runtime: zero tmpbuf between len and cap

Zero the entire buffer so we don't need to
lower its capacity upon return.  This lets callers
do some appending without allocation.

Zeroing is cheap, the byte buffer requires only
4 extra instructions.

Fixes #14235

Change-Id: I970d7badcef047dafac75ac17130030181f18fe2
Reviewed-on: https://go-review.googlesource.com/22424
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 112ce5d588..ef28ba9828 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -139,7 +139,8 @@ func slicebytetostringtmp(b []byte) string {
 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
 	var b []byte
 	if buf != nil && len(s) <= len(buf) {
-		b = buf[:len(s):len(s)]
+		*buf = tmpBuf{}
+		b = buf[:len(s)]
 	} else {
 		b = rawbyteslice(len(s))
 	}
@@ -171,7 +172,8 @@ func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	}
 	var a []rune
 	if buf != nil && n <= len(buf) {
-		a = buf[:n:n]
+		*buf = [tmpStringBufSize]rune{}
+		a = buf[:n]
 	} else {
 		a = rawruneslice(n)
 	}