tree 2c85b3b45d6b66a45071273a6e508d987f6365f6
parent e5c86b740520111558eaaca59ec1692d5fa9bb30
author David Benjamin <davidben@google.com> 1668552263 -0500
committer Boringssl LUCI CQ <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com> 1669159808 +0000

Update x86_64-mont5.pl and RSAZ comments a bit.

Back in https://boringssl-review.googlesource.com/c/boringssl/+/33268, I
wrote that I had no idea what the mont5 assembly was doing. In
preparation for fixing up some comments around
BN_mod_exp_mont_consttime, I wanted to understand whether we were still
making assumptions about cache lines.

Happily, for the mont5 code, the answer is no, we are not. We just make
a bunch of masks and apply them in the natural way. But we do require
16-byte alignment on the table, because we use movdqa to read out of it.

I didn't look as closely at RSAZ, but I believe it too is fine. It
fairly quickly tosses $power into an XMM register and builds up masks,
rather than incorporating it into address computations.

(Both scatter5 functions incorporate it into the address, but that's
part of table building, where the index is public. I've updated the
comments to note when the index is secret or public.)

There is one reference to cache lines in the comments of mont5.pl, in
computing $N. However, $N has been unused since
https://boringssl-review.googlesource.com/c/boringssl/+/7244. (There are
references to $N[0] and friends, but those refer to @N, which is a
completely unrelated variable.) Remove it.

Change-Id: I1fac0660dffcd1380572029de2e5baece60cddf6
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/55225
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>