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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Enable `-fstrict-overflow`","articleBody":"At the moment we compile releases with `-fwrapv` which makes the code a bit safer, but disables certain optimizations.  From the GCC docs:\r\n\r\n\u003e 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.\r\n\r\nMy [experiments with running sanitisers](https://github.com/python/cpython/issues/96678#issuecomment-1243499193) 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.\r\n\r\nIt might be worthwhile to get a few benchmarks.\r\n\r\n(To be extra precise, we give `-fwrapv` for clang and gcc for any build that doesn't get `--with-pydebug`.)\r\n\r\n# Pitch\r\n\r\nMy 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`.\r\n\r\nWe already have config machinery that can add specific `CFLAGS` for specific modules only.\r\n\r\nPerhaps the whole thing can be gated behind a configure flag, like `--with-strict-overflow`.\r\n\r\nIf everything goes well, and this improves performance we can consider adding this functionality to one of the standard optimization options.\r\n\r\nWe can also work on making more modules `-fstrict-overflow` safe.\r\n\r\n# Previous discussion\r\n\r\n@markshannon @ericsnowcurrently \r\n\r\nBrought up on https://github.com/faster-cpython/ideas/issues/458 and inspired by https://github.com/python/cpython/issues/96678\r\n\r\nSome previous issues around `-fwrapv`:\r\n\r\n* https://bugs.python.org/issue11149\r\n* https://bugs.python.org/issue1621\r\n* https://bugs.python.org/issue1608\r\n\r\nI'm sure there are more.\r\n\r\n# Progress so far\r\n\r\nAs far as is currently known, the three remaining modules that rely on defined integer overflow are fixed by:\r\n\r\n* `_struct`: https://github.com/python/cpython/pull/96739\r\n* `audioop`: https://github.com/python/cpython/pull/96923\r\n* `_ctypes`: https://github.com/python/cpython/pull/96925\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-96823\n* gh-139595\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/matthiasgoergens","@type":"Person","name":"matthiasgoergens"},"datePublished":"2022-09-14T09:33:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":14},"url":"https://github.com/96821/cpython/issues/96821"}