runtime: use bytes.IndexByte in findnull

bytes.IndexByte is heavily optimized. Use it in findnull.
This is second attempt, similar to CL97523.
In this version we never call IndexByte on region of memory,
that crosses page boundary. A bit slower than CL97523,
but still fast:

name        old time/op  new time/op  delta
GoString-6   164ns ± 2%   118ns ± 0%  -28.00%  (p=0.000 n=10+6)

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

Fixes #23830

Change-Id: Id843dd4f65a34309d92bdd8df229e484d26b0cb2
Reviewed-on: https://go-review.googlesource.com/98015
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@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

@@ -87,5 +87,7 @@ func Test6907(t *testing.T)                  { test6907(t) }
 func Test6907Go(t *testing.T)                { test6907Go(t) }
 func Test21897(t *testing.T)                 { test21897(t) }
 func Test22906(t *testing.T)                 { test22906(t) }
+func Test24206(t *testing.T)                 { test24206(t) }
 
-func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }
+func BenchmarkCgoCall(b *testing.B)  { benchCgoCall(b) }
+func BenchmarkGoString(b *testing.B) { benchGoString(b) }

misc/cgo/test/issue24206.go

+// +build amd64,linux
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+// Test that C.GoString uses IndexByte in safe manner.
+
+/*
+#include <sys/mman.h>
+
+// Returns string with null byte at the last valid address
+char* dangerousString1() {
+	int pageSize = 4096;
+	char *data = mmap(0, 2 * pageSize, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, 0, 0);
+	mprotect(data + pageSize,pageSize,PROT_NONE);
+	int start = pageSize - 123 - 1; // last 123 bytes of first page + 1 null byte
+	int i = start;
+	for (; i < pageSize; i++) {
+	data[i] = 'x';
+	}
+	data[pageSize -1 ] = 0;
+	return data+start;
+}
+
+char* dangerousString2() {
+	int pageSize = 4096;
+	char *data = mmap(0, 3 * pageSize, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, 0, 0);
+	mprotect(data + 2 * pageSize,pageSize,PROT_NONE);
+	int start = pageSize - 123 - 1; // last 123 bytes of first page + 1 null byte
+	int i = start;
+	for (; i < 2 * pageSize; i++) {
+	data[i] = 'x';
+	}
+	data[2*pageSize -1 ] = 0;
+	return data+start;
+}
+*/
+import "C"
+
+import (
+	"testing"
+)
+
+func test24206(t *testing.T) {
+	if l := len(C.GoString(C.dangerousString1())); l != 123 {
+		t.Errorf("Incorrect string length - got %d, want 123", l)
+	}
+	if l := len(C.GoString(C.dangerousString2())); l != 4096+123 {
+		t.Errorf("Incorrect string length - got %d, want %d", l, 4096+123)
+	}
+}

misc/cgo/test/issue24206_generic.go

+// +build !amd64 !linux
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+import "testing"
+
+func test24206(t *testing.T) {
+	t.Skip("Skipping on non-amd64 or non-linux system")
+}

src/runtime/string.go

@@ -4,7 +4,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"internal/bytealg"
+	"unsafe"
+)
 
 // The constant is known to the compiler.
 // There is no fundamental theory behind this number.
@@ -407,12 +410,31 @@ 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++
+
+	// pageSize is the unit we scan at a time looking for NULL.
+	// It must be the minimum page size for any architecture Go
+	// runs on. It's okay (just a minor performance loss) if the
+	// actual system page size is larger than this value.
+	const pageSize = 4096
+
+	offset := 0
+	ptr := unsafe.Pointer(s)
+	// IndexByteString uses wide reads, so we need to be careful
+	// with page boundaries. Call IndexByteString on
+	// [ptr, endOfPage) interval.
+	safeLen := int(pageSize - uintptr(ptr)%pageSize)
+
+	for {
+		t := *(*string)(unsafe.Pointer(&stringStruct{ptr, safeLen}))
+		// Check one page at a time.
+		if i := bytealg.IndexByteString(t, 0); i != -1 {
+			return offset + i
+		}
+		// Move to next page
+		ptr = unsafe.Pointer(uintptr(ptr) + uintptr(safeLen))
+		offset += safeLen
+		safeLen = pageSize
 	}
-	return l
 }
 
 func findnullw(s *uint16) int {