| commit | 67c2dcbc592f3ee03aa89ddcb1f07715b047f4b2 | [log] [tgz] |
|---|---|---|
| author | Michael Anthony Knyszek <mknyszek@google.com> | Mon Jan 27 20:41:38 2020 +0000 |
| committer | Michael Knyszek <mknyszek@google.com> | Mon Mar 23 17:07:22 2020 +0000 |
| tree | 926964cc6b29712cadf20e85c3d352d39454c64f | |
| parent | 7af6a31b48c5cb4d8cc9ca346949d1143ece76ca [diff] |
runtime: use OnesCount64 to count allocated objects in a span This change modifies the implementation of (*mspan).countAlloc by using OnesCount64 (which on many systems is intrinsified). It does so by using an unsafe pointer cast, but in this case we don't care about endianness because we're just counting bits set. This change means we no longer need the popcnt table which was redundant in the runtime anyway. We can also simplify the logic here significantly by observing that mark bits allocations are always 8-byte aligned, so we don't need to handle any edge-cases due to the fact that OnesCount64 operates on 64 bits at a time: all irrelevant bits will be zero. Overall, this implementation is significantly faster than the old one on amd64, and should be similarly faster (or better!) on other systems which support the intrinsic. On systems which do not, it should be roughly the same performance because OnesCount64 is implemented using a table in the general case. Results on linux/amd64: name old time/op new time/op delta MSpanCountAlloc/bits=64-4 16.8ns ± 0% 12.7ns ± 0% -24.40% (p=0.000 n=5+4) MSpanCountAlloc/bits=128-4 23.5ns ± 0% 12.8ns ± 0% -45.70% (p=0.000 n=4+5) MSpanCountAlloc/bits=256-4 43.5ns ± 0% 12.8ns ± 0% -70.67% (p=0.000 n=4+5) MSpanCountAlloc/bits=512-4 59.5ns ± 0% 15.4ns ± 0% -74.12% (p=0.008 n=5+5) MSpanCountAlloc/bits=1024-4 116ns ± 1% 23ns ± 0% -79.84% (p=0.000 n=5+4) Change-Id: Id4c994be22224653af5333683a69b0937130ed04 Reviewed-on: https://go-review.googlesource.com/c/go/+/216558 Run-TryBot: Michael Knyszek <mknyszek@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Keith Randall <khr@golang.org>
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