gc: fix order of operations for f() < g().

Also, 6g was passing uninitialized
Node &n2 to regalloc, causing non-deterministic
register collisions (but only when both left and
right hand side of comparison had function calls).

Fixes #1728.

R=ken2
CC=golang-dev
https://golang.org/cl/4425070

src/cmd/5g/cgen.c

@@ -962,7 +962,7 @@ bgen(Node *n, int true, Prog *to)
 		}
 
 		// make simplest on right
-		if(nl->op == OLITERAL || nl->ullman < nr->ullman) {
+		if(nl->op == OLITERAL || (nl->ullman < UINF && nl->ullman < nr->ullman)) {
 			a = brrev(a);
 			r = nl;
 			nl = nr;
@@ -1073,18 +1073,18 @@ bgen(Node *n, int true, Prog *to)
 		a = optoas(a, nr->type);
 
 		if(nr->ullman >= UINF) {
-			regalloc(&n1, nr->type, N);
-			cgen(nr, &n1);
+			regalloc(&n1, nl->type, N);
+			cgen(nl, &n1);
 
-			tempname(&tmp, nr->type);
+			tempname(&tmp, nl->type);
 			gmove(&n1, &tmp);
 			regfree(&n1);
 
-			regalloc(&n1, nl->type, N);
-			cgen(nl, &n1);
-
 			regalloc(&n2, nr->type, N);
-			cgen(&tmp, &n2);
+			cgen(nr, &n2);
+
+			regalloc(&n1, nl->type, N);
+			cgen(&tmp, &n1);
 
 			gcmp(optoas(OCMP, nr->type), &n1, &n2);
 			patch(gbranch(a, nr->type), to);

src/cmd/6g/cgen.c

@@ -829,7 +829,7 @@ bgen(Node *n, int true, Prog *to)
 		}
 
 		// make simplest on right
-		if(nl->op == OLITERAL || nl->ullman < nr->ullman) {
+		if(nl->op == OLITERAL || (nl->ullman < nr->ullman && nl->ullman < UINF)) {
 			a = brrev(a);
 			r = nl;
 			nl = nr;
@@ -879,18 +879,18 @@ bgen(Node *n, int true, Prog *to)
 		}
 
 		if(nr->ullman >= UINF) {
-			regalloc(&n1, nr->type, N);
-			cgen(nr, &n1);
+			regalloc(&n1, nl->type, N);
+			cgen(nl, &n1);
 
-			tempname(&tmp, nr->type);
+			tempname(&tmp, nl->type);
 			gmove(&n1, &tmp);
 			regfree(&n1);
 
-			regalloc(&n1, nl->type, N);
-			cgen(nl, &n1);
+			regalloc(&n2, nr->type, N);
+			cgen(nr, &n2);
 
-			regalloc(&n2, nr->type, &n2);
-			cgen(&tmp, &n2);
+			regalloc(&n1, nl->type, N);
+			cgen(&tmp, &n1);
 
 			goto cmp;
 		}

src/cmd/8g/cgen.c

@@ -900,7 +900,7 @@ bgen(Node *n, int true, Prog *to)
 		}
 
 		// make simplest on right
-		if(nl->op == OLITERAL || nl->ullman < nr->ullman) {
+		if(nl->op == OLITERAL || (nl->ullman < nr->ullman && nl->ullman < UINF)) {
 			a = brrev(a);
 			r = nl;
 			nl = nr;
@@ -1025,8 +1025,8 @@ bgen(Node *n, int true, Prog *to)
 		if(nr->ullman >= UINF) {
 			tempname(&n1, nl->type);
 			tempname(&tmp, nr->type);
-			cgen(nr, &tmp);
 			cgen(nl, &n1);
+			cgen(nr, &tmp);
 			regalloc(&n2, nr->type, N);
 			cgen(&tmp, &n2);
 			goto cmp;

test/func7.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+package main
+
+var calledf = false
+
+func f() int {
+	calledf = true
+	return 1
+}
+
+func g() int {
+	if !calledf {
+		println("BUG: func7 - called g before f")
+	}
+	return 0
+}
+
+func main() {
+	// 6g, 8g, 5g all used to evaluate g() before f().
+	if f() < g() {
+		panic("wrong answer")
+	}
+}
+