runtime: use bytes.IndexByte in findnull

bytes.IndexByte is heavily optimized.
Use it in findnull.

name        old time/op  new time/op  delta
GoString-8  65.5ns ± 1%  40.2ns ± 1%  -38.62%  (p=0.000 n=19+19)

findnull is also used in gostringnocopy,
which is used in many hot spots in the runtime.

Fixes #23830

Change-Id: I2e6cb279c7d8078f8844065de684cc3567fe89d7
Reviewed-on: https://go-review.googlesource.com/97523
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/basic.go

@@ -31,6 +31,8 @@ struct S {
 	int x;
 };
 
+const char *cstr = "abcefghijklmnopqrstuvwxyzABCEFGHIJKLMNOPQRSTUVWXYZ1234567890";
+
 extern enum E myConstFunc(struct S* const ctx, int const id, struct S **const filter);
 
 enum E myConstFunc(struct S *const ctx, int const id, struct S **const filter) { return 0; }
@@ -149,6 +151,18 @@ func benchCgoCall(b *testing.B) {
 	}
 }
 
+var sinkString string
+
+func benchGoString(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		sinkString = C.GoString(C.cstr)
+	}
+	const want = "abcefghijklmnopqrstuvwxyzABCEFGHIJKLMNOPQRSTUVWXYZ1234567890"
+	if sinkString != want {
+		b.Fatalf("%q != %q", sinkString, want)
+	}
+}
+
 // Issue 2470.
 func testUnsignedInt(t *testing.T) {
 	a := (int64)(C.UINT32VAL)

misc/cgo/test/cgo_test.go

@@ -89,3 +89,4 @@ func Test21897(t *testing.T)                 { test21897(t) }
 func Test22906(t *testing.T)                 { test22906(t) }
 
 func BenchmarkCgoCall(b *testing.B)  { benchCgoCall(b) }
+func BenchmarkGoString(b *testing.B) { benchGoString(b) }

src/runtime/error.go

@@ -121,6 +121,7 @@ func printany(i interface{}) {
 // strings.IndexByte is implemented in runtime/asm_$goarch.s
 // but amusingly we need go:linkname to get access to it here in the runtime.
 //go:linkname stringsIndexByte strings.IndexByte
+//go:noescape
 func stringsIndexByte(s string, c byte) int
 
 // panicwrap generates a panic for a call to a wrapped value method

src/runtime/string.go

@@ -407,12 +407,9 @@ func findnull(s *byte) int {
 	if s == nil {
 		return 0
 	}
-	p := (*[maxAlloc/2 - 1]byte)(unsafe.Pointer(s))
-	l := 0
-	for p[l] != 0 {
-		l++
-	}
-	return l
+	ss := stringStruct{unsafe.Pointer(s), maxAlloc/2 - 1}
+	t := *(*string)(unsafe.Pointer(&ss))
+	return stringsIndexByte(t, 0)
 }
 
 func findnullw(s *uint16) int {