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gh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW #148999

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gh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW #148999

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4 changes: 2 additions & 2 deletions Lib/test/test_dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -542,7 +542,7 @@ def _with(c):
dis_with = """\
%4d RESUME 0

%4d LOAD_FAST_BORROW 0 (c)
%4d LOAD_FAST 0 (c)
COPY 1
LOAD_SPECIAL 1 (__exit__)
SWAP 2
Expand Down Expand Up @@ -1922,7 +1922,7 @@ def _prepare_test_cases():
make_inst(opname='LOAD_SMALL_INT', arg=0, argval=0, argrepr='', offset=244, start_offset=244, starts_line=False, line_number=21),
make_inst(opname='BINARY_OP', arg=11, argval=11, argrepr='/', offset=246, start_offset=246, starts_line=False, line_number=21, cache_info=[('counter', 1, b'\x00\x00'), ('descr', 4, b'\x00\x00\x00\x00\x00\x00\x00\x00')]),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=258, start_offset=258, starts_line=False, line_number=21),
make_inst(opname='LOAD_FAST_BORROW', arg=0, argval='i', argrepr='i', offset=260, start_offset=260, starts_line=True, line_number=25),
make_inst(opname='LOAD_FAST', arg=0, argval='i', argrepr='i', offset=260, start_offset=260, starts_line=True, line_number=25),
make_inst(opname='COPY', arg=1, argval=1, argrepr='', offset=262, start_offset=262, starts_line=False, line_number=25),
make_inst(opname='LOAD_SPECIAL', arg=1, argval=1, argrepr='__exit__', offset=264, start_offset=264, starts_line=False, line_number=25),
make_inst(opname='SWAP', arg=2, argval=2, argrepr='', offset=266, start_offset=266, starts_line=False, line_number=25),
Expand Down
93 changes: 93 additions & 0 deletions Lib/test/test_peepholer.py
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Expand Up @@ -2598,6 +2598,36 @@ def test_for_iter(self):
("LOAD_CONST", 0, 7),
("RETURN_VALUE", None, 8),
]
expected = [
("LOAD_FAST_BORROW", 0, 1),
top := self.Label(),
("FOR_ITER", end := self.Label(), 2),
("STORE_FAST", 2, 3),
("JUMP", top, 4),
end,
("END_FOR", None, 5),
("POP_TOP", None, 6),
("LOAD_CONST", 0, 7),
("RETURN_VALUE", None, 8),
]
self.cfg_optimization_test(insts, expected, consts=[None])

def test_for_iter_checks_fallthrough(self):
insts = [
("LOAD_FAST", 0, 1),
("GET_ITER", 0, 2),
top := self.Label(),
("FOR_ITER", end := self.Label(), 3),
("STORE_FAST", 1, 4),
("LOAD_CONST", 0, 5),
("STORE_FAST", 0, 6),
("JUMP", top, 7),
end,
("END_FOR", None, 8),
("POP_ITER", None, 9),
("LOAD_CONST", 0, 10),
("RETURN_VALUE", None, 11),
]
self.cfg_optimization_test(insts, insts, consts=[None])

def test_load_attr(self):
Expand Down Expand Up @@ -2675,6 +2705,69 @@ def test_send(self):
]
self.cfg_optimization_test(insts, expected, consts=[None])

def test_borrow_checks_exception_handler_edges(self):
insts = [
("LOAD_FAST", 0, 1),
("JUMP", start := self.Label(), 2),
start,
("SETUP_FINALLY", handler := self.Label(), 3),
("LOAD_CONST", 0, 4),
("BINARY_OP", 0, 5),
("POP_BLOCK", None, 6),
("RETURN_VALUE", None, 7),
handler,
("STORE_FAST", 1, 8),
("LOAD_CONST", 0, 9),
("STORE_FAST", 0, 10),
("POP_TOP", None, 11),
("LOAD_CONST", 0, 12),
("RETURN_VALUE", None, 13),
]
expected = [
("LOAD_FAST", 0, 1),
("NOP", None, 2),
("SETUP_FINALLY", handler := self.Label(), 3),
("LOAD_CONST", 0, 4),
("BINARY_OP", 0, 5),
("NOP", None, 6),
("RETURN_VALUE", None, 7),
handler,
("STORE_FAST", 1, 8),
("LOAD_CONST", 0, 9),
("STORE_FAST", 0, 10),
("POP_TOP", None, 11),
("LOAD_CONST", 0, 12),
("RETURN_VALUE", None, 13),
]
self.cfg_optimization_test(insts, expected, consts=[None])

def test_copied_reference_is_not_borrowed(self):
insts = [
("LOAD_FAST", 0, 1),
("COPY", 1, 1),
("TO_BOOL", None, 1),
("POP_JUMP_IF_TRUE", target := self.Label(), 1),
("POP_TOP", None, 1),
("LOAD_FAST", 1, 1),
target,
("STORE_FAST", 0, 1),
("LOAD_CONST", 0, 2),
("RETURN_VALUE", None, 2),
]
expected = [
("LOAD_FAST", 0, 1),
("COPY", 1, 1),
("TO_BOOL", None, 1),
("POP_JUMP_IF_TRUE", target := self.Label(), 1),
("POP_TOP", None, 1),
("LOAD_FAST", 1, 1),
target,
("STORE_FAST", 0, 1),
("LOAD_CONST", 0, 2),
("RETURN_VALUE", None, 2),
]
self.cfg_optimization_test(insts, expected, consts=[None])

def test_format_simple(self):
# FORMAT_SIMPLE will leave its operand on the stack if it's a unicode
# object. We treat it conservatively and assume that it always leaves
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-05-09-00-06-52.gh-issue-133672.y6QwwH.rst
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@@ -0,0 +1 @@
The peephole optimizer now converts LOAD_FAST to LOAD_FAST_BORROW more aggressively. This optimization is applied even if the loaded variable remains live on the stack at the end of a basic block, as long as the borrow is otherwise determined to be safe. This can reduce reference counting overhead in certain code patterns, such as with iterables in loops.
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