internal/runtime/atomic: add Xchg8 for amd64

For #68578

Change-Id: Idecfdbb793f46560dd69287af9170c07cf4ee973
Reviewed-on: https://go-review.googlesource.com/c/go/+/606900
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
Auto-Submit: Rhys Hiltner <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

src/internal/runtime/atomic/atomic_amd64.go

@@ -57,6 +57,9 @@ func Xadd64(ptr *uint64, delta int64) uint64
 //go:noescape
 func Xadduintptr(ptr *uintptr, delta uintptr) uintptr
 
+//go:noescape
+func Xchg8(ptr *uint8, new uint8) uint8
+
 //go:noescape
 func Xchg(ptr *uint32, new uint32) uint32
 

src/internal/runtime/atomic/atomic_amd64.s

@@ -117,6 +117,18 @@ TEXT ·Xaddint64(SB), NOSPLIT, $0-24
 TEXT ·Xadduintptr(SB), NOSPLIT, $0-24
 	JMP	·Xadd64(SB)
 
+// uint8 Xchg(ptr *uint8, new uint8)
+// Atomically:
+//	old := *ptr;
+//	*ptr = new;
+//	return old;
+TEXT ·Xchg8(SB), NOSPLIT, $0-17
+	MOVQ	ptr+0(FP), BX
+	MOVB	new+8(FP), AX
+	XCHGB	AX, 0(BX)
+	MOVB	AX, ret+16(FP)
+	RET
+
 // uint32 Xchg(ptr *uint32, new uint32)
 // Atomically:
 //	old := *ptr;

src/internal/runtime/atomic/xchg8_test.go

@@ -0,0 +1,39 @@
+// Copyright 2024 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.
+
+//go:build amd64
+
+package atomic_test
+
+import (
+	"internal/runtime/atomic"
+	"testing"
+)
+
+func TestXchg8(t *testing.T) {
+	var a [16]uint8
+	for i := range a {
+		next := uint8(i + 50)
+		a[i] = next
+	}
+	b := a
+
+	// Compare behavior against non-atomic implementation. Expect the operation
+	// to work at any byte offset and to not clobber neighboring values.
+	for i := range a {
+		next := uint8(i + 100)
+		pa := atomic.Xchg8(&a[i], next)
+		pb := b[i]
+		b[i] = next
+		if pa != pb {
+			t.Errorf("atomic.Xchg8(a[%d]); %d != %d", i, pa, pb)
+		}
+		if a != b {
+			t.Errorf("after atomic.Xchg8(a[%d]); %d != %d", i, a, b)
+		}
+		if t.Failed() {
+			break
+		}
+	}
+}