cmd/8g: introduce temporaries in byte multiplication.

Also restore the smallintconst case for binary ops.

Fixes #3835.

R=daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/6999043

src/cmd/8g/cgen.c

@@ -395,7 +395,15 @@ sbop:	// symmetric binary
 	}
 
 abop:	// asymmetric binary
-	if(nl->ullman >= nr->ullman) {
+	if(smallintconst(nr)) {
+		mgen(nl, &n1, res);
+		regalloc(&n2, nl->type, &n1);
+		gmove(&n1, &n2);
+		gins(a, nr, &n2);
+		gmove(&n2, res);
+		regfree(&n2);
+		mfree(&n1);
+	} else if(nl->ullman >= nr->ullman) {
 		tempname(&nt, nl->type);
 		cgen(nl, &nt);
 		mgen(nr, &n2, N);

src/cmd/8g/ggen.c

@@ -748,7 +748,7 @@ cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 void
 cgen_bmul(int op, Node *nl, Node *nr, Node *res)
 {
-	Node n1, n2, *tmp;
+	Node n1, n2, nt, *tmp;
 	Type *t;
 	int a;
 
@@ -764,10 +764,12 @@ cgen_bmul(int op, Node *nl, Node *nr, Node *res)
 		nr = tmp;
 	}
 
+	tempname(&nt, nl->type);
+	cgen(nl, &nt);
 	regalloc(&n1, t, res);
-	cgen(nl, &n1);
+	cgen(nr, &n1);
 	regalloc(&n2, t, N);
-	cgen(nr, &n2);
+	gmove(&nt, &n2);
 	a = optoas(op, t);
 	gins(a, &n2, &n1);
 	regfree(&n2);

test/torture.go

@@ -333,3 +333,7 @@ func ChainDivConst(a int) int {
 		17 / 17 / 17 / 17 /
 		17 / 17 / 17 / 17
 }
+
+func ChainMulBytes(a, b, c byte) byte {
+	return a*(a*(a*(a*(a*(a*(a*(a*(a*b+c)+c)+c)+c)+c)+c)+c)+c) + c
+}