tree e2e8fc69cd888477eb155c68279e2ad32c5b0d0d
parent 7cb8df579329b70cd4ede09d6d228636b8e31e89
author David Benjamin <davidben@google.com> 1702347807 -0500
committer Boringssl LUCI CQ <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com> 1706292901 +0000

sha: Move Armv7 dispatching to C (reland)

This is a reland of
https://boringssl-review.googlesource.com/c/boringssl/+/64749, which was
reverted in
https://boringssl-review.googlesource.com/c/boringssl/+/65328 due to
issues in Arm mode (i.e. not Thumb mode) builds that target Armv6+
instead of Armv7+.

The issue was that sha256_block_data_order_nohw has slightly different
sizes depending on __ARM_ARCH. Prior to moving the dispatch, the sizes
worked out such that they were always encodable in ADR. After moving the
dispatch, the instructions got shorter, such that the Armv7+ build still
worked, but the Armv6+ build needed to encode an offset of 0x1060
(previously 0x1080), which does not fit.

See https://alisdair.mcdiarmid.org/arm-immediate-value-encoding/ for
details on Arm's very fussy immediate value encoding. It's not the only
form used for ADR (Thumb2 works very differently), but it's the
applicable one here.

While we could shuffle things around, this is all far too fragile. Just
use the LDR; ADD pattern we used for the other function. ADRL would
avoid a load (it splits the offset into two constants without a constant
bank), but that's a pseudo-instruction that's only supported by gas.
clang-assembler didn't want to implement it. Android have a macro at
https://android.googlesource.com/platform/ndk/+/refs/heads/master/docs/ClangMigration.md#arm,
but it didn't work for me when I tried it. Also, searching around, it
sounds like ADRL in gas only works in Arm mode and not Thumb mode?

We could probably work through all that, but the compiler emits constant
banks on 32-bit Arm all the time. (I got this pattern from Clang's
output.) This is probably not worth the trouble.

Bug: 673
Change-Id: I165544764a931b293aa66fb3fc9bb8f01eeb8092
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/65808
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>