Date: Sun, 6 Sep 2015 15:15:11 +0200 From: magnum <john.magnum@...hmail.com> To: john-dev@...ts.openwall.com Subject: Re: Large stack alignment On 2015-09-06 13:20, magnum wrote: > On 2015-09-05 20:48, magnum wrote: >> On 2015-09-05 04:47, Solar Designer wrote: >>> Guess not confirmed. We use buffers on the stack, and they are properly >>> aligned for 128-bit SIMD. This is unreliable for AVX2 and above, >>> though. >> >> Are you sure it's unreliable? As far as I can google it, gcc had >> problems with large alignments but it was fixed long before 4.7, which >> is the minimum version you need to even build for AVX2. >> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16660 >> http://stackoverflow.com/questions/841433/gcc-attribute-alignedx-explanation >> > > So Lei reminded me of this: > http://www.openwall.com/lists/john-dev/2015/08/24/8 > > We have an issue for changing all stack allocs using MEM_ALIGN_SIMD to > align ourselves. This is fixed in 1bd8d9d. In order to mitigate performance regression I inlined a copy of mem_align() in simd-intrinsics.c. Lei, I also changed definitions of vloadu_emu and vstoreu_emu in pseudo_intrinsics.h (also with an inlined function). This might mean a significant performance drop. Maybe we should have autoconf figure out whether large alignments work, and define some macro. magnum
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