cmd/compile: avoid slicebytetostring call in len(string([]byte))

Change-Id: Ie04503e61400a793a6a29a4b58795254deabe472
Reviewed-on: https://go-review.googlesource.com/c/go/+/497276
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/walk/builtin.go

@@ -250,6 +250,10 @@ func walkLenCap(n *ir.UnaryExpr, init *ir.Nodes) ir.Node {
 		// Replace len([]rune(string)) with runtime.countrunes(string).
 		return mkcall("countrunes", n.Type(), init, typecheck.Conv(n.X.(*ir.ConvExpr).X, types.Types[types.TSTRING]))
 	}
+	if isByteCount(n) {
+		_, len := backingArrayPtrLen(cheapExpr(n.X.(*ir.ConvExpr).X, init))
+		return len
+	}
 
 	n.X = walkExpr(n.X, init)
 
@@ -862,3 +866,9 @@ func writebarrierfn(name string, l *types.Type, r *types.Type) ir.Node {
 func isRuneCount(n ir.Node) bool {
 	return base.Flag.N == 0 && !base.Flag.Cfg.Instrumenting && n.Op() == ir.OLEN && n.(*ir.UnaryExpr).X.Op() == ir.OSTR2RUNES
 }
+
+// isByteCount reports whether n is of the form len(string([]byte)).
+func isByteCount(n ir.Node) bool {
+	return base.Flag.N == 0 && !base.Flag.Cfg.Instrumenting && n.Op() == ir.OLEN &&
+		(n.(*ir.UnaryExpr).X.Op() == ir.OBYTES2STR || n.(*ir.UnaryExpr).X.Op() == ir.OBYTES2STRTMP)
+}

test/codegen/strings.go

@@ -14,6 +14,11 @@ func CountRunes(s string) int { // Issue #24923
 	return len([]rune(s))
 }
 
+func CountBytes(s []byte) int {
+	// amd64:-`.*runtime.slicebytetostring`
+	return len(string(s))
+}
+
 func ToByteSlice() []byte { // Issue #24698
 	// amd64:`LEAQ\ttype:\[3\]uint8`
 	// amd64:`CALL\truntime\.newobject`