cmd/gc: don't copy string in range []byte(str)

Using benchmark from the issue:

benchmark                    old ns/op     new ns/op     delta
BenchmarkRangeStringCast     2162          1152          -46.72%

benchmark                    old allocs     new allocs     delta
BenchmarkRangeStringCast     1              0              -100.00%

Fixes #2204

Change-Id: I92c5edd2adca4a7b6fba00713a581bf49dc59afe
Reviewed-on: https://go-review.googlesource.com/3790
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 58198d0e1b..46c3502f77 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -135,6 +135,18 @@ func stringtoslicebyte(s string) []byte {
 	return b
 }
 
+func stringtoslicebytetmp(s string) []byte {
+	// Return a slice referring to the actual string bytes.
+	// This is only for use by internal compiler optimizations
+	// that know that the slice won't be mutated.
+	// The only such case today is:
+	// for i, c := range []byte(str)
+
+	str := (*stringStruct)(unsafe.Pointer(&s))
+	ret := slice{array: (*byte)(str.str), len: uint(str.len), cap: uint(str.len)}
+	return *(*[]byte)(unsafe.Pointer(&ret))
+}
+
 func stringtoslicerune(s string) []rune {
 	// two passes.
 	// unlike slicerunetostring, no race because strings are immutable.