runtime: fix slicecopy return value for zero-width elements

Fixes #8620

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

1 files changed, 5 insertions, 1 deletions

diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 4fb2adc1f9..e427a8b7cc 100644
--- a/src/runtime/slice.go
+++ b/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)