GH-125837: Split LOAD_CONST into three.

[markshannon]

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

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

[picnixz]

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]

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]

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%.

[iritkatriel]

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.

[markshannon]

That makes sense. LOAD_COMMON_CONST is not in hasconst.

[Eclips4]

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

[markshannon]

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