGIN_CRITICAL_SECTION(self);
    if (Py_SIZE(self) > 1)
        reverse_slice(self->ob_item, self->ob_item + Py_SIZE(self));
    Py_END_CRITICAL_SECTION;
    return 0;
}

PyObject *
PyList_AsTuple(PyObject *v)
{
    if (v == NULL || !PyList_Check(v)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    PyObject *ret;
    PyListObject *self = (PyListObject *)v;
    Py_BEGIN_CRITICAL_SECTION(self);
    ret = _PyTuple_FromArray(((PyListObject *)v)->ob_item, Py_SIZE(v));
    Py_END_CRITICAL_SECTION;
    return ret;
}

PyObject *
_PyList_FromArraySteal(PyObject *const *src, Py_ssize_t n)
{
    if (n == 0) {
        return PyList_New(0);
    }

    PyListObject *list = (PyListObject *)PyList_New(n);
    if (list == NULL) {
        for (Py_ssize_t i = 0; i < n; i++) {
            Py_DECREF(src[i]);
        }
        return NULL;
    }

    PyObject **dst = list->ob_item;
    memcpy(dst, src, n * sizeof(PyObject *));

    return (PyObject *)list;
}

static PyObject *
list_slice_wrap(PyListObject *a, Py_ssize_t start, Py_ssize_t stop, Py_ssize_t step)
{
    PyObject *res = NULL;
    Py_BEGIN_CRITICAL_SECTION(a);
    Py_ssize_t len = PySlice_AdjustIndices(Py_SIZE(a), &start, &stop, step);
    if (len <= 0) {
        res = PyList_New(0);
    }
    else if (step == 1) {
        res = list_slice_locked(a, start, len);
    }
    else {
        res = list_slice_step_locked(a, start, step, len);
    }
    Py_END_CRITICAL_SECTION;
    return res;
}

/*[clinic input]
list.index

    value: object
    start: slice_index(accept={int}) = 0
    stop: slice_index(accept={int}, c_default="PY_SSIZE_T_MAX") = sys.maxsize
    /

Return first index of value.

Raises ValueError if the value is not present.
[clinic start generated code]*/

static PyObject *
list_index_impl(PyListObject *self, PyObject *value, Py_ssize_t start,
                Py_ssize_t stop)
/*[clinic end generated code: output=ec51b88787e4e481 input=40ec5826303a0eb1]*/
{
    Py_ssize_t i;

    if (start < 0) {
        start += Py_SIZE(self);
        if (start < 0)
            start = 0;
    }
    if (stop < 0) {
        stop += Py_SIZE(self);
        if (stop < 0)
            stop = 0;
    }
    for (i = start; i < stop && i < Py_SIZE(self); i++) {
        PyObject *obj = list_fetch_item(self, i);
        if (obj == NULL) {
            break;
        }
        int cmp = PyObject_RichCompareBool(obj, value, Py_EQ);
        Py_DECREF(obj);
        if (cmp > 0)
            return PyLong_FromSsize_t(i);
        else if (cmp < 0)
            return NULL;
    }
    PyErr_Format(PyExc_ValueError, "%R is not in list", value);
    return NULL;
}

/*[clinic input]
list.count

     value: object
     /

Return number of occurrences of value.
[clinic start generated code]*/

static PyObject *
list_count(PyListObject *self, PyObject *value)
/*[clinic end generated code: output=b1f5d284205ae714 input=3bdc3a5e6f749565]*/
{
    Py_ssize_t count = 0;
    for (Py_ssize_t i = 0; i < Py_SIZE(self); i++) {
        PyObject *obj = list_fetch_item(self, i);
        if (!obj) {
            break;
        }
        if (obj == value) {
            count++;
            Py_DECREF(obj);
            continue;
        }
        int cmp = PyObject_RichCompareBool(obj, value, Py_EQ);
        Py_DECREF(obj);
        if (cmp > 0)
            count++;
        else if (cmp < 0)
            return NULL;
    }
    return PyLong_FromSsize_t(count);
}

/*[clinic input]
list.remove

     value: object
     /

Remove first occurrence of value.

Raises ValueError if the value is not present.
[clinic start generated code]*/

static PyObject *
list_remove(PyListObject *self, PyObject *value)
/*[clinic end generated code: output=f087e1951a5e30d1 input=2dc2ba5bb2fb1f82]*/
{
    PyObject *ret = NULL;
    Py_ssize_t i;

    Py_BEGIN_CRITICAL_SECTION(self);
    for (i = 0; i < Py_SIZE(self); i++) {
        PyObject *obj = Py_NewRef(self->ob_item[i]);
        int cmp = PyObject_RichCompareBool(obj, value, Py_EQ);
        Py_DECREF(obj);
        if (cmp == 0) {
            continue;
        }
        if (cmp > 0) {
            if (list_del_slice_locked(self, i, i+1) == 0) {
                ret = Py_None;
            }
            goto exit;
        }
        goto exit;
    }
    PyErr_SetString(PyExc_ValueError, "list.remove(x): x not in list");
exit:
    Py_END_CRITICAL_SECTION;
    return ret;
}

static int
list_traverse(PyListObject *o, visitproc visit, void *arg)
{
    Py_ssize_t i;

    for (i = Py_SIZE(o); --i >= 0; )
        Py_VISIT(o->ob_item[i]);
    return 0;
}

static PyObject *
list_richcompare_locked(PyListObject *vl, PyListObject *wl, int op)
{
    Py_ssize_t i;

    if (Py_SIZE(vl) != Py_SIZE(wl) && (op == Py_EQ || op == Py_NE)) {
        /* Shortcut: if the lengths differ, the lists differ */
        if (op == Py_EQ)
            Py_RETURN_FALSE;
        else
            Py_RETURN_TRUE;
    }

    /* Search for the first index where items are different */
    for (i = 0; i < Py_SIZE(vl) && i < Py_SIZE(wl); i++) {
        PyObject *vitem = vl->ob_item[i];
        PyObject *witem = wl->ob_item[i];
        if (vitem == witem) {
            continue;
        }

        Py_INCREF(vitem);
        Py_INCREF(witem);
        int k = PyObject_RichCompareBool(vitem, witem, Py_EQ);
        Py_DECREF(vitem);
        Py_DECREF(witem);
        if (k < 0)
            return NULL;
        if (!k)
            break;
    }

    if (i >= Py_SIZE(vl) || i >= Py_SIZE(wl)) {
        /* No more items to compare -- compare sizes */
        Py_RETURN_RICHCOMPARE(Py_SIZE(vl), Py_SIZE(wl), op);
    }

    /* We have an item that differs -- shortcuts for EQ/NE */
    if (op == Py_EQ) {
        Py_RETURN_FALSE;
    }
    if (op == Py_NE) {
        Py_RETURN_TRUE;
    }

    /* Compare the final item again using the proper operator */
    return PyObject_RichCompare(vl->ob_item[i], wl->ob_item[i], op);
}

static PyObject *
list_richcompare(PyObject *v, PyObject *w, int op)
{
    PyListObject *vl, *wl;

    if (!PyList_Check(v) || !PyList_Check(w))
        Py_RETURN_NOTIMPLEMENTED;

    vl = (PyListObject *)v;
    wl = (PyListObject *)w;

    PyObject *ret;
    Py_BEGIN_CRITICAL_SECTION2(vl, wl);
    ret = list_richcompare_locked(vl, wl, op);
    Py_END_CRITICAL_SECTION2;
    return ret;
}

/*[clinic input]
list.__init__

    iterable: object(c_default="NULL") = ()
    /

Built-in mutable sequence.

If no argument is given, the constructor creates a new empty list.
The argument must be an iterable if specified.
[clinic start generated code]*/

static int
list___init___impl(PyListObject *self, PyObject *iterable)
/*[clinic end generated code: output=0f3c21379d01de48 input=b3f3fe7206af8f6b]*/
{
    /* Verify list invariants established by PyType_GenericAlloc() */
    assert(0 <= Py_SIZE(self));
    assert(Py_SIZE(self) <= self->allocated || self->allocated == -1);
    assert(self->ob_item != NULL ||
           self->allocated == 0 || self->allocated == -1);

    /* Empty previous contents */
    if (self->ob_item != NULL) {
        _list_clear(self);
    }
    if (iterable != NULL) {
        PyObject *rv = list_extend(self, iterable);
        if (rv == NULL)
            return -1;
        Py_DECREF(rv);
    }
    return 0;
}

static PyObject *
list_vectorcall(PyObject *type, PyObject * const*args,
                size_t nargsf, PyObject *kwnames)
{
    if (!_PyArg_NoKwnames("list", kwnames)) {
        return NULL;
    }
    Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
    if (!_PyArg_CheckPositional("list", nargs, 0, 1)) {
        return NULL;
    }

    PyObject *list = PyType_GenericAlloc(_PyType_CAST(type), 0);
    if (list == NULL) {
        return NULL;
    }
    if (nargs) {
        if (list___init___impl((PyListObject *)list, args[0])) {
            Py_DECREF(list);
            return NULL;
        }
    }
    return list;
}


/*[clinic input]
list.__sizeof__

Return the size of the list in memory, in bytes.
[clinic start generated code]*/

static PyObject *
list___sizeof___impl(PyListObject *self)
/*[clinic end generated code: output=3417541f95f9a53e input=b8030a5d5ce8a187]*/
{
    size_t res = _PyObject_SIZE(Py_TYPE(self));
    Py_ssize_t allocated = _Py_atomic_load_ssize_relaxed(&self->allocated);
    res += (size_t)allocated * sizeof(void*);
    return PyLong_FromSize_t(res);
}

static PyObject *list_iter(PyObject *seq);
static PyObject *list_subscript(PyListObject*, PyObject*);

static PyMethodDef list_methods[] = {
    {"__getitem__", (PyCFunction)list_subscript, METH_O|METH_COEXIST,
     PyDoc_STR("__getitem__($self, index, /)\n--\n\nReturn self[index].")},
    LIST___REVERSED___METHODDEF
    LIST___SIZEOF___METHODDEF
    LIST_CLEAR_METHODDEF
    LIST_COPY_METHODDEF
    LIST_APPEND_METHODDEF
    LIST_INSERT_METHODDEF
    LIST_EXTEND_METHODDEF
    LIST_POP_METHODDEF
    LIST_REMOVE_METHODDEF
    LIST_INDEX_METHODDEF
    LIST_COUNT_METHODDEF
    LIST_REVERSE_METHODDEF
    LIST_SORT_METHODDEF
    {"__class_getitem__", Py_GenericAlias, METH_O|METH_CLASS, PyDoc_STR("See PEP 585")},
    {NULL,              NULL}           /* sentinel */
};

static PySequenceMethods list_as_sequence = {
    (lenfunc)list_length,                       /* sq_length */
    (binaryfunc)list_concat,                    /* sq_concat */
    (ssizeargfunc)list_repeat,                  /* sq_repeat */
    (ssizeargfunc)list_item,                    /* sq_item */
    0,                                          /* sq_slice */
    (ssizeobjargproc)list_ass_item,             /* sq_ass_item */
    0,                                          /* sq_ass_slice */
    (objobjproc)list_contains,                  /* sq_contains */
    (binaryfunc)list_inplace_concat,            /* sq_inplace_concat */
    (ssizeargfunc)list_inplace_repeat,          /* sq_inplace_repeat */
};

static PyObject *
list_subscript(PyListObject* self, PyObject* item)
{
    if (_PyIndex_Check(item)) {
        Py_ssize_t i;
        i = PyNumber_AsSsize_t(item, PyExc_IndexError);
        if (i == -1 && PyErr_Occurred())
            return NULL;
        if (i < 0)
            i += PyList_GET_SIZE(self);
        return list_item(self, i);
    }
    else if (PySlice_Check(item)) {
        Py_ssize_t start, stop, step;
        if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
            return NULL;
        }
        return list_slice_wrap(self, start, stop, step);
    }
    else {
        PyErr_Format(PyExc_TypeError,
                     "list indices must be integers or slices, not %.200s",
                     Py_TYPE(item)->tp_name);
        return NULL;
    }
}

static int
list_ass_subscript(PyListObject* self, PyObject* item, PyObject* value)
{
    if (_PyIndex_Check(item)) {
        Py_ssize_t i = PyNumber_AsSsize_t(item, PyExc_IndexError);
        if (i == -1 && PyErr_Occurred())
            return -1;
        if (i < 0)
            i += PyList_GET_SIZE(self);
        return list_ass_item(self, i, value);
    }
    else if (PySlice_Check(item)) {
        Py_ssize_t start, stop, step;
        if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
            return -1;
        }
        return list_ass_slice(self, start, stop, step, WRAP_INDICES, value);
    }
    else {
        PyErr_Format(PyExc_TypeError,
                     "list indices must be integers or slices, not %.200s",
                     Py_TYPE(item)->tp_name);
        return -1;
    }
}

static PyMappingMethods list_as_mapping = {
    (lenfunc)list_length,
    (binaryfunc)list_subscript,
    (objobjargproc)list_ass_subscript
};

PyTypeObject PyList_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "list",
    sizeof(PyListObject),
    0,
    (destructor)list_dealloc,                   /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    (reprfunc)list_repr,                        /* tp_repr */
    0,                                          /* tp_as_number */
    &list_as_sequence,                          /* tp_as_sequence */
    &list_as_mapping,                           /* tp_as_mapping */
    PyObject_HashNotImplemented,                /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
        Py_TPFLAGS_BASETYPE | Py_TPFLAGS_LIST_SUBCLASS |
        _Py_TPFLAGS_MATCH_SELF | Py_TPFLAGS_SEQUENCE,  /* tp_flags */
    list___init____doc__,                       /* tp_doc */
    (traverseproc)list_traverse,                /* tp_traverse */
    (inquiry)list_tp_clear,                    /* tp_clear */
    list_richcompare,                           /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    list_iter,                                  /* tp_iter */
    0,                                          /* tp_iternext */
    list_methods,                               /* tp_methods */
    0,                                          /* tp_members */
    0,                                          /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    (initproc)list___init__,                    /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    PyType_GenericNew,                          /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
    .tp_vectorcall = list_vectorcall,
};

/*********************** List Iterator **************************/

static void listiter_dealloc(_PyListIterObject *);
static int listiter_traverse(_PyListIterObject *, visitproc, void *);
static PyObject *listiter_next(_PyListIterObject *);
static PyObject *listiter_len(_PyListIterObject *, PyObject *);
static PyObject *listiter_reduce_general(void *_it, int forward);
static PyObject *listiter_reduce(_PyListIterObject *, PyObject *);
static PyObject *listiter_setstate(_PyListIterObject *, PyObject *state);

PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(it)).");
PyDoc_STRVAR(reduce_doc, "Return state information for pickling.");
PyDoc_STRVAR(setstate_doc, "Set state information for unpickling.");

static PyMethodDef listiter_methods[] = {
    {"__length_hint__", (PyCFunction)listiter_len, METH_NOARGS, length_hint_doc},
    {"__reduce__", (PyCFunction)listiter_reduce, METH_NOARGS, reduce_doc},
    {"__setstate__", (PyCFunction)listiter_setstate, METH_O, setstate_doc},
    {NULL,              NULL}           /* sentinel */
};

PyTypeObject PyListIter_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "list_iterator",                            /* tp_name */
    sizeof(_PyListIterObject),                  /* tp_basicsize */
    0,                                          /* tp_itemsize */
    /* methods */
    (destructor)listiter_dealloc,               /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    0,                                          /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)listiter_traverse,            /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    PyObject_SelfIter,                          /* tp_iter */
    (iternextfunc)listiter_next,                /* tp_iternext */
    listiter_methods,                           /* tp_methods */
    0,                                          /* tp_members */
};


static PyObject *
list_iter(PyObject *seq)
{
    _PyListIterObject *it;

    if (!PyList_Check(seq)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    it = PyObject_GC_New(_PyListIterObject, &PyListIter_Type);
    if (it == NULL)
        return NULL;
    it->it_index = 0;
    it->it_seq = (PyListObject *)Py_NewRef(seq);
    _PyObject_GC_TRACK(it);
    return (PyObject *)it;
}

static void
listiter_dealloc(_PyListIterObject *it)
{
    _PyObject_GC_UNTRACK(it);
    Py_XDECREF(it->it_seq);
    PyObject_GC_Del(it);
}

static int
listiter_traverse(_PyListIterObject *it, visitproc visit, void *arg)
{
    Py_VISIT(it->it_seq);
    return 0;
}

static PyObject *
listiter_next(_PyListIterObject *it)
{
    assert(it != NULL);
    assert(PyList_Check(it->it_seq));

    Py_ssize_t index = _Py_atomic_load_ssize_relaxed(&it->it_index);
    if (index < 0) {
        return NULL;
    }

    PyObject *item = list_fetch_item(it->it_seq, index);
    if (!item) {
        // out-of-bounds
        _Py_atomic_store_ssize_relaxed(&it->it_index, -1);
        return NULL;
    }
    _Py_atomic_store_ssize_relaxed(&it->it_index, index + 1);
    return item;
}

static PyObject *
listiter_len(_PyListIterObject *it, PyObject *Py_UNUSED(ignored))
{
    Py_ssize_t len;
    Py_ssize_t index = _Py_atomic_load_ssize_relaxed(&it->it_index);
    if (index >= 0) {
        len = PyList_GET_SIZE(it->it_seq) - index;
        if (len >= 0)
            return PyLong_FromSsize_t(len);
    }
    return PyLong_FromLong(0);
}

static PyObject *
listiter_reduce(_PyListIterObject *it, PyObject *Py_UNUSED(ignored))
{
    return listiter_reduce_general(it, 1);
}

static PyObject *
listiter_setstate(_PyListIterObject *it, PyObject *state)
{
    Py_ssize_t index = PyLong_AsSsize_t(state);
    if (index == -1 && PyErr_Occurred())
        return NULL;
    if (it->it_seq != NULL) {
        if (index < -1)
            index = -1;
        else if (index > PyList_GET_SIZE(it->it_seq))
            index = PyList_GET_SIZE(it->it_seq); /* iterator exhausted */
        it->it_index = index;
    }
    Py_RETURN_NONE;
}

/*********************** List Reverse Iterator **************************/

typedef struct {
    PyObject_HEAD
    Py_ssize_t it_index;
    PyListObject *it_seq; /* Set to NULL when iterator is exhausted */
} listreviterobject;

static void listreviter_dealloc(listreviterobject *);
static int listreviter_traverse(listreviterobject *, visitproc, void *);
static PyObject *listreviter_next(listreviterobject *);
static PyObject *listreviter_len(listreviterobject *, PyObject *);
static PyObject *listreviter_reduce(listreviterobject *, PyObject *);
static PyObject *listreviter_setstate(listreviterobject *, PyObject *);

static PyMethodDef listreviter_methods[] = {
    {"__length_hint__", (PyCFunction)listreviter_len, METH_NOARGS, length_hint_doc},
    {"__reduce__", (PyCFunction)listreviter_reduce, METH_NOARGS, reduce_doc},
    {"__setstate__", (PyCFunction)listreviter_setstate, METH_O, setstate_doc},
    {NULL,              NULL}           /* sentinel */
};

PyTypeObject PyListRevIter_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "list_reverseiterator",                     /* tp_name */
    sizeof(listreviterobject),                  /* tp_basicsize */
    0,                                          /* tp_itemsize */
    /* methods */
    (destructor)listreviter_dealloc,            /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    0,                                          /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)listreviter_traverse,         /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    PyObject_SelfIter,                          /* tp_iter */
    (iternextfunc)listreviter_next,             /* tp_iternext */
    listreviter_methods,                /* tp_methods */
    0,
};

/*[clinic input]
list.__reversed__

Return a reverse iterator over the list.
[clinic start generated code]*/

static PyObject *
list___reversed___impl(PyListObject *self)
/*[clinic end generated code: output=b166f073208c888c input=eadb6e17f8a6a280]*/
{
    listreviterobject *it;

    it = PyObject_GC_New(listreviterobject, &PyListRevIter_Type);
    if (it == NULL)
        return NULL;
    assert(PyList_Check(self));
    it->it_index = PyList_GET_SIZE(self) - 1;
    it->it_seq = (PyListObject*)Py_NewRef(self);
    PyObject_GC_Track(it);
    return (PyObject *)it;
}

static void
listreviter_dealloc(listreviterobject *it)
{
    PyObject_GC_UnTrack(it);
    Py_XDECREF(it->it_seq);
    PyObject_GC_Del(it);
}

static int
listreviter_traverse(listreviterobject *it, visitproc visit, void *arg)
{
    Py_VISIT(it->it_seq);
    return 0;
}

static PyObject *
listreviter_next(listreviterobject *it)
{
    PyObject *item;

    assert(it != NULL);
    PyListObject *seq = it->it_seq;
    assert(PyList_Check(seq));

    Py_ssize_t index = _Py_atomic_load_ssize_relaxed(&it->it_index);
    if (index >= 0 && index < PyList_GET_SIZE(seq)) {
        item = list_fetch_item(seq, index);
        if (item != NULL) {
            _Py_atomic_store_ssize_relaxed(&it->it_index, index - 1);
            return item;
        }
    }
    _Py_atomic_store_ssize_relaxed(&it->it_index, -1);
    return NULL;
}

static PyObject *
listreviter_len(listreviterobject *it, PyObject *Py_UNUSED(ignored))
{
    Py_ssize_t len = _Py_atomic_load_ssize_relaxed(&it->it_index) + 1;
    if (PyList_GET_SIZE(it->it_seq) < len)
        len = 0;
    return PyLong_FromSsize_t(len);
}

static PyObject *
listreviter_reduce(listreviterobject *it, PyObject *Py_UNUSED(ignored))
{
    return listiter_reduce_general(it, 0);
}

static PyObject *
listreviter_setstate(listreviterobject *it, PyObject *state)
{
    Py_ssize_t index = PyLong_AsSsize_t(state);
    if (index == -1 && PyErr_Occurred())
        return NULL;
    if (it->it_seq != NULL) {
        if (index < -1)
            index = -1;
        else if (index > PyList_GET_SIZE(it->it_seq) - 1)
            index = PyList_GET_SIZE(it->it_seq) - 1;
        it->it_index = index;
    }
    Py_RETURN_NONE;
}

/* common pickling support */

static PyObject *
listiter_reduce_general(void *_it, int forward)
{
    PyObject *list;
    PyObject *iter;

    /* the objects are not the same, index is of different types! */
    if (forward) {
        iter = _PyEval_GetBuiltin(&_Py_ID(iter));
        _PyListIterObject *it = (_PyListIterObject *)_it;
        if (it->it_index >= 0) {
            return Py_BuildValue("N(O)n", iter,
                                 it->it_seq, it->it_index);
        }
    } else {
        iter = _PyEval_GetBuiltin(&_Py_ID(reversed));
        listreviterobject *it = (listreviterobject *)_it;
        if (it->it_index >= 0) {
            return Py_BuildValue("N(O)n", iter,
                                 it->it_seq, it->it_index);
        }
    }
    /* empty iterator, create an empty list */
    list = PyList_New(0);
    if (list == NULL)
        return NULL;
    return Py_BuildValue("N(N)", iter, list);
}