Date: Mon, 19 Sep 2016 15:29:53 +0200 From: Georg Sauthoff <mail@...rg.so> To: musl@...ts.openwall.com Subject: Re: memchr() performance 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. Best regards Georg
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