Revert "runtime: use bytes.IndexByte in findnull"

This reverts commit 7365fac2.

Reason for revert: breaks the build on some architectures, reading unmapped pages?

Change-Id: I3a8c02dc0b649269faacea79ecd8213defa97c54
Reviewed-on: https://go-review.googlesource.com/97995Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/basic.go

@@ -31,8 +31,6 @@ 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; }
@@ -151,18 +149,6 @@ 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

@@ -89,4 +89,3 @@ 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

@@ -121,7 +121,6 @@ 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

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