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 <khr@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

diff --git a/src/internal/runtime/atomic/atomic_amd64.go b/src/internal/runtime/atomic/atomic_amd64.go
index b439954093cd28b686d493cbba59042e35853b01..2a2d07e51191391a69a97643042e44c2fb607334 100644 (file)
--- a/src/internal/runtime/atomic/atomic_amd64.go
+++ b/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
 

diff --git a/src/internal/runtime/atomic/atomic_amd64.s b/src/internal/runtime/atomic/atomic_amd64.s
index ec75bf9332f79c7a90fd7269125efa482801ac81..d6dc7a32d668fecc96f17c7baf2fa07851f3970a 100644 (file)
--- a/src/internal/runtime/atomic/atomic_amd64.s
+++ b/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;

diff --git a/src/internal/runtime/atomic/xchg8_test.go b/src/internal/runtime/atomic/xchg8_test.go
new file mode 100644 (file)
index 0000000..1390624
--- /dev/null
+++ b/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
+               }
+       }
+}