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Message-ID: <01d80a530e367a0a4612b90960c52a69@smtp.hushmail.com>
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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