cmd/compile: fix magic multiply smashing AX

Fixes #12411.

Change-Id: I2202a754c7750e3b2119e3744362c98ca0d2433e
Reviewed-on: https://go-review.googlesource.com/17818Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/amd64/ggen.go

@@ -365,24 +365,25 @@ func cgen_hmul(nl *gc.Node, nr *gc.Node, res *gc.Node) {
 	gc.Cgenr(nl, &n1, res)
 	var n2 gc.Node
 	gc.Cgenr(nr, &n2, nil)
-	var ax gc.Node
-	gc.Nodreg(&ax, t, x86.REG_AX)
+	var ax, oldax, dx, olddx gc.Node
+	savex(x86.REG_AX, &ax, &oldax, res, gc.Types[gc.TUINT64])
+	savex(x86.REG_DX, &dx, &olddx, res, gc.Types[gc.TUINT64])
 	gmove(&n1, &ax)
 	gins(a, &n2, nil)
 	gc.Regfree(&n2)
 	gc.Regfree(&n1)
 
-	var dx gc.Node
 	if t.Width == 1 {
 		// byte multiply behaves differently.
-		gc.Nodreg(&ax, t, x86.REG_AH)
-
-		gc.Nodreg(&dx, t, x86.REG_DX)
-		gmove(&ax, &dx)
+		var byteAH, byteDX gc.Node
+		gc.Nodreg(&byteAH, t, x86.REG_AH)
+		gc.Nodreg(&byteDX, t, x86.REG_DX)
+		gmove(&byteAH, &byteDX)
 	}
-
-	gc.Nodreg(&dx, t, x86.REG_DX)
 	gmove(&dx, res)
+
+	restx(&ax, &oldax)
+	restx(&dx, &olddx)
 }
 
 /*

src/cmd/compile/internal/x86/ggen.go

@@ -531,24 +531,21 @@ func cgen_bmul(op gc.Op, nl *gc.Node, nr *gc.Node, res *gc.Node) bool {
 func cgen_hmul(nl *gc.Node, nr *gc.Node, res *gc.Node) {
 	var n1 gc.Node
 	var n2 gc.Node
-	var ax gc.Node
-	var dx gc.Node
 
 	t := nl.Type
 	a := optoas(gc.OHMUL, t)
 
 	// gen nl in n1.
 	gc.Tempname(&n1, t)
-
 	gc.Cgen(nl, &n1)
 
 	// gen nr in n2.
 	gc.Regalloc(&n2, t, res)
-
 	gc.Cgen(nr, &n2)
 
-	// multiply.
-	gc.Nodreg(&ax, t, x86.REG_AX)
+	var ax, oldax, dx, olddx gc.Node
+	savex(x86.REG_AX, &ax, &oldax, res, gc.Types[gc.TUINT32])
+	savex(x86.REG_DX, &dx, &olddx, res, gc.Types[gc.TUINT32])
 
 	gmove(&n2, &ax)
 	gins(a, &n1, nil)
@@ -556,14 +553,16 @@ func cgen_hmul(nl *gc.Node, nr *gc.Node, res *gc.Node) {
 
 	if t.Width == 1 {
 		// byte multiply behaves differently.
-		gc.Nodreg(&ax, t, x86.REG_AH)
-
-		gc.Nodreg(&dx, t, x86.REG_DX)
-		gmove(&ax, &dx)
+		var byteAH, byteDX gc.Node
+		gc.Nodreg(&byteAH, t, x86.REG_AH)
+		gc.Nodreg(&byteDX, t, x86.REG_DX)
+		gmove(&byteAH, &byteDX)
 	}
 
-	gc.Nodreg(&dx, t, x86.REG_DX)
 	gmove(&dx, res)
+
+	restx(&ax, &oldax)
+	restx(&dx, &olddx)
 }
 
 /*

test/fixedbugs/issue12411.go

+// +build !386
+// run
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 12411. Loss of AX during %.
+
+package main
+
+func main() {
+	x := f(4)
+	if x != 0 {
+		println("BUG: x=", x)
+	}
+}
+
+//go:noinline
+func f(x int) int {
+	// AX was live on entry to one of the % code generations,
+	// and the % code generation smashed it.
+	return ((2 * x) % 3) % (2 % ((x << 2) ^ (x % 3)))
+}