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Date: Tue, 02 Jun 2015 17:30:41 +0200
From: magnum <john.magnum@...hmail.com>
To: john-dev@...ts.openwall.com
Subject: Re: Interleaving of intrinsics
On 2015-06-02 16:57, magnum wrote:
> On 2015-06-02 13:01, Solar Designer wrote:
>> magnum -
>>
>> On Mon, Jun 01, 2015 at 01:37:14PM +0200, magnum wrote:
>>> Or perhaps as soon as we use interleaving, things like tmp[SIMD_PARA]
>>> end up being stack arrays? That should hurt a lot.
>>
>> This is quite possible. In general, one of the things limiting the
>> interleaving factor is register pressure - and the compiler might in
>> fact do a worse job at register allocation when we use arrays.
>>
>>> Actually, here's a bug we have: Using the wide loops as in SHA2, we
>>> don't need to use "tmp[i]" at all - we do fine with just "tmp".
>>
>> Huh? Doesn't this defeat interleaving, replacing it with sequential
>> processing, because our source code sort of hints to the compiler to
>> reuse the same register across instances? Or are we hoping that the
>> compiler or the CPU will recognize that we're reusing the variable, and
>> actually allocate a new register or a new rename register, respectively?
>> The compiler might and a CPU capable of register renaming at all
>> probably will, but didn't we intend to reduce rather then increase our
>> reliance on luck?
>>
>> I just took a look at commit cde0fb470f35ef6dc5949d3b11137dd27ca2672b,
>> and it does look as problematic as I had thought from reading your
>> message. :-(
>
> I see what you mean and maybe we never got proper interleaving anyway.
> But MD4 and MD5 are faster at the same x3 as before. Anyway reverting to
> use tmp arrays again is easy.
I reverted just the use of tmp arrays in d85f8fd and the speedups I had
are still there. I guess the actual cause of it was just the fewer/wider
loops that are still in there now.
magnum
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