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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Define `_Py_TYPEOF` on MSVC and other compilers supporting `__typeof__`","articleBody":"# Feature or enhancement\r\n\r\n### Proposal:\r\n\r\nRecently, MSVC [implemented `__typeof__`](https://learn.microsoft.com/en-us/cpp/c-language/typeof-c), which currently is defined as `_Py_TYPEOF` only on GCC and Clang.\r\nhttps://github.com/python/cpython/blob/65fededf9cc1780d5edbef8a6e0a7cf9bc15aea6/Include/pyport.h#L553-L560\r\n\r\nThe only users of `_Py_TYPEOF` are macros [`Py_CLEAR()`](https://github.com/python/cpython/blob/65fededf9cc1780d5edbef8a6e0a7cf9bc15aea6/Include/refcount.h#L422), [`Py_SETREF()` and `Py_XSETREF()`](https://github.com/python/cpython/blob/65fededf9cc1780d5edbef8a6e0a7cf9bc15aea6/Include/cpython/object.h#L326-L365).\r\nIf `_Py_TYPEOF` is not defined, these macros fall back to slightly uglier implementations with `memcpy` calls.\r\nhttps://github.com/python/cpython/blob/65fededf9cc1780d5edbef8a6e0a7cf9bc15aea6/Include/refcount.h#L421-L442\r\n\r\nThe proposal is to define  `_Py_TYPEOF` on:\r\n - latest MSVC versions that support [`__typeof__`](https://learn.microsoft.com/en-us/cpp/c-language/typeof-c)\r\n - arbitrary compilers that support `__typeof__`\r\n - arbitrary compilers that support C23 `typeof` or C++11 `decltype`\r\n\r\nBy defining `_Py_TYPEOF`, uses of `Py_CLEAR()`, `Py_SETREF()` and `Py_XSETREF()` are switched to the `_Py_TYPEOF` implementation; on these compilers, the benefits would be:\r\n - Less [worrying about miscompilation and machine code quality](https://github.com/python/cpython/issues/98724#issuecomment-1342256667).\r\n - Fewer surprises with hidden `memcpy` as seen in \r\n - - #100121 \r\n - - #121617\r\n - Reduced variance between optimization settings thanks to more straightforward implementation.\r\n\r\nAdditionally, this removes a differing detail between MSVC and GCC/Clang builds, which should be generally desirable.\r\n\r\n### Has this already been discussed elsewhere?\r\n\r\nNo response given\r\n\r\n### Links to previous discussion of this feature:\r\n\r\n- #98724\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-121648\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/fuhsnn","@type":"Person","name":"fuhsnn"},"datePublished":"2024-07-12T13:16:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/121647/cpython/issues/121647"}