Date: Sat, 24 Oct 2015 13:57:04 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: _STRING_ARCH_unaligned or equivalent definition * P?draig Brady <P@...igBrady.com> [2015-10-24 10:21:31 +0100]: > I was writing some code which uses faster word at a time > but possibly unaligned access when we know that unaligned > access is OK, i.e. #if _STRING_ARCH_unaligned provided by glibc. > > I would like not to fall back to the slower path on musl. > Is there a similar define for musl. Is is worth proposing > a standard define somewhere? > you have to negotiate with the compiler about this, not with the libc. this violates pointer aliasing rules and only the compiler can give you guaranteed semantics (with language extensions like attribute mayalias). i don't think gcc has a predefined macro about unaligned access (it is non-trivial to provide it in a useful way because a platform may allow it, but at a significant performance cost). i think _STRING_ARCH_unaligned in glibc was not supposed to be public api, it is used internally to provide more efficient implementations (some functions are implemented in the string header hence it is publicly visible.. but don't assume such variables will remain there after a glibc update). when you do such low-level optimizations i think you have to manually maintain the list of targets the optimization is known to be valid for.
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