Enable `-fstrict-overflow`

從為模組加入特定 CFLAGS 的建置系統設定機制開始，然後檢視 gh-96823 和 gh-139595 中連結的工作。提議的變更是針對符合條件的模組、由 configure 控制的 strict-overflow 建置，同時安全地處理其餘依賴 overflow 的模組，並透過基準測試檢查效能。

At the moment we compile releases with -fwrapv which makes the code a bit safer, but disables certain optimizations. From the GCC docs:

This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others.

My experiments with running sanitisers seem to suggest that we are nearly already ready for -fno-wrapv (or -fstrict-overflow in general). Doing so could lead to quite a few speedups, but we would need to be more careful with the code we write.

It might be worthwhile to get a few benchmarks.

(To be extra precise, we give -fwrapv for clang and gcc for any build that doesn't get --with-pydebug.)

Pitch

My plan right now is to adapt the build system so that only the modules that need it are build with -fwrapv, and the rest can be build with -fstrict-overflow.

We already have config machinery that can add specific CFLAGS for specific modules only.

Perhaps the whole thing can be gated behind a configure flag, like --with-strict-overflow.

If everything goes well, and this improves performance we can consider adding this functionality to one of the standard optimization options.

We can also work on making more modules -fstrict-overflow safe.

Previous discussion

@markshannon @ericsnowcurrently

Brought up on https://github.com/faster-cpython/ideas/issues/458 and inspired by https://github.com/python/cpython/issues/96678

Some previous issues around -fwrapv:

• https://bugs.python.org/issue11149
• https://bugs.python.org/issue1621
• https://bugs.python.org/issue1608

I'm sure there are more.

Progress so far

As far as is currently known, the three remaining modules that rely on defined integer overflow are fixed by:

• _struct: https://github.com/python/cpython/pull/96739
• audioop: https://github.com/python/cpython/pull/96923
• _ctypes: https://github.com/python/cpython/pull/96925

Linked PRs

• gh-96823
• gh-139595