6g,8g: make constant propagation inlining-friendly.

This changes makes constant propagation compare 'from' values using node
pointers rather than symbol names when checking to see whether a set
operation is redundant. When a function is inlined multiple times in a
calling function its arguments will share symbol names even though the values
are different. Prior to this fix the bug409 test would hit a case with 6g
where an LEAQ instruction was incorrectly eliminated from the second inlined
function call. 8g appears to have had the same bug, but the test did not fail
there.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5646044

src/cmd/6g/peep.c

@@ -987,7 +987,7 @@ loop:
 	case 3:	// set
 		if(p->as == p0->as)
 		if(p->from.type == p0->from.type)
-		if(p->from.sym == p0->from.sym)
+		if(p->from.node == p0->from.node)
 		if(p->from.offset == p0->from.offset)
 		if(p->from.scale == p0->from.scale)
 		if(p->from.dval == p0->from.dval)

src/cmd/8g/peep.c

@@ -878,7 +878,7 @@ loop:
 	case 3:	// set
 		if(p->as == p0->as)
 		if(p->from.type == p0->from.type)
-		if(p->from.sym == p0->from.sym)
+		if(p->from.node == p0->from.node)
 		if(p->from.offset == p0->from.offset)
 		if(p->from.scale == p0->from.scale)
 		if(p->from.dval == p0->from.dval)

test/fixedbugs/bug409.go

+// $G $D/$F.go && $L $F.$A && ./$A.out 2>&1 | cmp - $D/$F.out
+
+// Copyright 2012 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.
+
+// Multiple inlined calls to a function that causes
+// redundant address loads.
+
+package main
+
+func F(v [2]float64) [2]float64 {
+	return [2]float64{v[0], v[1]}
+}
+
+func main() {
+	a := F([2]float64{1, 2})
+	b := F([2]float64{3, 4})
+	println(a[0], a[1], b[0], b[1])
+}

test/fixedbugs/bug409.out

++1.000000e+000 +2.000000e+000 +3.000000e+000 +4.000000e+000