Date: Sat, 25 Apr 2015 11:09:59 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: yescrypt AVX2

Hi,

So I thought that AVX2 could be of significant help even when pwxform is
tuned for 128-bit gather lanes, like we want it to be for better
bcrypt-like anti-GPU on the (currently widespread) machines that can
only do 128-bit SIMD and not wider yet.

It appears that I was mostly wrong, speaking of AVX2 as implemented in
Haswell.  My expectation for AVX2 working reasonably well even when
"misused" like that was based on my previous experiments (unreleased)
with 64-bit bcrypt-like S-box lookups in 128-bit SIMD code, on SSE*/AVX
where we have _mm_loadl_epi64() and _mm_loadh_pi() to efficiently load
64-bit halves of a 128-bit register.  On SSE4.1, there are also
instructions to extract 32- or 64-bit words from anywhere in a 128-bit
register.  I thought it'd be similar for 128-bit loads to 256-bit
registers.  However, it turns out there's a 3 cycle latency to access
the upper 128-bit halves of 256-bit registers on Haswell, such as with
_mm256_inserti128_si256() and _mm256_extracti128_si256() or
_mm256_permute4x64_epi64(), and the word-extract instructions do not
directly operate on the upper 128 bits.  So it's 3 cycles to extract
would-be S-box indices from there, and it's another 3 cycles to load the
S-box lookup result in there.  Oops.

Now, there may be hacks around some of this, such as by doing a 256-bit
load from 16 bytes below the desired offset (thereby loading the lower
128 bits with garbage and the upper 128 bits with what's needed) and
then loading the lower 128 bits with what's needed there.  I've tried
this, and it works, but at least without a change to data layout to make
those 256-bit loads always (rather than half of the time) 256-bit
aligned the performance is no better than with
_mm256_inserti128_si256().  Also, when implementing at intrinsics level,
I resorted to using a combination of _mm256_loadu_si256() and
_mm256_blend_epi32(), since I guess _mm256_castsi128_si256() is not
meant to be used on the left-hand side.  When implementing in assembly,
I think it can be two loads (no need for a blend instruction), but I
doubt it'd be any faster (same instruction count, and blend is fast).
And I doubt the alignment would make all the difference, as I think
Haswell has fast unaligned loads from cache (we are occasionally
crossing a cache line, though).

Samuel - any thoughts on accessing the upper 128 bits faster?

So far, the best I achieved when going from AVX to AVX2 yet insisting on
128-bit S-box lookups is:

AVX:
enchmarking 1 thread ...
930 c/s real, 930 c/s virtual (1023 hashes in 1.10 seconds)
Benchmarking 8 threads ...
3410 c/s real, 433 c/s virtual (7161 hashes in 2.10 seconds)

AVX2:
Benchmarking 1 thread ...
710 c/s real, 710 c/s virtual (1023 hashes in 1.44 seconds)
Benchmarking 8 threads ...
3580 c/s real, 458 c/s virtual (7161 hashes in 2.00 seconds)

at 2 MB per hash on i7-4770K, with otherwise currently default yescrypt
settings (PWXrounds=6, etc.)  As you can see, single thread performance
went down by 24%, and 8-thread went up only by 5%.

It's good news that attackers probably also can't use Haswell to attack
yescrypt hashes tuned for older CPUs (with 128-bit S-box lookups) much
faster.  It's bad news that defenders can't benefit from Haswell either.

It looks like I might introduce separate pwxform settings for AVX2,
using 256-bit S-box lookups.  Maybe I'll try 512-bit as well, and see if
it becomes much weaker than bcrypt on Haswell or not (maybe target L2
cache with the S-boxes, then).  (256-bit would definitely be fine in
terms of bcrypt-like anti-GPU on Haswell.)  If it's fine then 512-bit
would be preferable in order to avoid introducing yet another
combination of settings for MIC and AVX-512.  Note: all of this fits
into yescrypt and pwxform as currently specified.  We're only talking of
adjusting the PWX* compile-time settings (and possibly making them
runtime tunable), deviating from the current defaults, and producing
optimized code for those (the -ref will run fine with only #define's
changed).

OTOH, we may be satisfied with the current defaults as being a good fit
for up to x86/AVX and ARM/NEON, and appearing to defeat attacks with
newer CPUs so far.  With these settings, Haswell performs almost like
Sandy/Ivy Bridge do - no worse, but also no better.  Maybe that's fine.

I've attached a patch containing my experiments, including several AVX2
code versions of PWXFORM_SIMD.  All of those use 128-bit S-boxes so far.

Alexander

View attachment "yescrypt-0.7.1-avx2-hack1.diff" of type "text/plain" (19341 bytes)

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