tree a48a017dcf6c673122cb98026ed383135fcc0f75
parent fc23300164d6e36599c1ffe4bd50e878826fcdec
author David Benjamin <davidben@google.com> 1612905208 -0500
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1612913127 +0000

Skip runtime NEON checks if __ARM_NEON is defined.

When a feature is enabled statically in the build config, the compiler
defines __ARM_NEON and also considers itself free to emit NEON code.
In this case, there is no need to check for NEON support at runtime
since the binary will not work without NEON anyway. Moving the check to
compile time lets us drop unused code.

Chrome has required NEON on Android for nearly five years now. However,
historically there was a bad CPU which broke on some NEON code, but not
others. See https://crbug.com/341598 and https://crbug.com/606629. We
worked around that CPU by parsing /proc/cpuinfo and intentionally
dropping the optimization. This is not a stable situation, however, as
we're hoping the compiler is not good enough at emitting NEON to trigger
this bug.

Since then, the number of affected devices has dropped, and Chrome has
raised the minimum Android requirement to L. The Net.HasBrokenNEON
metric from Chrome is now well in the noise.

This CL stops short of removing the workaround altogether because some
consumers of Cronet are unsure whether they needed this workaround.
Those consumers also build without __ARM_NEON, so gating on that works
out. We'll decide what to do with it pending metrics from them.

Update-Note: Builds with __ARM_NEON (-mfpu=neon) will now drop about
30KiB of dead code, but no longer work (if they even did before) on a
particular buggy CPU. Builds without __ARM_NEON are not affected.

Change-Id: Id8f7bccfb75afe0a1594572ea20c51d275b0a256
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45484
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>