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GH-123044: Give the `POP_TOP` after a case test a location in the body, not the pattern. by markshannon · Pull Request #130627 · python/cpython · GitHub

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GH-123044: Give the POP_TOP after a case test a location in the body, not the pattern. #130627

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Merged
markshannon merged 10 commits into from
Mar 10, 2025
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GH-123044: Give the POP_TOP after a case test a location in the body, not the pattern. #130627

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fd5d1e7
Use case body location for POP_TOP after case test, to give better lo…
markshannon Feb 27, 2025
1ce7f7f
Add news
markshannon Feb 27, 2025
f98fe7a
Include a default case in test
markshannon Feb 27, 2025
7084af0
Fix copy and paste
markshannon Feb 27, 2025
9332b19
Fix formatting in NEWS file for match cases
markshannon Feb 27, 2025
0d82f1b
Use pseudo location to mark instruction as having the location of the…
markshannon Feb 28, 2025
ab6fa4c
Use -2s for NEXT_LOCATION. Modify basicblock_remove_redundant_nops an…
markshannon Mar 6, 2025
1229037
Add a couple of asserts for NEXT_LOCATION
markshannon Mar 6, 2025
a077433
Replace NEXT_LOCATION with NO_LOCATION if there is no meaningful loca…
markshannon Mar 7, 2025
00380b3
Merge branch 'main' into location-pop-top-after-case
markshannon Mar 10, 2025
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1 change: 1 addition & 0 deletions Include/internal/pycore_symtable.h
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,7 @@ typedef struct {
.end_col_offset = (n)->end_col_offset }

static const _Py_SourceLocation NO_LOCATION = {-1, -1, -1, -1};
static const _Py_SourceLocation NEXT_LOCATION = {-2, -2, -2, -2};

/* __future__ information */
typedef struct {
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29 changes: 29 additions & 0 deletions Lib/test/test_monitoring.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1681,6 +1681,35 @@ async def foo():
('branch left', 'func', 12, 12)])


def test_match(self):

def func(v=1):
x = 0
for v in range(4):
match v:
case 1:
x += 1
case 2:
x += 2
case _:
x += 3
return x

self.check_events(func, recorders = BRANCHES_RECORDERS, expected = [
('branch left', 'func', 2, 2),
('branch right', 'func', 4, 6),
('branch right', 'func', 6, 8),
('branch left', 'func', 2, 2),
('branch left', 'func', 4, 5),
('branch left', 'func', 2, 2),
('branch right', 'func', 4, 6),
('branch left', 'func', 6, 7),
('branch left', 'func', 2, 2),
('branch right', 'func', 4, 6),
('branch right', 'func', 6, 8),
('branch right', 'func', 2, 10)])


class TestBranchConsistency(MonitoringTestBase, unittest.TestCase):

def check_branches(self, run_func, test_func=None, tool=TEST_TOOL, recorders=BRANCH_OFFSET_RECORDERS):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-02-27-10-47-09.gh-issue-123044.8182Un.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Make sure that the location of branch targets in ``match`` cases is in the
body, not the pattern.
13 changes: 13 additions & 0 deletions Python/assemble.c
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Original file line number Diff line number Diff line change
Expand Up @@ -291,6 +291,7 @@ write_location_info_entry(struct assembler* a, location loc, int isize)
RETURN_IF_ERROR(_PyBytes_Resize(&a->a_linetable, len*2));
}
if (loc.lineno < 0) {
assert(loc.lineno == NO_LOCATION.lineno);
write_location_info_none(a, isize);
return SUCCESS;
}
Expand Down Expand Up @@ -341,6 +342,18 @@ assemble_location_info(struct assembler *a, instr_sequence *instrs,
{
a->a_lineno = firstlineno;
location loc = NO_LOCATION;
for (int i = instrs->s_used-1; i >= 0; i--) {
instruction *instr = &instrs->s_instrs[i];
if (same_location(instr->i_loc, NEXT_LOCATION)) {
if (IS_TERMINATOR_OPCODE(instr->i_opcode)) {
instr->i_loc = NO_LOCATION;
}
else {
assert(i < instrs->s_used-1);
instr->i_loc = instr[1].i_loc;
}
}
}

iritkatriel Mar 3, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think this transformation should be in propagate_line_numbers in flowgraph.c. Otherwise the last instruction may remain without location, when it could have received a location from an earlier instruction in its block.

markshannon Mar 3, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Wouldn't it be too early in propagate_line_numbers?
If the NEXT_LOCATION instruction is at the end of the block where would it get the location information from?

iritkatriel Mar 3, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

From the next block (fall through or jump). If there is more than one successor, then we have a problem anyway, right, so what do we do?

markshannon Mar 5, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

We should only use NEXT_LOCATION if instruction is not the last instruction in the block. I'll add an assert for that.
int size = 0;
for (int i = 0; i < instrs->s_used; i++) {
instruction *instr = &instrs->s_instrs[i];
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3 changes: 2 additions & 1 deletion Python/codegen.c
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Original file line number Diff line number Diff line change
Expand Up @@ -6124,7 +6124,8 @@ codegen_match_inner(compiler *c, stmt_ty s, pattern_context *pc)
}
// Success! Pop the subject off, we're done with it:
if (i != cases - has_default - 1) {
ADDOP(c, LOC(m->pattern), POP_TOP);
/* Use the next location to give better locations for branch events */
ADDOP(c, NEXT_LOCATION, POP_TOP);
}
VISIT_SEQ(c, stmt, m->body);
ADDOP_JUMP(c, NO_LOCATION, JUMP, end);
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10 changes: 5 additions & 5 deletions Python/flowgraph.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1078,8 +1078,8 @@ basicblock_remove_redundant_nops(basicblock *bb) {
location next_loc = NO_LOCATION;
for (int next_i=0; next_i < next->b_iused; next_i++) {
cfg_instr *instr = &next->b_instr[next_i];
if (instr->i_opcode == NOP && instr->i_loc.lineno == NO_LOCATION.lineno) {
/* Skip over NOPs without location, they will be removed */
if (instr->i_opcode == NOP && instr->i_loc.lineno < 0) {
/* Skip over NOPs without a location, they will be removed */
continue;
}
next_loc = instr->i_loc;
Expand Down Expand Up @@ -2976,7 +2976,7 @@ propagate_line_numbers(basicblock *entryblock) {

location prev_location = NO_LOCATION;
for (int i = 0; i < b->b_iused; i++) {
if (b->b_instr[i].i_loc.lineno < 0) {
if (b->b_instr[i].i_loc.lineno == NO_LOCATION.lineno) {
b->b_instr[i].i_loc = prev_location;
}
else {
Expand All @@ -2985,15 +2985,15 @@ propagate_line_numbers(basicblock *entryblock) {
}
if (BB_HAS_FALLTHROUGH(b) && b->b_next->b_predecessors == 1) {
if (b->b_next->b_iused > 0) {
if (b->b_next->b_instr[0].i_loc.lineno < 0) {
if (b->b_next->b_instr[0].i_loc.lineno == NO_LOCATION.lineno) {
b->b_next->b_instr[0].i_loc = prev_location;
}
}
}
if (is_jump(last)) {
basicblock *target = last->i_target;
if (target->b_predecessors == 1) {
if (target->b_instr[0].i_loc.lineno < 0) {
if (target->b_instr[0].i_loc.lineno == NO_LOCATION.lineno) {
target->b_instr[0].i_loc = prev_location;
}
}
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