Re: [PATCH 2/3] [CRYPTO] Add optimized SHA-1 implementation for i486+

From: Matt Mackall
Date: Sat Jun 09 2007 - 17:35:25 EST

On Sat, Jun 09, 2007 at 04:23:27PM -0400, Jeff Garzik wrote:
> Matt Mackall wrote:
> >On Fri, Jun 08, 2007 at 05:42:53PM -0400, Benjamin Gilbert wrote:
> >>Add x86-optimized implementation of the SHA-1 hash function, taken from
> >>Nettle under the LGPL. This code will be enabled on kernels compiled for
> >>486es or better; kernels which support 386es will use the generic
> >>implementation (since we need BSWAP).
> >>
> >>We disable building lib/sha1.o when an optimized implementation is
> >>available, as the library link order for x86 (and x86_64) would otherwise
> >>ignore the optimized version. The existing optimized implementation for
> >>ARM
> >>does not do this; the library link order for that architecture appears to
> >>favor the arch/arm/ version automatically. I've left this situation alone
> >>since I'm not familiar with the ARM code, but a !ARM condition could be
> >>added to CONFIG_SHA1_GENERIC if it makes sense.
> >>
> >>The code has been tested with tcrypt and the NIST test vectors.
> >
> >Have you benchmarked this against lib/sha1.c? Please post the results.
> >Until then, I'm frankly skeptical that your unrolled version is faster
> >because when I introduced lib/sha1.c the rolled version therein won by
> >a significant margin and had 1/10th the cache footprint.
> Yes. And it also depends on the CPU as well. Testing on a server-class
> x86 CPU (often with bigger L2, and perhaps even L1, cache) will produce
> different result than from popular but less-capable "value" CPUs.

In particular, any optimization made for "486+" CPUs is highly suspect
on any machine which runs the core at >1x the memory bus speeds.

Mathematics is the supreme nostalgia of our time.
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