On 2022-03-21 at 17:06:12, Ævar Arnfjörð Bjarmason wrote:
> Remove the PPC_SHA1 implementation added in a6ef3518f9a ([PATCH] PPC
> assembly implementation of SHA1, 2005-04-22). When this was added
> Apple consumer hardware used the PPC architecture, and the
> implementation was intended to improve SHA-1 speed there.
> 
> Since it was added we've moved to DC_SHA1 by default, and anyone
> wanting hard-rolled non-DC SHA-1 implementation can use OpenSSL's via
> the OPENSSL_SHA1 knob.
> 
> I'm unsure if this was ever supposed to work on 64-bit PPC. It clearly
> originally targeted 32 bit PPC, but there's some mailing list
> references to this being tried on G5 (PPC 970). I can't get it to do
> anything but segfault on the BE POWER8 machine in the GCC compile
> farm. Anyone caring about speed on PPC these days is likely to be
> using IBM's POWER, not PPC 970.
> 
> There have been proposals to entirely remove non-DC_SHA1
> implementations from the tree[1]. I think per [2] that would be a bit
> overzealous. I.e. there are various set-ups git's speed is going to be
> more important than the relatively implausible SHA-1 collision attack,
> or where such attacks are entirely mitigated by other means (e.g. by
> incoming objects being checked with DC_SHA1).
> 
> The main reason for doing so at this point is to simplify follow-up
> Makefile change. Since PPC_SHA1 included the only in-tree *.S assembly
> file we needed to keep around special support for building objects
> from it. By getting rid of it we know we'll always build *.o from *.c
> files, which makes the build process simpler.
> 
> As an aside the code being removed here was also throwing warnings
> with the "-pedantic" flag, but let's remove it instead of fixing it,
> as 544d93bc3b4 (block-sha1: remove use of obsolete x86 assembly,
> 2022-03-10) did for block-sha1/*.

While I don't agree that we shouldn't remove the other non-DC SHA-1
implementations, I do agree that we should remove this one.  Given the
testing you've done and the fact that almost everyone desiring speed is
using a 64-bit machine these days, I think it's unlikely that anyone is
using this in the real world.
-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA