Date: Tue, 21 Oct 2014 17:26:00 +0200 From: magnum <john.magnum@...hmail.com> To: john-dev@...ts.openwall.com Subject: Re: nVidia Maxwell support On 2014-09-24 00:00, magnum wrote: > Is it safe to assume that OpenCL's bitselect() function will boil down > to the new instructions on Maxwell? Or is it not that simple? > > For example, we have many cases like this: > > #ifdef USE_BITSELECT > #define F(x, y, z) bitselect((z), (y), (x)) > #define G(x, y, z) bitselect((y), (x), (z)) > #else > #define F(x, y, z) ((z) ^ ((x) & ((y) ^ (z)))) > #define G(x, y, z) ((y) ^ ((z) & ((x) ^ (y)))) > #endif I had a chance to try out a GTX980. Like I suspected, it seems to unpredictably affect "optimizer transparency" so using bitselect for the above gains a few percent with some versions of SHA1 and lose a few with others. > The thing is, we currently only define USE_BITSELECT for AMD devices. > Would it be safer, in the nvidia case, to leave the non-bitselect > versions for the optimizer to consider? Or would it be safer to use > bitselect, or should it really not matter? It seems to still matter on AMD. > > If use of bitselect() increases the chance for better low-level code for > nvidia too, maybe we should always define USE_BITSELECT (I'd still keep > the #ifdefs for quick benchmarks with/without them, as well as for > reference). I only tested WPAPSK for now because it's very flexible, so I could try many combinations of alternate code just messing around with #ifdefs. I ended up using the same SHA1 we use for AMD (Milen's, not Lukas' code with a separate "SHA1SHORT" hard-coded for short input) and enabling bitselect, for a ~5% gain (exceeding 3 billion SHA1/s). Using bitselect gained some with Milen's code and ruined some with Lukas' code. Another curious observation was that compared to the default scalar mode, Lukas' code gained a little speed with --force-vector=2 (ie. using uint2) while Milen's code lost some. Any larger vector size would ruin performance, as expected (perhaps more than expected). magnum
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