Date: Mon, 19 Sep 2016 09:37:35 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: memchr() performance On Mon, Sep 19, 2016 at 03:29:53PM +0200, Georg Sauthoff wrote: > On Sun, Sep 18, 2016 at 10:40:36PM +0200, Szabolcs Nagy wrote: > > Hello, > > > * Georg Sauthoff <mail@...rg.so> [2016-09-18 20:54:22 +0200]: > > [..] > > > > On recent Intel CPUs it is even slower than a naive implementation: > > > > > > https://gms.tf/stdfind-and-memchr-optimizations.html#measurements > > > https://gms.tf/sparc-and-ppc-find-benchmark-results.html > > > > > > Of course, on x86, other implementations that use SIMD instructions > > > perform even better. > > > yes simd is expected to be faster. > > > but that needs asm which is expensive to maintain (there is no > > portable simd language extension for c and there is the aliasing > > issue: the reinterpret_cast in your code is formally ub). > > you mean because the vector-word pointer returned by reinterpret_cast is > used to access vector-words in the memory passed via a char pointer and > this is not covered by the ISO C++ strict aliasing rules? > > Yes. Sure, ub means that anything can happen, but this case should be ok > with GCC - if the function is compiled in isolation in its own > translation unit. > > I mean, there isn't much possibiltiy for reordering due to the > application of strict-aliasing-rules that would yield a different > result. > > There are no aliased write accesses. > > Btw, the current musl memchr() implementation has similar aliased > accesses - there, unsigned characters are aliased via a size_t pointer. Yes, that's a known bug with a pending patch - see: http://www.openwall.com/lists/musl/2016/04/27/8 Rich
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