cmd/compile: in cse, allow for new ssa values

The table of rewrites in ssa/cse is not sized appropriately for
ssa IDs that are created during copying of selects into new blocks.

Fixes #17918

Change-Id: I65fe86c6aab5efa679aa473aadc4ee6ea882cd41
Reviewed-on: https://go-review.googlesource.com/33240
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -188,7 +188,10 @@ func cse(f *Func) {
 	for _, b := range f.Blocks {
 	out:
 		for _, v := range b.Values {
-			if rewrite[v.ID] != nil {
+			// New values are created when selectors are copied to
+			// a new block. We can safely ignore those new values,
+			// since they have already been copied (issue 17918).
+			if int(v.ID) >= len(rewrite) || rewrite[v.ID] != nil {
 				continue
 			}
 			if v.Op != OpSelect0 && v.Op != OpSelect1 {

test/fixedbugs/issue17918.go

@@ -0,0 +1,41 @@
+// compile
+
+// Copyright 2016 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 17918: slice out-of-bounds in ssa/cse
+
+package dead
+
+import (
+	"fmt"
+	"time"
+)
+
+var (
+	units = []struct {
+		divisor time.Duration
+		unit    rune
+	}{
+		{1000000, 's'},
+		{60, 'm'},
+		{60, 'h'},
+		{24, 'd'},
+		{7, 'w'},
+	}
+)
+
+func foobar(d time.Duration) string {
+	d /= time.Microsecond
+	unit := 'u'
+
+	for _, f := range units {
+		if d%f.divisor != 0 {
+			break
+		}
+		d /= f.divisor
+		unit = f.unit
+	}
+	return fmt.Sprintf("%d%c", d, unit)
+}