Re: x86 asm SHA1 (draft)

The series to revamp SHA1 is good but judging the merit of each is outside my expertise, so I'd appreciate help to evaluate these changes. For example, I cannot choose between competing three implementations for ppc without having access to a variety of ppc machines, and even if I did, ppc is not what I normally use, so incentive to try picking the best one for everybody is relatively low on my part.

What we would want are obviously (1) no regression, (2) an easy way for people who build git for their own machine to decide which one suits them the best, and (3) an easy way to tell the build mechanism to use the one that is chosen.

The only external interface for the set of SHA1 implementation alternatives to the outside world is a well established SHA_CTX type, and three functions SHA1_Init(), SHA1_Update() and SHA1_Final(), and the alternative implementations are supposed to be drop-in replaceable.

So let's do something like this:

 - When adding a new SHA1 implementation, we need to come up
   with a Makefile symbol (similar to PPC_SHA1, MOZILLA_SHA1 and
   friends) and set up the build machinery to use the one that
   is chosen;

 - We need a test program and a build rule in the Makefile that
   links with the chosen SHA1 implementation.

 - We need a test script that feeds the above program with known
   vectors to validate the SHA1 implementation (make sure the
   test covers large input to avoid the recent half-gig-limit
   problem), and bench the speed on the platform it was built.

 - If we wanted to go fancier, another script that builds all
   applicable alternatives on the building platform, run the
   bench for all of them and automatically pick the right one
   for the platform would be a plus.

We probably would want to collect the benchmark results from popular platforms, have a summary to help people to choose a sensible one in the toplevel INSTALL file, and include the raw numbers in Documentation/technical/sha1-implementations.txt.

Once we go the above path, we may want to include both of the the two new ppc implementations as separate choices as I understand their performance depends on which ppc you are talking about.

Any takers?