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runtime: zero upper bit of Y registers in asyncPreempt on darwin/amd64

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Apparently, the signal handling code path in darwin kernel leaves
the upper bits of Y registers in a dirty state, which causes many
SSE operations (128-bit and narrower) become much slower. Clear
the upper bits to get to a clean state.

We do it at the entry of asyncPreempt, which is immediately
following exiting from the kernel's signal handling code, if we
actually injected a call. It does not cover other exits where we
don't inject a call, e.g. failed preemption, profiling signal, or
other async signals. But it does cover an important use case of
async signals, preempting a tight numerical loop, which we
introduced in this cycle.

Running the benchmark in issue #37174:

name    old time/op  new time/op  delta
Fast-8  90.0ns ± 1%  46.8ns ± 3%  -47.97%  (p=0.000 n=10+10)
Slow-8   188ns ± 5%    49ns ± 1%  -73.82%  (p=0.000 n=10+9)

There is no more slowdown due to preemption signals.

For #37174.

Change-Id: I8b83d083fade1cabbda09b4bc25ccbadafaf7605
Reviewed-on: https://go-review.googlesource.com/c/go/+/219131
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Cherry Zhang 2020-02-11 18:54:30 -05:00
parent a0c9fb6bd3
commit 123f7dd3e1
2 changed files with 12 additions and 0 deletions
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9
src/runtime/mkpreempt.go
View File

@ -244,6 +244,15 @@ func genAMD64() {
// TODO: MXCSR register?
// Apparently, the signal handling code path in darwin kernel leaves
// the upper bits of Y registers in a dirty state, which causes
// many SSE operations (128-bit and narrower) become much slower.
// Clear the upper bits to get to a clean state. See issue #37174.
// It is safe here as Go code don't use the upper bits of Y registers.
p("#ifdef GOOS_darwin")
p("VZEROUPPER")
p("#endif")
p("PUSHQ BP")
p("MOVQ SP, BP")
p("// Save flags before clobbering them")

3
src/runtime/preempt_amd64.s
View File

@ -4,6 +4,9 @@
#include "textflag.h"
TEXT ·asyncPreempt(SB),NOSPLIT|NOFRAME,$0-0
#ifdef GOOS_darwin
VZEROUPPER
#endif
PUSHQ BP
MOVQ SP, BP
// Save flags before clobbering them