Optimize int add/sub for wide exact ints

CPython has a fast path for compact integers in binary add/sub, but wide exact ints still go through the generic long arithmetic path even when both operands fit in int64_t.

This issue proposes adding a separate fast path for exact PyLong operands that fit in signed 64-bit integers, while preserving the existing compact-int path.

Suggested implementation:

• Keep the current compact-int specialization unchanged.
• Add a separate wide-int path for exact ints that fit in int64_t.
• Preserve current behavior for overflow, subclasses, and other non-exact-int cases.

Motivation:

• Improve performance for wide integer add/sub without affecting the common compact-int hot path.
• Avoid adding new opcodes in the compact-int path.
• Fit within the current interpreter and specialization structure.

Benchmark evidence:

• I prototyped this locally with a benchmark covering compact and wide add/sub cases.
• Wide cases improved substantially, while compact cases remained effectively flat.
• Representative interpreter-only results with JIT disabled:
  • add_wide: about 25% faster
  • sub_wide: about 35% faster
  • add_compact/sub_compact: effectively unchanged

Benchmark script used locally:

"""Microbenchmark compact vs wide int add/sub with pyperf.

Use this with PYTHON_JIT=0 and -S if you want a stable interpreter-only run:

    PYTHON_JIT=0 ./python.exe -S Tools/scripts/bench_wide_int_pyperf.py
"""

from __future__ import annotations

import pyperf


def bench_add_compact() -> int:
    a = 1
    b = 2
    return a + b


def bench_add_wide() -> int:
    a = 10_000_000_000
    b = 1
    return a + b


def bench_sub_compact() -> int:
    a = 1
    b = 2
    return a - b


def bench_sub_wide() -> int:
    a = 10_000_000_000
    b = 1
    return a - b


def main() -> None:
    runner = pyperf.Runner()
    runner.bench_func("add_compact", bench_add_compact)
    runner.bench_func("add_wide", bench_add_wide)
    runner.bench_func("sub_compact", bench_sub_compact)
    runner.bench_func("sub_wide", bench_sub_wide)


if __name__ == "__main__":
    main()

Linked PRs

• gh-151290

Bắt đầu bằng cách xem xét PR được liên kết gh-151290, sau đó kiểm tra cấu trúc chuyên biệt hóa của trình thông dịch được mô tả trong issue. Chạy Tools/scripts/bench_wide_int_pyperf.py với PYTHON_JIT=0 và -S để so sánh các trường hợp add/sub compact và wide. Được xem là hoàn tất khi các toán hạng wide tương thích với int64 sử dụng fast path, trong khi hành vi của compact, overflow, subclass và non-exact-int vẫn không thay đổi.