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Merged
markshannon merged 5 commits into from
Oct 22, 2026
+87 −45
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gh-159493: insert super instructions after borrow analysis in compiler #160032

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f627d4e
fix bug
kumaraditya303 Oct 21, 2026
f17201c
fix error handling
kumaraditya303 Oct 21, 2026
041662e
Update Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-14-00-00.gh-issu…
kumaraditya303 Oct 21, 2026
2808c53
fix lint
kumaraditya303 Oct 21, 2026
69419b0
fix comments
kumaraditya303 Oct 22, 2026
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75 changes: 36 additions & 39 deletions Lib/test/test_dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1817,26 +1817,25 @@ def _prepare_test_cases():
make_inst(opname='MAKE_CELL', arg=1, argval='b', argrepr='b', offset=2, start_offset=2, starts_line=False, line_number=None),
make_inst(opname='RESUME', arg=0, argval=0, argrepr='', offset=4, start_offset=4, starts_line=True, line_number=1, cache_info=[('counter', 1, b'\x00\x00')]),
make_inst(opname='LOAD_CONST', arg=3, argval=(3, 4), argrepr='(3, 4)', offset=8, start_offset=8, starts_line=True, line_number=2),
make_inst(opname='LOAD_FAST_BORROW', arg=0, argval='a', argrepr='a', offset=10, start_offset=10, starts_line=False, line_number=2),
make_inst(opname='LOAD_FAST_BORROW', arg=1, argval='b', argrepr='b', offset=12, start_offset=12, starts_line=False, line_number=2),
make_inst(opname='BUILD_TUPLE', arg=2, argval=2, argrepr='', offset=14, start_offset=14, starts_line=False, line_number=2),
make_inst(opname='LOAD_CONST', arg=1, argval=code_object_f, argrepr=repr(code_object_f), offset=16, start_offset=16, starts_line=False, line_number=2),
make_inst(opname='MAKE_FUNCTION', arg=None, argval=None, argrepr='', offset=18, start_offset=18, starts_line=False, line_number=2),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=8, argval=8, argrepr='closure', offset=20, start_offset=20, starts_line=False, line_number=2),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=1, argval=1, argrepr='defaults', offset=22, start_offset=22, starts_line=False, line_number=2),
make_inst(opname='STORE_FAST', arg=2, argval='f', argrepr='f', offset=24, start_offset=24, starts_line=False, line_number=2),
make_inst(opname='LOAD_GLOBAL', arg=1, argval='print', argrepr='print + NULL', offset=26, start_offset=26, starts_line=True, line_number=7, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_DEREF', arg=0, argval='a', argrepr='a', offset=36, start_offset=36, starts_line=False, line_number=7),
make_inst(opname='LOAD_DEREF', arg=1, argval='b', argrepr='b', offset=38, start_offset=38, starts_line=False, line_number=7),
make_inst(opname='LOAD_COMMON_CONSTANT', arg=8, argval='', argrepr="''", offset=40, start_offset=40, starts_line=False, line_number=7),
make_inst(opname='LOAD_SMALL_INT', arg=1, argval=1, argrepr='', offset=42, start_offset=42, starts_line=False, line_number=7),
make_inst(opname='BUILD_LIST', arg=0, argval=0, argrepr='', offset=44, start_offset=44, starts_line=False, line_number=7),
make_inst(opname='BUILD_MAP', arg=0, argval=0, argrepr='', offset=46, start_offset=46, starts_line=False, line_number=7),
make_inst(opname='LOAD_CONST', arg=2, argval='Hello world!', argrepr="'Hello world!'", offset=48, start_offset=48, starts_line=False, line_number=7),
make_inst(opname='CALL', arg=7, argval=7, argrepr='', offset=50, start_offset=50, starts_line=False, line_number=7, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=58, start_offset=58, starts_line=False, line_number=7),
make_inst(opname='LOAD_FAST_BORROW', arg=2, argval='f', argrepr='f', offset=60, start_offset=60, starts_line=True, line_number=8),
make_inst(opname='RETURN_VALUE', arg=None, argval=None, argrepr='', offset=62, start_offset=62, starts_line=False, line_number=8),
make_inst(opname='LOAD_FAST_BORROW_LOAD_FAST_BORROW', arg=1, argval=('a', 'b'), argrepr='a, b', offset=10, start_offset=10, starts_line=False, line_number=2),
make_inst(opname='BUILD_TUPLE', arg=2, argval=2, argrepr='', offset=12, start_offset=12, starts_line=False, line_number=2),
make_inst(opname='LOAD_CONST', arg=1, argval=code_object_f, argrepr=repr(code_object_f), offset=14, start_offset=14, starts_line=False, line_number=2),
make_inst(opname='MAKE_FUNCTION', arg=None, argval=None, argrepr='', offset=16, start_offset=16, starts_line=False, line_number=2),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=8, argval=8, argrepr='closure', offset=18, start_offset=18, starts_line=False, line_number=2),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=1, argval=1, argrepr='defaults', offset=20, start_offset=20, starts_line=False, line_number=2),
make_inst(opname='STORE_FAST', arg=2, argval='f', argrepr='f', offset=22, start_offset=22, starts_line=False, line_number=2),
make_inst(opname='LOAD_GLOBAL', arg=1, argval='print', argrepr='print + NULL', offset=24, start_offset=24, starts_line=True, line_number=7, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_DEREF', arg=0, argval='a', argrepr='a', offset=34, start_offset=34, starts_line=False, line_number=7),
make_inst(opname='LOAD_DEREF', arg=1, argval='b', argrepr='b', offset=36, start_offset=36, starts_line=False, line_number=7),
make_inst(opname='LOAD_COMMON_CONSTANT', arg=8, argval='', argrepr="''", offset=38, start_offset=38, starts_line=False, line_number=7),
make_inst(opname='LOAD_SMALL_INT', arg=1, argval=1, argrepr='', offset=40, start_offset=40, starts_line=False, line_number=7),
make_inst(opname='BUILD_LIST', arg=0, argval=0, argrepr='', offset=42, start_offset=42, starts_line=False, line_number=7),
make_inst(opname='BUILD_MAP', arg=0, argval=0, argrepr='', offset=44, start_offset=44, starts_line=False, line_number=7),
make_inst(opname='LOAD_CONST', arg=2, argval='Hello world!', argrepr="'Hello world!'", offset=46, start_offset=46, starts_line=False, line_number=7),
make_inst(opname='CALL', arg=7, argval=7, argrepr='', offset=48, start_offset=48, starts_line=False, line_number=7, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=56, start_offset=56, starts_line=False, line_number=7),
make_inst(opname='LOAD_FAST_BORROW', arg=2, argval='f', argrepr='f', offset=58, start_offset=58, starts_line=True, line_number=8),
make_inst(opname='RETURN_VALUE', arg=None, argval=None, argrepr='', offset=60, start_offset=60, starts_line=False, line_number=8),
]

expected_opinfo_f = [
Expand All @@ -1845,25 +1844,23 @@ def _prepare_test_cases():
make_inst(opname='MAKE_CELL', arg=1, argval='d', argrepr='d', offset=4, start_offset=4, starts_line=False, line_number=None),
make_inst(opname='RESUME', arg=0, argval=0, argrepr='', offset=6, start_offset=6, starts_line=True, line_number=2, cache_info=[('counter', 1, b'\x00\x00')]),
make_inst(opname='LOAD_CONST', arg=2, argval=(5, 6), argrepr='(5, 6)', offset=10, start_offset=10, starts_line=True, line_number=3),
make_inst(opname='LOAD_FAST_BORROW', arg=3, argval='a', argrepr='a', offset=12, start_offset=12, starts_line=False, line_number=3),
make_inst(opname='LOAD_FAST_BORROW', arg=4, argval='b', argrepr='b', offset=14, start_offset=14, starts_line=False, line_number=3),
make_inst(opname='LOAD_FAST_BORROW', arg=0, argval='c', argrepr='c', offset=16, start_offset=16, starts_line=False, line_number=3),
make_inst(opname='LOAD_FAST_BORROW', arg=1, argval='d', argrepr='d', offset=18, start_offset=18, starts_line=False, line_number=3),
make_inst(opname='BUILD_TUPLE', arg=4, argval=4, argrepr='', offset=20, start_offset=20, starts_line=False, line_number=3),
make_inst(opname='LOAD_CONST', arg=1, argval=code_object_inner, argrepr=repr(code_object_inner), offset=22, start_offset=22, starts_line=False, line_number=3),
make_inst(opname='MAKE_FUNCTION', arg=None, argval=None, argrepr='', offset=24, start_offset=24, starts_line=False, line_number=3),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=8, argval=8, argrepr='closure', offset=26, start_offset=26, starts_line=False, line_number=3),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=1, argval=1, argrepr='defaults', offset=28, start_offset=28, starts_line=False, line_number=3),
make_inst(opname='STORE_FAST', arg=2, argval='inner', argrepr='inner', offset=30, start_offset=30, starts_line=False, line_number=3),
make_inst(opname='LOAD_GLOBAL', arg=1, argval='print', argrepr='print + NULL', offset=32, start_offset=32, starts_line=True, line_number=5, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_DEREF', arg=3, argval='a', argrepr='a', offset=42, start_offset=42, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=4, argval='b', argrepr='b', offset=44, start_offset=44, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=0, argval='c', argrepr='c', offset=46, start_offset=46, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=1, argval='d', argrepr='d', offset=48, start_offset=48, starts_line=False, line_number=5),
make_inst(opname='CALL', arg=4, argval=4, argrepr='', offset=50, start_offset=50, starts_line=False, line_number=5, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=58, start_offset=58, starts_line=False, line_number=5),
make_inst(opname='LOAD_FAST_BORROW', arg=2, argval='inner', argrepr='inner', offset=60, start_offset=60, starts_line=True, line_number=6),
make_inst(opname='RETURN_VALUE', arg=None, argval=None, argrepr='', offset=62, start_offset=62, starts_line=False, line_number=6),
make_inst(opname='LOAD_FAST_BORROW_LOAD_FAST_BORROW', arg=52, argval=('a', 'b'), argrepr='a, b', offset=12, start_offset=12, starts_line=False, line_number=3),
make_inst(opname='LOAD_FAST_BORROW_LOAD_FAST_BORROW', arg=1, argval=('c', 'd'), argrepr='c, d', offset=14, start_offset=14, starts_line=False, line_number=3),
make_inst(opname='BUILD_TUPLE', arg=4, argval=4, argrepr='', offset=16, start_offset=16, starts_line=False, line_number=3),
make_inst(opname='LOAD_CONST', arg=1, argval=code_object_inner, argrepr=repr(code_object_inner), offset=18, start_offset=18, starts_line=False, line_number=3),
make_inst(opname='MAKE_FUNCTION', arg=None, argval=None, argrepr='', offset=20, start_offset=20, starts_line=False, line_number=3),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=8, argval=8, argrepr='closure', offset=22, start_offset=22, starts_line=False, line_number=3),
make_inst(opname='SET_FUNCTION_ATTRIBUTE', arg=1, argval=1, argrepr='defaults', offset=24, start_offset=24, starts_line=False, line_number=3),
make_inst(opname='STORE_FAST', arg=2, argval='inner', argrepr='inner', offset=26, start_offset=26, starts_line=False, line_number=3),
make_inst(opname='LOAD_GLOBAL', arg=1, argval='print', argrepr='print + NULL', offset=28, start_offset=28, starts_line=True, line_number=5, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_DEREF', arg=3, argval='a', argrepr='a', offset=38, start_offset=38, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=4, argval='b', argrepr='b', offset=40, start_offset=40, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=0, argval='c', argrepr='c', offset=42, start_offset=42, starts_line=False, line_number=5),
make_inst(opname='LOAD_DEREF', arg=1, argval='d', argrepr='d', offset=44, start_offset=44, starts_line=False, line_number=5),
make_inst(opname='CALL', arg=4, argval=4, argrepr='', offset=46, start_offset=46, starts_line=False, line_number=5, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=54, start_offset=54, starts_line=False, line_number=5),
make_inst(opname='LOAD_FAST_BORROW', arg=2, argval='inner', argrepr='inner', offset=56, start_offset=56, starts_line=True, line_number=6),
make_inst(opname='RETURN_VALUE', arg=None, argval=None, argrepr='', offset=58, start_offset=58, starts_line=False, line_number=6),
]

expected_opinfo_inner = [
Expand Down
25 changes: 25 additions & 0 deletions Lib/test/test_peepholer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -855,6 +855,31 @@ def f():
y = x + x
self.assertInBytecode(f, 'LOAD_FAST_BORROW_LOAD_FAST_BORROW')

def test_store_fast_load_fast_borrow(self):
# gh-159493: a LOAD_FAST that can borrow must not be fused into
# STORE_FAST_LOAD_FAST, regardless of how the source is laid out.
def f(shared, n):
for i in range(n): shared

def g(shared, n):
for i in range(n):
shared

for func in (f, g):
with self.subTest(func=func.__name__):
self.assertNotInBytecode(func, 'STORE_FAST_LOAD_FAST')
self.assertInBytecode(func, 'LOAD_FAST_BORROW', 'shared')
self.assertEqual([i.opname for i in dis.get_instructions(f)],
[i.opname for i in dis.get_instructions(g)])

def test_store_fast_load_fast_not_borrowable(self):
def f(x):
y = x; z = y
return z
# The load of y is stored into z, so it needs a strong reference and
# can still be fused.
self.assertInBytecode(f, 'STORE_FAST_LOAD_FAST', ('y', 'y'))

def test_load_fast_unknown_simple(self):
def f():
if condition():
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-14-00-00.gh-issue-159493.sfLf1b.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Superinstructions are now formed after the compiler decides which
:opcode:`LOAD_FAST` instructions can borrow their reference.
This fixes reference count contention slowdown on the free-threaded build for single-line loops and comprehensions.
29 changes: 23 additions & 6 deletions Python/flowgraph.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2719,6 +2719,12 @@ insert_superinstructions(cfg_builder *g)
make_super_instruction(inst, &b->b_instr[i + 1], LOAD_FAST_LOAD_FAST);
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}
break;
case LOAD_FAST_BORROW:
if (nextop == LOAD_FAST_BORROW) {
make_super_instruction(inst, &b->b_instr[i + 1],
LOAD_FAST_BORROW_LOAD_FAST_BORROW);
}
break;
case STORE_FAST:
switch (nextop) {
case LOAD_FAST:
Expand All @@ -2732,9 +2738,9 @@ insert_superinstructions(cfg_builder *g)
}
}
}
int res = remove_redundant_nops(g);
RETURN_IF_ERROR(remove_redundant_nops(g));
assert(no_redundant_nops(g));
return res;
return SUCCESS;
}

#define NOT_LOCAL -1
Expand Down Expand Up @@ -3834,8 +3840,6 @@ _PyCfg_OptimizeCodeUnit(cfg_builder *g, PyObject *consts, PyObject *const_cache,
RETURN_IF_ERROR(
add_checks_for_loads_of_uninitialized_variables(
g->g_entryblock, nlocals, nparams));
RETURN_IF_ERROR(insert_superinstructions(g));

RETURN_IF_ERROR(push_cold_blocks_to_end(g));
RETURN_IF_ERROR(resolve_line_numbers(g, firstlineno));
// temporarily remove assert. See https://github.com/python/cpython/issues/125845
Expand Down Expand Up @@ -4157,11 +4161,19 @@ _PyCfg_OptimizedCfgToInstructionSequence(cfg_builder *g,
RETURN_IF_ERROR(normalize_jumps(g));
assert(no_redundant_jumps(g));

/* Can't modify the bytecode after inserting instructions that produce
* borrowed references.
/* After this point the bytecode may only be modified in ways that
* preserve the borrowed references computed here.
*
* NOTE: keep the passes below in sync with _PyCompile_OptimizeCfg().
*/
RETURN_IF_ERROR(optimize_load_fast(g));

/* Superinstructions are inserted after optimize_load_fast() so that a
* LOAD_FAST which can be borrowed is converted to LOAD_FAST_BORROW
* instead of being fused into a STORE_FAST_LOAD_FAST. Fusing adjacent
* instructions does not change which references are borrowed. */
RETURN_IF_ERROR(insert_superinstructions(g));

/* Can't modify the bytecode after computing jump offsets. */
if (_PyCfg_ToInstructionSequence(g, seq) < 0) {
return ERROR;
Expand Down Expand Up @@ -4254,10 +4266,15 @@ _PyCompile_OptimizeCfg(PyObject *seq, PyObject *consts, int nlocals)
goto error;
}

/* NOTE: keep in sync with _PyCfg_OptimizedCfgToInstructionSequence(). */
if (optimize_load_fast(g) != SUCCESS) {
goto error;
}

if (insert_superinstructions(g) < 0) {
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goto error;
}

res = cfg_to_instruction_sequence(g);
error:
Py_DECREF(const_cache);
Expand Down
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