unicode/utf8: optimize Valid to parity with ValidString
The benchmarks added in this change revealed that ValidString
runs ~17% faster than Valid([]byte) on the ASCII prefix
of the input. Inspection of the assembly revealed that the
code generated for p[8:] required recomputing the slice capacity
to handle the cap=0 special case, which added an ADD -8 instruction.
By making len=cap, the capacity becomes a common subexpression
with the length, saving the ADD instruction.
(Thanks to khr for the tip.)
Incidentally, I tried a number of other optimizations but was
unable to make consistent gains across all benchmarks. The most
promising was to retain the bitmask of non-ASCII bytes from the
fast loop; the slow loop would shift it, and when it becomes zero,
return to the fast loop. This made the MostlyASCII benchmark 4x
faster, but made the other cases slower by up to 10%.
cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
benchmark old ns/op new ns/op delta
BenchmarkValidTenASCIIChars-16 4.09 4.06 -0.85%
BenchmarkValid100KASCIIChars-16 9325 7747 -16.92%
BenchmarkValidTenJapaneseChars-16 27.0 27.2 +0.85%
BenchmarkValidLongMostlyASCII-16 57277 58361 +1.89%
BenchmarkValidLongJapanese-16 94002 93131 -0.93%
BenchmarkValidStringTenASCIIChars-16 4.15 4.07 -1.74%
BenchmarkValidString100KASCIIChars-16 7980 8019 +0.49%
BenchmarkValidStringTenJapaneseChars-16 26.0 25.9 -0.38%
BenchmarkValidStringLongMostlyASCII-16 58550 58006 -0.93%
BenchmarkValidStringLongJapanese-16 97964 100038 +2.12%
Change-Id: Ic9d585dedd9af83c27dd791ecd805150ac949f15
Reviewed-on: https://go-review.googlesource.com/c/go/+/375594
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Alex Rakoczy <alex@golang.org>