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arena.c
495 lines (415 loc) · 17.5 KB
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arena.c
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// Protocol Buffers - Google's data interchange format
// Copyright 2023 Google LLC. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd
#include "upb/mem/arena.h"
#include <stddef.h>
#include <stdint.h>
#include "upb/mem/alloc.h"
#include "upb/mem/internal/arena.h"
#include "upb/port/atomic.h"
// Must be last.
#include "upb/port/def.inc"
typedef struct upb_MemBlock {
// Atomic only for the benefit of SpaceAllocated().
UPB_ATOMIC(struct upb_MemBlock*) next;
uint32_t size;
// Data follows.
} upb_MemBlock;
typedef struct upb_ArenaInternal {
// upb_alloc* together with a low bit which signals if there is an initial
// block.
uintptr_t block_alloc;
// When multiple arenas are fused together, each arena points to a parent
// arena (root points to itself). The root tracks how many live arenas
// reference it.
// The low bit is tagged:
// 0: pointer to parent
// 1: count, left shifted by one
UPB_ATOMIC(uintptr_t) parent_or_count;
// All nodes that are fused together are in a singly-linked list.
// == NULL at end of list.
UPB_ATOMIC(struct upb_ArenaInternal*) next;
// The last element of the linked list. This is present only as an
// optimization, so that we do not have to iterate over all members for every
// fuse. Only significant for an arena root. In other cases it is ignored.
// == self when no other list members.
UPB_ATOMIC(struct upb_ArenaInternal*) tail;
// Linked list of blocks to free/cleanup. Atomic only for the benefit of
// upb_Arena_SpaceAllocated().
UPB_ATOMIC(upb_MemBlock*) blocks;
} upb_ArenaInternal;
// All public + private state for an arena.
typedef struct {
upb_Arena head;
upb_ArenaInternal body;
} upb_ArenaState;
typedef struct {
upb_ArenaInternal* root;
uintptr_t tagged_count;
} upb_ArenaRoot;
static const size_t kUpb_MemblockReserve =
UPB_ALIGN_UP(sizeof(upb_MemBlock), UPB_MALLOC_ALIGN);
// Extracts the (upb_ArenaInternal*) from a (upb_Arena*)
static upb_ArenaInternal* upb_Arena_Internal(const upb_Arena* a) {
return &((upb_ArenaState*)a)->body;
}
static bool _upb_Arena_IsTaggedRefcount(uintptr_t parent_or_count) {
return (parent_or_count & 1) == 1;
}
static bool _upb_Arena_IsTaggedPointer(uintptr_t parent_or_count) {
return (parent_or_count & 1) == 0;
}
static uintptr_t _upb_Arena_RefCountFromTagged(uintptr_t parent_or_count) {
UPB_ASSERT(_upb_Arena_IsTaggedRefcount(parent_or_count));
return parent_or_count >> 1;
}
static uintptr_t _upb_Arena_TaggedFromRefcount(uintptr_t refcount) {
uintptr_t parent_or_count = (refcount << 1) | 1;
UPB_ASSERT(_upb_Arena_IsTaggedRefcount(parent_or_count));
return parent_or_count;
}
static upb_ArenaInternal* _upb_Arena_PointerFromTagged(
uintptr_t parent_or_count) {
UPB_ASSERT(_upb_Arena_IsTaggedPointer(parent_or_count));
return (upb_ArenaInternal*)parent_or_count;
}
static uintptr_t _upb_Arena_TaggedFromPointer(upb_ArenaInternal* ai) {
uintptr_t parent_or_count = (uintptr_t)ai;
UPB_ASSERT(_upb_Arena_IsTaggedPointer(parent_or_count));
return parent_or_count;
}
static upb_alloc* _upb_ArenaInternal_BlockAlloc(upb_ArenaInternal* ai) {
return (upb_alloc*)(ai->block_alloc & ~0x1);
}
static uintptr_t _upb_Arena_MakeBlockAlloc(upb_alloc* alloc, bool has_initial) {
uintptr_t alloc_uint = (uintptr_t)alloc;
UPB_ASSERT((alloc_uint & 1) == 0);
return alloc_uint | (has_initial ? 1 : 0);
}
static bool _upb_ArenaInternal_HasInitialBlock(upb_ArenaInternal* ai) {
return ai->block_alloc & 0x1;
}
static upb_ArenaRoot _upb_Arena_FindRoot(upb_Arena* a) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
uintptr_t poc = upb_Atomic_Load(&ai->parent_or_count, memory_order_acquire);
while (_upb_Arena_IsTaggedPointer(poc)) {
upb_ArenaInternal* next = _upb_Arena_PointerFromTagged(poc);
UPB_ASSERT(ai != next);
uintptr_t next_poc =
upb_Atomic_Load(&next->parent_or_count, memory_order_acquire);
if (_upb_Arena_IsTaggedPointer(next_poc)) {
// To keep complexity down, we lazily collapse levels of the tree. This
// keeps it flat in the final case, but doesn't cost much incrementally.
//
// Path splitting keeps time complexity down, see:
// https://en.wikipedia.org/wiki/Disjoint-set_data_structure
//
// We can safely use a relaxed atomic here because all threads doing this
// will converge on the same value and we don't need memory orderings to
// be visible.
//
// This is true because:
// - If no fuses occur, this will eventually become the root.
// - If fuses are actively occurring, the root may change, but the
// invariant is that `parent_or_count` merely points to *a* parent.
//
// In other words, it is moving towards "the" root, and that root may move
// further away over time, but the path towards that root will continue to
// be valid and the creation of the path carries all the memory orderings
// required.
UPB_ASSERT(ai != _upb_Arena_PointerFromTagged(next_poc));
upb_Atomic_Store(&ai->parent_or_count, next_poc, memory_order_relaxed);
}
ai = next;
poc = next_poc;
}
return (upb_ArenaRoot){.root = ai, .tagged_count = poc};
}
size_t upb_Arena_SpaceAllocated(upb_Arena* arena, size_t* fused_count) {
upb_ArenaInternal* ai = _upb_Arena_FindRoot(arena).root;
size_t memsize = 0;
size_t local_fused_count = 0;
while (ai != NULL) {
upb_MemBlock* block = upb_Atomic_Load(&ai->blocks, memory_order_relaxed);
while (block != NULL) {
memsize += sizeof(upb_MemBlock) + block->size;
block = upb_Atomic_Load(&block->next, memory_order_relaxed);
}
ai = upb_Atomic_Load(&ai->next, memory_order_relaxed);
local_fused_count++;
}
if (fused_count) *fused_count = local_fused_count;
return memsize;
}
uint32_t upb_Arena_DebugRefCount(upb_Arena* a) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
// These loads could probably be relaxed, but given that this is debug-only,
// it's not worth introducing a new variant for it.
uintptr_t poc = upb_Atomic_Load(&ai->parent_or_count, memory_order_acquire);
while (_upb_Arena_IsTaggedPointer(poc)) {
ai = _upb_Arena_PointerFromTagged(poc);
poc = upb_Atomic_Load(&ai->parent_or_count, memory_order_acquire);
}
return _upb_Arena_RefCountFromTagged(poc);
}
static void _upb_Arena_AddBlock(upb_Arena* a, void* ptr, size_t size) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
upb_MemBlock* block = ptr;
// Insert into linked list.
block->size = (uint32_t)size;
upb_Atomic_Init(&block->next, ai->blocks);
upb_Atomic_Store(&ai->blocks, block, memory_order_release);
a->UPB_PRIVATE(ptr) = UPB_PTR_AT(block, kUpb_MemblockReserve, char);
a->UPB_PRIVATE(end) = UPB_PTR_AT(block, size, char);
UPB_POISON_MEMORY_REGION(a->UPB_PRIVATE(ptr),
a->UPB_PRIVATE(end) - a->UPB_PRIVATE(ptr));
}
static bool _upb_Arena_AllocBlock(upb_Arena* a, size_t size) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
if (!ai->block_alloc) return false;
upb_MemBlock* last_block = upb_Atomic_Load(&ai->blocks, memory_order_acquire);
size_t last_size = last_block != NULL ? last_block->size : 128;
size_t block_size = UPB_MAX(size, last_size * 2) + kUpb_MemblockReserve;
upb_MemBlock* block =
upb_malloc(_upb_ArenaInternal_BlockAlloc(ai), block_size);
if (!block) return false;
_upb_Arena_AddBlock(a, block, block_size);
UPB_ASSERT(UPB_PRIVATE(_upb_ArenaHas)(a) >= size);
return true;
}
void* UPB_PRIVATE(_upb_Arena_SlowMalloc)(upb_Arena* a, size_t size) {
if (!_upb_Arena_AllocBlock(a, size)) return NULL; // OOM
return upb_Arena_Malloc(a, size - UPB_ASAN_GUARD_SIZE);
}
static upb_Arena* _upb_Arena_InitSlow(upb_alloc* alloc) {
const size_t first_block_overhead =
sizeof(upb_ArenaState) + kUpb_MemblockReserve;
upb_ArenaState* a;
// We need to malloc the initial block.
char* mem;
size_t n = first_block_overhead + 256;
if (!alloc || !(mem = upb_malloc(alloc, n))) {
return NULL;
}
a = UPB_PTR_AT(mem, n - sizeof(upb_ArenaState), upb_ArenaState);
n -= sizeof(upb_ArenaState);
a->body.block_alloc = _upb_Arena_MakeBlockAlloc(alloc, 0);
upb_Atomic_Init(&a->body.parent_or_count, _upb_Arena_TaggedFromRefcount(1));
upb_Atomic_Init(&a->body.next, NULL);
upb_Atomic_Init(&a->body.tail, &a->body);
upb_Atomic_Init(&a->body.blocks, NULL);
_upb_Arena_AddBlock(&a->head, mem, n);
return &a->head;
}
upb_Arena* upb_Arena_Init(void* mem, size_t n, upb_alloc* alloc) {
UPB_ASSERT(sizeof(void*) * UPB_ARENA_SIZE_HACK >= sizeof(upb_ArenaState));
upb_ArenaState* a;
if (n) {
/* Align initial pointer up so that we return properly-aligned pointers. */
void* aligned = (void*)UPB_ALIGN_UP((uintptr_t)mem, UPB_MALLOC_ALIGN);
size_t delta = (uintptr_t)aligned - (uintptr_t)mem;
n = delta <= n ? n - delta : 0;
mem = aligned;
}
/* Round block size down to alignof(*a) since we will allocate the arena
* itself at the end. */
n = UPB_ALIGN_DOWN(n, UPB_ALIGN_OF(upb_ArenaState));
if (UPB_UNLIKELY(n < sizeof(upb_ArenaState))) {
return _upb_Arena_InitSlow(alloc);
}
a = UPB_PTR_AT(mem, n - sizeof(upb_ArenaState), upb_ArenaState);
upb_Atomic_Init(&a->body.parent_or_count, _upb_Arena_TaggedFromRefcount(1));
upb_Atomic_Init(&a->body.next, NULL);
upb_Atomic_Init(&a->body.tail, &a->body);
upb_Atomic_Init(&a->body.blocks, NULL);
a->body.block_alloc = _upb_Arena_MakeBlockAlloc(alloc, 1);
a->head.UPB_PRIVATE(ptr) = mem;
a->head.UPB_PRIVATE(end) = UPB_PTR_AT(mem, n - sizeof(upb_ArenaState), char);
return &a->head;
}
static void _upb_Arena_DoFree(upb_ArenaInternal* ai) {
UPB_ASSERT(_upb_Arena_RefCountFromTagged(ai->parent_or_count) == 1);
while (ai != NULL) {
// Load first since arena itself is likely from one of its blocks.
upb_ArenaInternal* next_arena =
(upb_ArenaInternal*)upb_Atomic_Load(&ai->next, memory_order_acquire);
upb_alloc* block_alloc = _upb_ArenaInternal_BlockAlloc(ai);
upb_MemBlock* block = upb_Atomic_Load(&ai->blocks, memory_order_acquire);
while (block != NULL) {
// Load first since we are deleting block.
upb_MemBlock* next_block =
upb_Atomic_Load(&block->next, memory_order_acquire);
upb_free(block_alloc, block);
block = next_block;
}
ai = next_arena;
}
}
void upb_Arena_Free(upb_Arena* a) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
uintptr_t poc = upb_Atomic_Load(&ai->parent_or_count, memory_order_acquire);
retry:
while (_upb_Arena_IsTaggedPointer(poc)) {
ai = _upb_Arena_PointerFromTagged(poc);
poc = upb_Atomic_Load(&ai->parent_or_count, memory_order_acquire);
}
// compare_exchange or fetch_sub are RMW operations, which are more
// expensive then direct loads. As an optimization, we only do RMW ops
// when we need to update things for other threads to see.
if (poc == _upb_Arena_TaggedFromRefcount(1)) {
_upb_Arena_DoFree(ai);
return;
}
if (upb_Atomic_CompareExchangeWeak(
&ai->parent_or_count, &poc,
_upb_Arena_TaggedFromRefcount(_upb_Arena_RefCountFromTagged(poc) - 1),
memory_order_release, memory_order_acquire)) {
// We were >1 and we decremented it successfully, so we are done.
return;
}
// We failed our update, so someone has done something, retry the whole
// process, but the failed exchange reloaded `poc` for us.
goto retry;
}
static void _upb_Arena_DoFuseArenaLists(upb_ArenaInternal* const parent,
upb_ArenaInternal* child) {
upb_ArenaInternal* parent_tail =
upb_Atomic_Load(&parent->tail, memory_order_relaxed);
do {
// Our tail might be stale, but it will always converge to the true tail.
upb_ArenaInternal* parent_tail_next =
upb_Atomic_Load(&parent_tail->next, memory_order_relaxed);
while (parent_tail_next != NULL) {
parent_tail = parent_tail_next;
parent_tail_next =
upb_Atomic_Load(&parent_tail->next, memory_order_relaxed);
}
upb_ArenaInternal* displaced =
upb_Atomic_Exchange(&parent_tail->next, child, memory_order_relaxed);
parent_tail = upb_Atomic_Load(&child->tail, memory_order_relaxed);
// If we displaced something that got installed racily, we can simply
// reinstall it on our new tail.
child = displaced;
} while (child != NULL);
upb_Atomic_Store(&parent->tail, parent_tail, memory_order_relaxed);
}
static upb_ArenaInternal* _upb_Arena_DoFuse(upb_Arena* a1, upb_Arena* a2,
uintptr_t* ref_delta) {
// `parent_or_count` has two disctint modes
// - parent pointer mode
// - refcount mode
//
// In parent pointer mode, it may change what pointer it refers to in the
// tree, but it will always approach a root. Any operation that walks the
// tree to the root may collapse levels of the tree concurrently.
upb_ArenaRoot r1 = _upb_Arena_FindRoot(a1);
upb_ArenaRoot r2 = _upb_Arena_FindRoot(a2);
if (r1.root == r2.root) return r1.root; // Already fused.
// Avoid cycles by always fusing into the root with the lower address.
if ((uintptr_t)r1.root > (uintptr_t)r2.root) {
upb_ArenaRoot tmp = r1;
r1 = r2;
r2 = tmp;
}
// The moment we install `r1` as the parent for `r2` all racing frees may
// immediately begin decrementing `r1`'s refcount (including pending
// increments to that refcount and their frees!). We need to add `r2`'s refs
// now, so that `r1` can withstand any unrefs that come from r2.
//
// Note that while it is possible for `r2`'s refcount to increase
// asynchronously, we will not actually do the reparenting operation below
// unless `r2`'s refcount is unchanged from when we read it.
//
// Note that we may have done this previously, either to this node or a
// different node, during a previous and failed DoFuse() attempt. But we will
// not lose track of these refs because we always add them to our overall
// delta.
uintptr_t r2_untagged_count = r2.tagged_count & ~1;
uintptr_t with_r2_refs = r1.tagged_count + r2_untagged_count;
if (!upb_Atomic_CompareExchangeStrong(
&r1.root->parent_or_count, &r1.tagged_count, with_r2_refs,
memory_order_release, memory_order_acquire)) {
return NULL;
}
// Perform the actual fuse by removing the refs from `r2` and swapping in the
// parent pointer.
if (!upb_Atomic_CompareExchangeStrong(
&r2.root->parent_or_count, &r2.tagged_count,
_upb_Arena_TaggedFromPointer(r1.root), memory_order_release,
memory_order_acquire)) {
// We'll need to remove the excess refs we added to r1 previously.
*ref_delta += r2_untagged_count;
return NULL;
}
// Now that the fuse has been performed (and can no longer fail) we need to
// append `r2` to `r1`'s linked list.
_upb_Arena_DoFuseArenaLists(r1.root, r2.root);
return r1.root;
}
static bool _upb_Arena_FixupRefs(upb_ArenaInternal* new_root,
uintptr_t ref_delta) {
if (ref_delta == 0) return true; // No fixup required.
uintptr_t poc =
upb_Atomic_Load(&new_root->parent_or_count, memory_order_relaxed);
if (_upb_Arena_IsTaggedPointer(poc)) return false;
uintptr_t with_refs = poc - ref_delta;
UPB_ASSERT(!_upb_Arena_IsTaggedPointer(with_refs));
return upb_Atomic_CompareExchangeStrong(&new_root->parent_or_count, &poc,
with_refs, memory_order_relaxed,
memory_order_relaxed);
}
bool upb_Arena_Fuse(upb_Arena* a1, upb_Arena* a2) {
if (a1 == a2) return true; // trivial fuse
upb_ArenaInternal* ai1 = upb_Arena_Internal(a1);
upb_ArenaInternal* ai2 = upb_Arena_Internal(a2);
// Do not fuse initial blocks since we cannot lifetime extend them.
// Any other fuse scenario is allowed.
if (_upb_ArenaInternal_HasInitialBlock(ai1) ||
_upb_ArenaInternal_HasInitialBlock(ai2)) {
return false;
}
// The number of refs we ultimately need to transfer to the new root.
uintptr_t ref_delta = 0;
while (true) {
upb_ArenaInternal* new_root = _upb_Arena_DoFuse(a1, a2, &ref_delta);
if (new_root != NULL && _upb_Arena_FixupRefs(new_root, ref_delta)) {
return true;
}
}
}
bool upb_Arena_IncRefFor(upb_Arena* a, const void* owner) {
upb_ArenaInternal* ai = upb_Arena_Internal(a);
if (_upb_ArenaInternal_HasInitialBlock(ai)) return false;
upb_ArenaRoot r;
retry:
r = _upb_Arena_FindRoot(a);
if (upb_Atomic_CompareExchangeWeak(
&r.root->parent_or_count, &r.tagged_count,
_upb_Arena_TaggedFromRefcount(
_upb_Arena_RefCountFromTagged(r.tagged_count) + 1),
memory_order_release, memory_order_acquire)) {
// We incremented it successfully, so we are done.
return true;
}
// We failed update due to parent switching on the arena.
goto retry;
}
void upb_Arena_DecRefFor(upb_Arena* a, const void* owner) { upb_Arena_Free(a); }
void UPB_PRIVATE(_upb_Arena_SwapIn)(upb_Arena* des, const upb_Arena* src) {
upb_ArenaInternal* desi = upb_Arena_Internal(des);
upb_ArenaInternal* srci = upb_Arena_Internal(src);
*des = *src;
desi->block_alloc = srci->block_alloc;
upb_MemBlock* blocks = upb_Atomic_Load(&srci->blocks, memory_order_relaxed);
upb_Atomic_Init(&desi->blocks, blocks);
}
void UPB_PRIVATE(_upb_Arena_SwapOut)(upb_Arena* des, const upb_Arena* src) {
upb_ArenaInternal* desi = upb_Arena_Internal(des);
upb_ArenaInternal* srci = upb_Arena_Internal(src);
*des = *src;
upb_MemBlock* blocks = upb_Atomic_Load(&srci->blocks, memory_order_relaxed);
upb_Atomic_Store(&desi->blocks, blocks, memory_order_relaxed);
}