Date: Sun, 6 Sep 2015 20:16:26 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: Large stack alignment On Sun, Sep 06, 2015 at 12:02:07PM -0500, JimF wrote: > I fixed things here. I guess you meant to reply to magnum's message about the performance impact, rather than to my message about this being either redundant or insufficient. > We still must use mem_align(), but now it is a > define macro, using no writes to memory variables to do this 'magic'. > Just simply pointer readjustment > > Here is the macro: > > #define mem_align(a,b) > (void*)(((char*)(a))+(((b)-1)-(((size_t)((char*)(a))-1)&((b)-1)))) > > Now to align, just build a buffer that contains align_wanted extra bytes: > > unsigned long _buf[whatever_size + (SIMD_ALIGN)/sizeof(unsigned long)]; > > then use the macro; > unsigned long *buf = mem_alloc(_buf, SIMD_ALIGN); > > buf will be properly aligned somewhere in the range of _buf to > &((char*)_buf) +SIMD_COEF That's great, but I think there's still performance impact potential: all those individual pointers may need their own registers, whereas without such per-pointer alignment gcc would simply address everything relative to a single frame pointer register. Alexander
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