When using a byte-encoding for matched group metadata we need to mask
down to a single bit in each matching byte to make the iteration of a
range of match indices work. In most cases, this mask can be folded into
the overall match computation, but for Arm Neon, there is avoidable
overhead from this. Instead, we can defer the mask until starting to
iterate. Doing more than one iteration is relative rare so this doesn't
accumulate much waste and makes common paths a bit faster.
For the M1 this makes the SIMD match path about 2-4% faster. This isn't
enough to catch the portable match code path on the M1 though.
For some Neoverse cores the difference here is more significant (>10%
improvement) and it makes the SIMD and scalar code paths have comparable
latency. Still not clear which is better as the latency is comparable
and beyond latency the factors are very hard to analyze -- port pressure
on different parts of the CPU, etc.
Leaving the selected code path as portable since that's so much better
on the M1, and I'm hoping to avoid different code paths for different
Arm CPUs for a while.
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Co-authored-by: Danila Kutenin <danilak@google.com>
Chandler CarruthandDanila Kutenin
3ae968a74c