cmd/6g, cmd/8g: fix, test byte-sized magic multiply

Credit to Rémy for finding and writing test case.

Fixes #8325.

LGTM=r
R=golang-codereviews, r
CC=dave, golang-codereviews, iant, remyoudompheng
https://golang.org/cl/124950043

src/cmd/6g/ggen.c

@@ -943,7 +943,7 @@ cgen_hmul(Node *nl, Node *nr, Node *res)
 	if(t->width == 1) {
 		// byte multiply behaves differently.
 		nodreg(&ax, t, D_AH);
-		nodreg(&dx, t, D_DL);
+		nodreg(&dx, t, D_DX);
 		gmove(&ax, &dx);
 	}
 	nodreg(&dx, t, D_DX);

src/cmd/6g/peep.c

@@ -838,6 +838,11 @@ copyu(Prog *p, Adr *v, Adr *s)
 static int
 copyas(Adr *a, Adr *v)
 {
+	if(D_AL <= a->type && a->type <= D_R15B)
+		fatal("use of byte register");
+	if(D_AL <= v->type && v->type <= D_R15B)
+		fatal("use of byte register");
+
 	if(a->type != v->type)
 		return 0;
 	if(regtyp(v))

src/cmd/8g/ggen.c

@@ -991,7 +991,7 @@ cgen_hmul(Node *nl, Node *nr, Node *res)
 	if(t->width == 1) {
 		// byte multiply behaves differently.
 		nodreg(&ax, t, D_AH);
-		nodreg(&dx, t, D_DL);
+		nodreg(&dx, t, D_DX);
 		gmove(&ax, &dx);
 	}
 	nodreg(&dx, t, D_DX);

src/cmd/8g/peep.c

@@ -636,6 +636,11 @@ copyu(Prog *p, Adr *v, Adr *s)
 static int
 copyas(Adr *a, Adr *v)
 {
+	if(D_AL <= a->type && a->type <= D_R15B)
+		fatal("use of byte register");
+	if(D_AL <= v->type && v->type <= D_R15B)
+		fatal("use of byte register");
+
 	if(a->type != v->type)
 		return 0;
 	if(regtyp(v))

test/fixedbugs/issue8325.go

@@ -0,0 +1,31 @@
+// run
+
+// Copyright 2014 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 8325: corrupted byte operations during optimization
+// pass.
+
+package main
+
+const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+func main() {
+	var bytes = []byte{10, 20, 30, 40, 50}
+
+	for i, b := range bytes {
+		bytes[i] = alphanum[b%byte(len(alphanum))]
+	}
+
+	for _, b := range bytes {
+		switch {
+		case '0' <= b && b <= '9',
+			'A' <= b && b <= 'Z':
+		default:
+			println("found a bad character", string(b))
+			panic("BUG")
+		}
+
+	}
+}