Re: x86 SHA1: Faster than OpenSSL

[Dmitry Potapov <dpotapov@gmail.com>]

On Tue, Aug 04, 2009 at 08:56:48AM -0400, Jon Smirl wrote:
> 
> A simple start would be to feed each byte as it is decompressed
> directly into the sha code and avoid the intermediate buffer. Removing
> the buffer reduces cache pressure.

First, you still have to preserve any decoded byte in the compress
window, which is 32Kb by default. Typical files in Git repositories are
not so big, many are under 32Kb and practically all of them fit to L2
cache of modern processors. Second, complication of assembler code from
the coupling of two algorithms will be enormous. It is not sufficient
registers on x86 for SHA-1 alone. Third, SHA-1 is very computationally
intensive and with predictable access pattern (linear), so you do not
wait for L2, because it will be in L1. So, I don't see where you can
gain significantly. Perhaps, you can win just from re-writing inflate in
assembler, but I do not expect any significant gains other than that.
And coupling has obvious disadvantages when it comes to maintenance...


Dmitry