runtime: save R15 before checking AVX state

When in dynlink mode, reading a global can clobber R15.
Just to be safe, save R15 before checking the AVX state to see
if we need to VZEROUPPER or not.

This could cause a problem in buildmodes that aren't supported yet.

Change-Id: I8fda62d3fbe808584774fa5e8d9810a4612a84e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/288452
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/mkpreempt.go

@@ -230,12 +230,16 @@ func genAMD64() {
 		if reg == "SP" || reg == "BP" {
 			continue
 		}
-		if strings.HasPrefix(reg, "X") {
-			l.add("MOVUPS", reg, 16)
-		} else {
+		if !strings.HasPrefix(reg, "X") {
 			l.add("MOVQ", reg, 8)
 		}
 	}
+	lSSE := layout{stack: l.stack, sp: "SP"}
+	for _, reg := range regNamesAMD64 {
+		if strings.HasPrefix(reg, "X") {
+			lSSE.add("MOVUPS", reg, 16)
+		}
+	}
 
 	// TODO: MXCSR register?
 
@@ -244,10 +248,12 @@ func genAMD64() {
 	p("// Save flags before clobbering them")
 	p("PUSHFQ")
 	p("// obj doesn't understand ADD/SUB on SP, but does understand ADJSP")
-	p("ADJSP $%d", l.stack)
+	p("ADJSP $%d", lSSE.stack)
 	p("// But vet doesn't know ADJSP, so suppress vet stack checking")
 	p("NOP SP")
 
+	l.save()
+
 	// 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.
@@ -259,10 +265,11 @@ func genAMD64() {
 	p("VZEROUPPER")
 	p("#endif")
 
-	l.save()
+	lSSE.save()
 	p("CALL ·asyncPreempt2(SB)")
+	lSSE.restore()
 	l.restore()
-	p("ADJSP $%d", -l.stack)
+	p("ADJSP $%d", -lSSE.stack)
 	p("POPFQ")
 	p("POPQ BP")
 	p("RET")

src/runtime/preempt_amd64.s

@@ -13,11 +13,6 @@ TEXT ·asyncPreempt<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-0
 	ADJSP $368
 	// But vet doesn't know ADJSP, so suppress vet stack checking
 	NOP SP
-	#ifdef GOOS_darwin
-	CMPB internal∕cpu·X86+const_offsetX86HasAVX(SB), $0
-	JE 2(PC)
-	VZEROUPPER
-	#endif
 	MOVQ AX, 0(SP)
 	MOVQ CX, 8(SP)
 	MOVQ DX, 16(SP)
@@ -32,6 +27,11 @@ TEXT ·asyncPreempt<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-0
 	MOVQ R13, 88(SP)
 	MOVQ R14, 96(SP)
 	MOVQ R15, 104(SP)
+	#ifdef GOOS_darwin
+	CMPB internal∕cpu·X86+const_offsetX86HasAVX(SB), $0
+	JE 2(PC)
+	VZEROUPPER
+	#endif
 	MOVUPS X0, 112(SP)
 	MOVUPS X1, 128(SP)
 	MOVUPS X2, 144(SP)