cmd/6g, cmd/8g: fix, test byte-sized magic multiply

Credit to R?my for finding and writing test case.

Fixes issue 8325.

LGTM=r
R=golang-codereviews, r
CC=dave, golang-codereviews, iant, remyoudompheng
https://codereview.appspot.com/124950043

1 files changed, 31 insertions, 0 deletions

diff --git a/test/fixedbugs/issue8325.go b/test/fixedbugs/issue8325.go
new file mode 100644
index 000000000..e22fd319d
--- /dev/null
+++ b/test/fixedbugs/issue8325.go
@@ -0,0 +1,31 @@
+// run
+
+// Copyright 2014 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 8325: corrupted byte operations during optimization
+// pass.
+
+package main
+
+const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+func main() {
+	var bytes = []byte{10, 20, 30, 40, 50}
+
+	for i, b := range bytes {
+		bytes[i] = alphanum[b%byte(len(alphanum))]
+	}
+
+	for _, b := range bytes {
+		switch {
+		case '0' <= b && b <= '9',
+			'A' <= b && b <= 'Z':
+		default:
+			println("found a bad character", string(b))
+			panic("BUG")
+		}
+
+	}
+}