runtime: fix slicecopy return value for zero-width elements

Fixes #8620

Change-Id: Idb49e586919d21d07e94a39ed9ebb0562f403460
Reviewed-on: https://go-review.googlesource.com/2221Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/slice.go

@@ -92,7 +92,7 @@ func growslice(t *slicetype, old sliceStruct, n int64) sliceStruct {
 }
 
 func slicecopy(to sliceStruct, fm sliceStruct, width uintptr) int {
-	if fm.len == 0 || to.len == 0 || width == 0 {
+	if fm.len == 0 || to.len == 0 {
 		return 0
 	}
 
@@ -101,6 +101,10 @@ func slicecopy(to sliceStruct, fm sliceStruct, width uintptr) int {
 		n = to.len
 	}
 
+	if width == 0 {
+		return n
+	}
+
 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&to))
 		pc := funcPC(slicecopy)

test/fixedbugs/issue8620.go

+// 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 8620. Used to fail with -race.
+
+package main
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func test(s1, s2 []struct{}) {
+	n := min(len(s1), len(s2))
+	if copy(s1, s2) != n {
+		panic("bad copy result")
+	}
+}
+
+func main() {
+	var b [100]struct{}
+	test(b[:], b[:])
+	test(b[1:], b[:])
+	test(b[:], b[2:])
+}