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

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>

src/runtime/mkpreempt.go

@@ -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")

src/runtime/preempt_amd64.s

@@ -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