Enable `-fstrict-overflow`

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#96,821 14 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
25/100
Tipo de issue
Nueva funcionalidad
Claridad
Bastante claro
Estado de actividad
Estancado
Stack tecnológico
c
Área
build-system, compilers, performance

Línea de trabajo

Comienza con la maquinaria de configuración del sistema de build que añade CFLAGS específicos de cada módulo y, a continuación, revisa el trabajo enlazado en gh-96823 y gh-139595. El cambio propuesto es un build strict-overflow condicionado por configure para los módulos elegibles, tratando de forma segura los módulos restantes que dependen de overflow y comprobando el rendimiento con benchmarks.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

build performance type-feature

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:

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:

Linked PRs
  • gh-96823
  • gh-139595
Lenguaje dominante
Python
Estrellas
77.2k
Forks
36k
Merge medio
1 d 9 h
PR fusionados (30 d)
558

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