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GH-93444: remove redundant fields from basicblock: b_nofallthrough, b…
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…_exit, b_return (GH-93445)
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@iritkatriel
iritkatriel committed Jun 3, 2022
1 parent f32e6b4 commit 1713ff0
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-06-02-23-00-08.gh-issue-93444.m63DIs.rst
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@@ -0,0 +1 @@
Removed redundant fields from the compiler's basicblock struct: ``b_nofallthrough``, ``b_exit``, ``b_return``. They can be easily calculated from the opcode of the last instruction of the block.
152 changes: 74 additions & 78 deletions Python/compile.c
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Expand Up @@ -101,6 +101,10 @@
(opcode) == POP_JUMP_IF_FALSE || \
(opcode) == POP_JUMP_IF_TRUE)

#define IS_JUMP_OPCODE(opcode) \
(IS_VIRTUAL_JUMP_OPCODE(opcode) || \
is_bit_set_in_table(_PyOpcode_Jump, opcode))

/* opcodes which are not emitted in codegen stage, only by the assembler */
#define IS_ASSEMBLER_OPCODE(opcode) \
((opcode) == JUMP_FORWARD || \
Expand All @@ -124,6 +128,17 @@
(opcode) == POP_JUMP_BACKWARD_IF_TRUE || \
(opcode) == POP_JUMP_BACKWARD_IF_FALSE)

#define IS_UNCONDITIONAL_JUMP_OPCODE(opcode) \
((opcode) == JUMP || \
(opcode) == JUMP_NO_INTERRUPT || \
(opcode) == JUMP_FORWARD || \
(opcode) == JUMP_BACKWARD || \
(opcode) == JUMP_BACKWARD_NO_INTERRUPT)

#define IS_SCOPE_EXIT_OPCODE(opcode) \
((opcode) == RETURN_VALUE || \
(opcode) == RAISE_VARARGS || \
(opcode) == RERAISE)

#define IS_TOP_LEVEL_AWAIT(c) ( \
(c->c_flags->cf_flags & PyCF_ALLOW_TOP_LEVEL_AWAIT) \
Expand All @@ -142,11 +157,6 @@ struct instr {
int i_end_col_offset;
};

typedef struct excepthandler {
struct instr *setup;
int offset;
} ExceptHandler;

typedef struct exceptstack {
struct basicblock_ *handlers[CO_MAXBLOCKS+1];
int depth;
Expand Down Expand Up @@ -187,8 +197,7 @@ is_block_push(struct instr *instr)
static inline int
is_jump(struct instr *i)
{
return IS_VIRTUAL_JUMP_OPCODE(i->i_opcode) ||
is_bit_set_in_table(_PyOpcode_Jump, i->i_opcode);
return IS_JUMP_OPCODE(i->i_opcode);
}

static int
Expand Down Expand Up @@ -254,16 +263,10 @@ typedef struct basicblock_ {
int b_startdepth;
/* instruction offset for block, computed by assemble_jump_offsets() */
int b_offset;
/* Basic block has no fall through (it ends with a return, raise or jump) */
unsigned b_nofallthrough : 1;
/* Basic block is an exception handler that preserves lasti */
unsigned b_preserve_lasti : 1;
/* Used by compiler passes to mark whether they have visited a basic block. */
unsigned b_visited : 1;
/* Basic block exits scope (it ends with a return or raise) */
unsigned b_exit : 1;
/* b_return is true if a RETURN_VALUE opcode is inserted. */
unsigned b_return : 1;
/* b_cold is true if this block is not perf critical (like an exception handler) */
unsigned b_cold : 1;
/* b_warm is used by the cold-detection algorithm to mark blocks which are definitely not cold */
Expand All @@ -279,6 +282,29 @@ basicblock_last_instr(basicblock *b) {
return NULL;
}

static inline int
basicblock_returns(basicblock *b) {
struct instr *last = basicblock_last_instr(b);
return last && last->i_opcode == RETURN_VALUE;
}

static inline int
basicblock_exits_scope(basicblock *b) {
struct instr *last = basicblock_last_instr(b);
return last && IS_SCOPE_EXIT_OPCODE(last->i_opcode);
}

static inline int
basicblock_nofallthrough(basicblock *b) {
struct instr *last = basicblock_last_instr(b);
return (last &&
(IS_SCOPE_EXIT_OPCODE(last->i_opcode) ||
IS_UNCONDITIONAL_JUMP_OPCODE(last->i_opcode)));
}

#define BB_NO_FALLTHROUGH(B) (basicblock_nofallthrough(B))
#define BB_HAS_FALLTHROUGH(B) (!basicblock_nofallthrough(B))

/* fblockinfo tracks the current frame block.
A frame block is used to handle loops, try/except, and try/finally.
Expand Down Expand Up @@ -852,7 +878,7 @@ compiler_copy_block(struct compiler *c, basicblock *block)
/* Cannot copy a block if it has a fallthrough, since
* a block can only have one fallthrough predecessor.
*/
assert(block->b_nofallthrough);
assert(BB_NO_FALLTHROUGH(block));
basicblock *result = compiler_new_block(c);
if (result == NULL) {
return NULL;
Expand All @@ -864,8 +890,6 @@ compiler_copy_block(struct compiler *c, basicblock *block)
}
result->b_instr[n] = block->b_instr[i];
}
result->b_exit = block->b_exit;
result->b_nofallthrough = 1;
return result;
}

Expand Down Expand Up @@ -1223,11 +1247,7 @@ static int
is_end_of_basic_block(struct instr *instr)
{
int opcode = instr->i_opcode;

return is_jump(instr) ||
opcode == RETURN_VALUE ||
opcode == RAISE_VARARGS ||
opcode == RERAISE;
return is_jump(instr) || IS_SCOPE_EXIT_OPCODE(opcode);
}

static int
Expand Down Expand Up @@ -1263,9 +1283,6 @@ basicblock_addop_line(basicblock *b, int opcode, int line,
struct instr *i = &b->b_instr[off];
i->i_opcode = opcode;
i->i_oparg = 0;
if (opcode == RETURN_VALUE) {
b->b_return = 1;
}
i->i_lineno = line;
i->i_end_lineno = end_line;
i->i_col_offset = col_offset;
Expand Down Expand Up @@ -7144,11 +7161,8 @@ stackdepth(struct compiler *c, basicblock *entry)
}
depth = new_depth;
assert(!IS_ASSEMBLER_OPCODE(instr->i_opcode));
if (instr->i_opcode == JUMP_NO_INTERRUPT ||
instr->i_opcode == JUMP ||
instr->i_opcode == RETURN_VALUE ||
instr->i_opcode == RAISE_VARARGS ||
instr->i_opcode == RERAISE)
if (IS_UNCONDITIONAL_JUMP_OPCODE(instr->i_opcode) ||
IS_SCOPE_EXIT_OPCODE(instr->i_opcode))
{
/* remaining code is dead */
next = NULL;
Expand All @@ -7159,7 +7173,7 @@ stackdepth(struct compiler *c, basicblock *entry)
}
}
if (next != NULL) {
assert(b->b_nofallthrough == 0);
assert(BB_HAS_FALLTHROUGH(b));
stackdepth_push(&sp, next, depth);
}
}
Expand Down Expand Up @@ -7314,7 +7328,7 @@ label_exception_targets(basicblock *entry) {
instr->i_except = handler;
assert(i == b->b_iused -1);
if (!instr->i_target->b_visited) {
if (b->b_nofallthrough == 0) {
if (BB_HAS_FALLTHROUGH(b)) {
ExceptStack *copy = copy_except_stack(except_stack);
if (copy == NULL) {
goto error;
Expand All @@ -7334,7 +7348,7 @@ label_exception_targets(basicblock *entry) {
instr->i_except = handler;
}
}
if (b->b_nofallthrough == 0 && !b->b_next->b_visited) {
if (BB_HAS_FALLTHROUGH(b) && !b->b_next->b_visited) {
assert(except_stack != NULL);
b->b_next->b_exceptstack = except_stack;
todo[0] = b->b_next;
Expand Down Expand Up @@ -7373,7 +7387,7 @@ mark_warm(basicblock *entry) {
assert(!b->b_except_predecessors);
b->b_warm = 1;
basicblock *next = b->b_next;
if (next && !b->b_nofallthrough && !next->b_visited) {
if (next && BB_HAS_FALLTHROUGH(b) && !next->b_visited) {
*sp++ = next;
next->b_visited = 1;
}
Expand Down Expand Up @@ -7417,7 +7431,7 @@ mark_cold(basicblock *entry) {
basicblock *b = *(--sp);
b->b_cold = 1;
basicblock *next = b->b_next;
if (next && !b->b_nofallthrough) {
if (next && BB_HAS_FALLTHROUGH(b)) {
if (!next->b_warm && !next->b_visited) {
*sp++ = next;
next->b_visited = 1;
Expand Down Expand Up @@ -7452,15 +7466,14 @@ push_cold_blocks_to_end(struct compiler *c, basicblock *entry, int code_flags) {
/* If we have a cold block with fallthrough to a warm block, add */
/* an explicit jump instead of fallthrough */
for (basicblock *b = entry; b != NULL; b = b->b_next) {
if (b->b_cold && !b->b_nofallthrough && b->b_next && b->b_next->b_warm) {
if (b->b_cold && BB_HAS_FALLTHROUGH(b) && b->b_next && b->b_next->b_warm) {
basicblock *explicit_jump = compiler_new_block(c);
if (explicit_jump == NULL) {
return -1;
}
basicblock_add_jump(explicit_jump, JUMP, -1, 0, 0, 0, b->b_next);

explicit_jump->b_cold = 1;
explicit_jump->b_nofallthrough = 1;
explicit_jump->b_next = b->b_next;
b->b_next = explicit_jump;
}
Expand Down Expand Up @@ -7953,7 +7966,7 @@ scan_block_for_local(int target, basicblock *b, bool unsafe_to_start,
if (unsafe) {
// unsafe at end of this block,
// so unsafe at start of next blocks
if (b->b_next && !b->b_nofallthrough) {
if (b->b_next && BB_HAS_FALLTHROUGH(b)) {
MAYBE_PUSH(b->b_next);
}
if (b->b_iused > 0) {
Expand Down Expand Up @@ -8281,9 +8294,10 @@ dump_instr(struct instr *i)
static void
dump_basicblock(const basicblock *b)
{
const char *b_return = b->b_return ? "return " : "";
const char *b_return = basicblock_returns(b) ? "return " : "";
fprintf(stderr, "[%d %d %d %p] used: %d, depth: %d, offset: %d %s\n",
b->b_cold, b->b_warm, b->b_nofallthrough, b, b->b_iused, b->b_startdepth, b->b_offset, b_return);
b->b_cold, b->b_warm, BB_NO_FALLTHROUGH(b), b, b->b_iused,
b->b_startdepth, b->b_offset, b_return);
if (b->b_instr) {
int i;
for (i = 0; i < b->b_iused; i++) {
Expand Down Expand Up @@ -8545,7 +8559,6 @@ remove_redundant_jumps(basicblock *entry) {
b_last_instr->i_opcode == JUMP_NO_INTERRUPT) {
if (b_last_instr->i_target == b->b_next) {
assert(b->b_next->b_iused);
b->b_nofallthrough = 0;
b_last_instr->i_opcode = NOP;
removed++;
}
Expand All @@ -8572,7 +8585,7 @@ assemble(struct compiler *c, int addNone)
}

/* Make sure every block that falls off the end returns None. */
if (!c->u->u_curblock->b_return) {
if (!basicblock_returns(c->u->u_curblock)) {
UNSET_LOC(c);
if (addNone)
ADDOP_LOAD_CONST(c, Py_None);
Expand Down Expand Up @@ -9064,7 +9077,6 @@ optimize_basic_block(struct compiler *c, basicblock *bb, PyObject *consts)
jump_if_true = nextop == POP_JUMP_IF_TRUE;
if (is_true == jump_if_true) {
bb->b_instr[i+1].i_opcode = JUMP;
bb->b_nofallthrough = 1;
}
else {
bb->b_instr[i+1].i_opcode = NOP;
Expand All @@ -9084,7 +9096,6 @@ optimize_basic_block(struct compiler *c, basicblock *bb, PyObject *consts)
jump_if_true = nextop == JUMP_IF_TRUE_OR_POP;
if (is_true == jump_if_true) {
bb->b_instr[i+1].i_opcode = JUMP;
bb->b_nofallthrough = 1;
}
else {
inst->i_opcode = NOP;
Expand Down Expand Up @@ -9273,7 +9284,7 @@ extend_block(basicblock *bb) {
last->i_opcode != JUMP_BACKWARD) {
return 0;
}
if (last->i_target->b_exit && last->i_target->b_iused <= MAX_COPY_SIZE) {
if (basicblock_exits_scope(last->i_target) && last->i_target->b_iused <= MAX_COPY_SIZE) {
basicblock *to_copy = last->i_target;
last->i_opcode = NOP;
for (int i = 0; i < to_copy->b_iused; i++) {
Expand All @@ -9283,7 +9294,6 @@ extend_block(basicblock *bb) {
}
bb->b_instr[index] = to_copy->b_instr[i];
}
bb->b_exit = 1;
}
return 0;
}
Expand Down Expand Up @@ -9341,34 +9351,21 @@ normalize_basic_block(basicblock *bb) {
/* Mark blocks as exit and/or nofallthrough.
Raise SystemError if CFG is malformed. */
for (int i = 0; i < bb->b_iused; i++) {
assert(!IS_ASSEMBLER_OPCODE(bb->b_instr[i].i_opcode));
switch(bb->b_instr[i].i_opcode) {
case RETURN_VALUE:
case RAISE_VARARGS:
case RERAISE:
bb->b_exit = 1;
bb->b_nofallthrough = 1;
break;
case JUMP:
case JUMP_NO_INTERRUPT:
bb->b_nofallthrough = 1;
/* fall through */
case POP_JUMP_IF_NOT_NONE:
case POP_JUMP_IF_NONE:
case POP_JUMP_IF_FALSE:
case POP_JUMP_IF_TRUE:
case JUMP_IF_FALSE_OR_POP:
case JUMP_IF_TRUE_OR_POP:
case FOR_ITER:
if (i != bb->b_iused-1) {
PyErr_SetString(PyExc_SystemError, "malformed control flow graph.");
return -1;
}
/* Skip over empty basic blocks. */
while (bb->b_instr[i].i_target->b_iused == 0) {
bb->b_instr[i].i_target = bb->b_instr[i].i_target->b_next;
}

int opcode = bb->b_instr[i].i_opcode;
assert(!IS_ASSEMBLER_OPCODE(opcode));
int is_jump = IS_JUMP_OPCODE(opcode);
int is_exit = IS_SCOPE_EXIT_OPCODE(opcode);
if (is_exit || is_jump) {
if (i != bb->b_iused-1) {
PyErr_SetString(PyExc_SystemError, "malformed control flow graph.");
return -1;
}
}
if (is_jump) {
/* Skip over empty basic blocks. */
while (bb->b_instr[i].i_target->b_iused == 0) {
bb->b_instr[i].i_target = bb->b_instr[i].i_target->b_next;
}
}
}
return 0;
Expand All @@ -9386,7 +9383,7 @@ mark_reachable(struct assembler *a) {
while (sp > stack) {
basicblock *b = *(--sp);
b->b_visited = 1;
if (b->b_next && !b->b_nofallthrough) {
if (b->b_next && BB_HAS_FALLTHROUGH(b)) {
if (!b->b_next->b_visited) {
assert(b->b_next->b_predecessors == 0);
*sp++ = b->b_next;
Expand Down Expand Up @@ -9475,7 +9472,7 @@ propagate_line_numbers(struct assembler *a) {
COPY_INSTR_LOC(b->b_instr[i], prev_instr);
}
}
if (!b->b_nofallthrough && b->b_next->b_predecessors == 1) {
if (BB_HAS_FALLTHROUGH(b) && b->b_next->b_predecessors == 1) {
assert(b->b_next->b_iused);
if (b->b_next->b_instr[0].i_lineno < 0) {
COPY_INSTR_LOC(prev_instr, b->b_next->b_instr[0]);
Expand Down Expand Up @@ -9523,7 +9520,6 @@ optimize_cfg(struct compiler *c, struct assembler *a, PyObject *consts)
for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
if (b->b_predecessors == 0) {
b->b_iused = 0;
b->b_nofallthrough = 0;
}
}
eliminate_empty_basic_blocks(a->a_entry);
Expand Down Expand Up @@ -9563,7 +9559,7 @@ trim_unused_consts(struct assembler *a, PyObject *consts)

static inline int
is_exit_without_lineno(basicblock *b) {
if (!b->b_exit) {
if (!basicblock_exits_scope(b)) {
return 0;
}
for (int i = 0; i < b->b_iused; i++) {
Expand Down Expand Up @@ -9614,7 +9610,7 @@ duplicate_exits_without_lineno(struct compiler *c)
/* Any remaining reachable exit blocks without line number can only be reached by
* fall through, and thus can only have a single predecessor */
for (basicblock *b = c->u->u_blocks; b != NULL; b = b->b_list) {
if (!b->b_nofallthrough && b->b_next && b->b_iused > 0) {
if (BB_HAS_FALLTHROUGH(b) && b->b_next && b->b_iused > 0) {
if (is_exit_without_lineno(b->b_next)) {
assert(b->b_next->b_iused > 0);
COPY_INSTR_LOC(b->b_instr[b->b_iused-1], b->b_next->b_instr[0]);
Expand Down

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