cmd/compile: intrinsify slicebytetostringtmp when not instrumenting

when not instrumenting:
- Intrinsify uses of slicebytetostringtmp within the runtime package
  in the ssa backend.
- Pass OARRAYBYTESTRTMP nodes to the compiler backends for lowering
  instead of generating calls to slicebytetostringtmp.

name                    old time/op  new time/op  delta
ConcatStringAndBytes-4  27.9ns ± 2%  24.7ns ± 2%  -11.52%  (p=0.000 n=43+43)

Fixes #17044

Change-Id: I51ce9c3b93284ce526edd0234f094e98580faf2d
Reviewed-on: https://go-review.googlesource.com/29017
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 13 insertions, 10 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 2263e16410..4c4b736c63 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -109,17 +109,20 @@ func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
 	return
 }
 
+// slicebytetostringtmp returns a "string" referring to the actual []byte bytes.
+//
+// Callers need to ensure that the returned string will not be used after
+// the calling goroutine modifies the original slice or synchronizes with
+// another goroutine.
+//
+// The function is only called when instrumenting
+// and otherwise intrinsified by the compiler.
+//
+// Some internal compiler optimizations use this function.
+// - Used for m[string(k)] lookup where m is a string-keyed map and k is a []byte.
+// - Used for "<"+string(b)+">" concatenation where b is []byte.
+// - Used for string(b)=="foo" comparison where b is []byte.
 func slicebytetostringtmp(b []byte) string {
-	// Return a "string" referring to the actual []byte bytes.
-	// This is only for use by internal compiler optimizations
-	// that know that the string form will be discarded before
-	// the calling goroutine could possibly modify the original
-	// slice or synchronize with another goroutine.
-	// First such case is a m[string(k)] lookup where
-	// m is a string-keyed map and k is a []byte.
-	// Second such case is "<"+string(b)+">" concatenation where b is []byte.
-	// Third such case is string(b)=="foo" comparison where b is []byte.
-
 	if raceenabled && len(b) > 0 {
 		racereadrangepc(unsafe.Pointer(&b[0]),
 			uintptr(len(b)),