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bpo-46528: Simplify BUILD_TUPLE/UNPACK_SEQUENCE folding #31039

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Merged
merged 1 commit into from
Feb 1, 2022
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bpo-46528: Simplify BUILD_TUPLE/UNPACK_SEQUENCE folding #31039

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2 changes: 1 addition & 1 deletion Lib/test/test_peepholer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -126,7 +126,7 @@ def test_pack_unpack(self):
code = compile(line,'','single')
self.assertInBytecode(code, elem)
self.assertNotInBytecode(code, 'BUILD_TUPLE')
self.assertNotInBytecode(code, 'UNPACK_TUPLE')
self.assertNotInBytecode(code, 'UNPACK_SEQUENCE')
self.check_lnotab(code)

def test_folding_of_tuples_of_constants(self):
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21 changes: 7 additions & 14 deletions Python/compile.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8650,29 +8650,22 @@ optimize_basic_block(struct compiler *c, basicblock *bb, PyObject *consts)
}

/* Try to fold tuples of constants.
Skip over BUILD_SEQN 1 UNPACK_SEQN 1.
Replace BUILD_SEQN 2 UNPACK_SEQN 2 with ROT2.
Replace BUILD_SEQN 3 UNPACK_SEQN 3 with ROT3 ROT2. */
Skip over BUILD_TUPLE(1) UNPACK_SEQUENCE(1).
Replace BUILD_TUPLE(2) UNPACK_SEQUENCE(2) with SWAP(2).
Replace BUILD_TUPLE(3) UNPACK_SEQUENCE(3) with SWAP(3). */
case BUILD_TUPLE:
if (nextop == UNPACK_SEQUENCE && oparg == bb->b_instr[i+1].i_oparg) {
switch(oparg) {
case 1:
inst->i_opcode = NOP;
bb->b_instr[i+1].i_opcode = NOP;
break;
continue;
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Why change this? Aren't they equivalent.
Having a continue here distracts from the flow of the code, as it is not the normal way to end a case and increases cognitive load (at least it did for me).

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They’re not quite equivalent: note that I also removed the break on old line 8675.

Previously, values other than 1, 2, or 3 wouldn’t get a chance to hit fold_tuple_on_constants. Now they do.

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👍 LGTM then.

case 2:
inst->i_opcode = SWAP;
inst->i_oparg = 2;
bb->b_instr[i+1].i_opcode = NOP;
i--;
break;
case 3:
inst->i_opcode = SWAP;
inst->i_oparg = 3;
bb->b_instr[i+1].i_opcode = NOP;
i--;
inst->i_opcode = NOP;
bb->b_instr[i+1].i_opcode = SWAP;
continue;
}
break;
}
if (i >= oparg) {
if (fold_tuple_on_constants(c, inst-oparg, oparg, consts)) {
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