cmd/6g, cmd/8g: mark used registers in indirect addressing.

Fixes #4094.
Fixes #4353.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6810090

src/cmd/6g/reg.c

@@ -247,6 +247,16 @@ regopt(Prog *firstp)
 			}
 		}
 
+		// Addressing makes some registers used.
+		if(p->from.type >= D_INDIR)
+			r->use1.b[0] |= RtoB(p->from.type-D_INDIR);
+		if(p->from.index != D_NONE)
+			r->use1.b[0] |= RtoB(p->from.index);
+		if(p->to.type >= D_INDIR)
+			r->use2.b[0] |= RtoB(p->to.type-D_INDIR);
+		if(p->to.index != D_NONE)
+			r->use2.b[0] |= RtoB(p->to.index);
+
 		bit = mkvar(r, &p->from);
 		if(bany(&bit))
 		switch(p->as) {

src/cmd/8g/reg.c

@@ -212,6 +212,16 @@ regopt(Prog *firstp)
 			}
 		}
 
+		// Addressing makes some registers used.
+		if(p->from.type >= D_INDIR)
+			r->use1.b[0] |= RtoB(p->from.type-D_INDIR);
+		if(p->from.index != D_NONE)
+			r->use1.b[0] |= RtoB(p->from.index);
+		if(p->to.type >= D_INDIR)
+			r->use2.b[0] |= RtoB(p->to.type-D_INDIR);
+		if(p->to.index != D_NONE)
+			r->use2.b[0] |= RtoB(p->to.index);
+
 		bit = mkvar(r, &p->from);
 		if(bany(&bit))
 		switch(p->as) {

test/fixedbugs/issue4353.go

+// run
+
+// 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.
+
+// Issue 4353. An optimizer bug in 8g triggers a runtime fault
+// instead of an out of bounds panic.
+
+package main
+
+var aib [100000]int
+var paib *[100000]int = &aib
+var i64 int64 = 100023
+
+func main() {
+	defer func() { recover() }()
+	_ = paib[i64]
+}