[dev.ssa] cmd/compile/internal/ssa: reuse Aux values for PEXTERN

This improves cse and works correctly now that divide by zero is checked
explicitly.

Change-Id: If54fbe403ed5230b897afc5def644ba9f0056dfd
Reviewed-on: https://go-review.googlesource.com/16454
Run-TryBot: Todd Neal <todd@tneal.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -2265,7 +2265,7 @@ func (s *state) addr(n *Node, bounded bool) *ssa.Value {
 		switch n.Class {
 		case PEXTERN:
 			// global variable
-			aux := &ssa.ExternSymbol{n.Type, n.Sym}
+			aux := s.lookupSymbol(n, &ssa.ExternSymbol{n.Type, n.Sym})
 			v := s.entryNewValue1A(ssa.OpAddr, t, aux, s.sb)
 			// TODO: Make OpAddr use AuxInt as well as Aux.
 			if n.Xoffset != 0 {

src/cmd/compile/internal/gc/testdata/loadstore_ssa.go

@@ -77,11 +77,39 @@ func testExtStore() {
 	}
 }
 
+var b int
+
+// testDeadStorePanic_ssa ensures that we don't optimize away stores
+// that could be read by after recover().  Modeled after fixedbugs/issue1304.
+func testDeadStorePanic_ssa(a int) (r int) {
+	switch {
+	}
+	defer func() {
+		recover()
+		r = a
+	}()
+	a = 2      // store
+	b := a - a // optimized to zero
+	c := 4
+	a = c / b // store, but panics
+	a = 3     // store
+	r = a
+	return
+}
+
+func testDeadStorePanic() {
+	if want, got := 2, testDeadStorePanic_ssa(1); want != got {
+		fmt.Println("testDeadStorePanic failed.  want =", want, ", got =", got)
+		failed = true
+	}
+}
+
 func main() {
 
 	testLoadStoreOrder()
 	testStoreSize()
 	testExtStore()
+	testDeadStorePanic()
 
 	if failed {
 		panic("failed")