Eli Zaretskii writes: >> From: Robert Pluim >> Cc: Po Lu >> Date: Thu, 30 Mar 2023 11:34:42 +0200 >> >> Fstring_lessp has: >> >> /* Check whether the platform allows access to unaligned addresses for >> size_t integers without trapping or undue penalty (a few cycles is OK). >> >> This whitelist is incomplete but since it is only used to improve >> performance, omitting cases is safe. */ >> #if defined __x86_64__|| defined __amd64__ \ >> || defined __i386__ || defined __i386 \ >> || defined __arm64__ || defined __aarch64__ \ >> || defined __powerpc__ || defined __powerpc \ >> || defined __ppc__ || defined __ppc \ >> || defined __s390__ || defined __s390x__ >> #define HAVE_FAST_UNALIGNED_ACCESS 1 >> #else >> #define HAVE_FAST_UNALIGNED_ACCESS 0 >> #endif >> >> but even if unaligned access is normally permitted by a machine, it is >> still undefined behavior to dereference an unaligned pointer. > > This is incorrect. There's nothing undefined about x86 unaligned > accesses. C standards can regard this as UB, but we are using > machine-specific knowledge here (and Emacs cannot be built with a > strict adherence to C standards anyway). Things can still go wrong on x86, particularly with SIMD: https://pzemtsov.github.io/2016/11/06/bug-story-alignment-on-x86.html. > >> Instead, HAVE_FAST_UNALIGNED_ACCESS and UNALIGNED_LOAD_SIZE should be >> removed and memcpy used instead: >> >> word_t a, c; >> >> memcpy (&a, w1 + b / ws, sizeof a); >> memcpy (&c, w2 + b / ws, sizeof c); >> >> doing so will make the compiler itself generate the right sequence of >> instructions for performing unaligned accesses, normally with only a few >> cycles penalty. > > We don't want that penalty here, that's all. > >> I would like to install such a change on emacs-29. > > No, please don't. > >> Emacs currently crashes when built with various compilers performing >> pointer alignment checks. > > Details, please. Which compilers, on what platforms, for what target > architectures, etc. Unconditionally removing the fast copy there is a > non-starter. I imagine they're referring to UBSAN. Emacs may want to add an annotation where HAVE_FAST_UNALIGNED_ACCESS is used. But for modern compilers, they will indeed DTRT anyway when they see memcpy. See https://github.com/WayneD/rsync/pull/428 for an example.