GH-125837: Split `LOAD_CONST` into three. #125972

markshannon · 2024-10-25T10:00:37Z

Splits LOAD_CONST into three instructions

LOAD_INT for ints in range(256). Avoids the need for a space in the co_consts tuple and avoids an incref
LOAD_CONST_IMMORTAL for other immortal objects. Avoids an incref
LOAD_CONST for the remaining constants

Issue: Refactor RETURN_VALUE and RETURN_CONST #125837

📚 Documentation preview 📚: https://cpython-previews--125972.org.readthedocs.build/

…_GEN

Co-authored-by: Tomas R. <[email protected]>

Python/bytecodes.c

Python/compile.c

Python/flowgraph.c

picnixz · 2024-10-25T10:29:12Z

LOAD_INT for ints in range(256)

I think the range is -5 -> 256 (included) right? (at least _PyLong_SMALL_INTS works that way, but maybe 256 should be excluded even though it's stored in the _PyLong_SMALL_INTS).

markshannon · 2024-10-25T12:53:43Z

I think the range is -5 -> 256 (included) right? (at least _PyLong_SMALL_INTS works that way, but maybe 256 should be excluded even though it's stored in the _PyLong_SMALL_INTS).

I want LOAD_INT 0 to push 0 not -5 to keep things simple.
So LOAD_INT will only cover range(256) even though the range of statically allocated ints is larger.

We can always add -5,-4,-3,-2 and -1 to LOAD_COMMON_CONST later if we want.

markshannon · 2024-10-25T14:30:29Z

Performance for tier 1 appears to be about 0.5% faster.

Stats show the following fraction of constants loaded:

Opcode	Fraction
LOAD_INT	47%
LOAD_CONST_IMMORTAL	42%
LOAD_CONST	11%

With a reduction in increfs of immortal objects of about 20%.

Python/codegen.c

Lib/test/test_ast/test_ast.py

Include/internal/pycore_magic_number.h

Lib/test/test_ast/test_ast.py

Misc/NEWS.d/next/Core_and_Builtins/2024-10-25-15-56-14.gh-issue-125837.KlCdgD.rst

Python/specialize.c

Python/flowgraph.c

Python/codegen.c

Doc/library/dis.rst

Python/codegen.c

iritkatriel · 2024-10-28T15:40:47Z

Python/flowgraph.c

@@ -2173,7 +2155,8 @@ remove_unused_consts(basicblock *entryblock, PyObject *consts)

    for (basicblock *b = entryblock; b != NULL; b = b->b_next) {
        for (int i = 0; i < b->b_iused; i++) {
-            if (OPCODE_HAS_CONST(b->b_instr[i].i_opcode)) {
+            int opcode = b->b_instr[i].i_opcode;
+            if (OPCODE_HAS_CONST(opcode) && opcode != LOAD_SMALL_INT) {


hasconst is defined in the dis docs as "Sequence of bytecodes that access a constant". I think it will make more sense if we clarify that it means "access a constant from co_consts" and then remove LOAD_SMALL_INT from hasconst and OPCODE_HAS_CONST.

That makes sense. LOAD_COMMON_CONST is not in hasconst.

Maybe it's worth testing whether creating integers within the range(256) will not affect co_consts?

I don't want to go overboard on testing internal details of the bytecode compiler.
LOAD_CONST with a small int is still correct, just a bit less efficient than LOAD_SMALL_INT.
We already have plenty of tests in test_dis that test this

…om code.co_consts

Lib/test/test_ast/test_ast.py

markshannon and others added 16 commits October 22, 2024 15:57

Replace RETURN_VLAUE/RETURN_CONST with RETURN_VALUE_FUNC/RETURN_VALUE…

2a61cfc

…_GEN

Add LOAD_CONST_IMMORTAL

42a526c

Fix up all the tests apart from test_dis

2a3b1e2

Update optimizer

031f320

Fix up test_dis

cb5252b

Fix up a couple of comments

892edbc

Apply suggestions from code review

5ce99ca

Co-authored-by: Tomas R. <[email protected]>

Clarify comment

325c5fe

Remove unused function

6149153

Update test

59a554a

Update dis docs

bcb4e65

Remove link to remove instruction

f3383e5

Restore RETURN_VALUE

68e97ba

Add LOAD_INT instruction

0b8a5ad

Fix up dis and some tests

1bfdd70

Fix up test dis

f934559

bedevere-app bot mentioned this pull request Oct 25, 2024

Refactor RETURN_VALUE and RETURN_CONST #125837

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markshannon added 2 commits October 25, 2024 15:15

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markshannon marked this pull request as ready for review October 25, 2024 14:39

markshannon requested review from Eclips4, ericsnowcurrently, brettcannon, ncoghlan and warsaw as code owners October 25, 2024 14:39

markshannon added 2 commits October 25, 2024 15:48

Remove the definitely last remnant of earlier PR

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implement review suggestion

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markshannon added 4 commits October 28, 2024 10:47

Remove extra argument

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Rename LOAD_INT to LOAD_SMALL_INT

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Regenerate test file

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Fix test_dis

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markshannon mentioned this pull request Oct 28, 2024

Add instructions for creating small objects. faster-cpython/ideas#609

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Don't mark LOAD_SMALL_INT as having a constant, as it doesn't load fr…

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…om code.co_consts

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markshannon mentioned this pull request Oct 29, 2024

gh-126072: do not add None to co_consts if no docstring #126101

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GH-125837: Split `LOAD_CONST` into three. #125972

GH-125837: Split `LOAD_CONST` into three. #125972

markshannon commented Oct 25, 2024 •

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iritkatriel Oct 28, 2024

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markshannon Oct 29, 2024

GH-125837: Split LOAD_CONST into three. #125972

GH-125837: Split LOAD_CONST into three. #125972

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GH-125837: Split `LOAD_CONST` into three. #125972

GH-125837: Split `LOAD_CONST` into three. #125972

markshannon commented Oct 25, 2024 •

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