cmd/compile: fix intrinsifying sync/atomic.Swap* on AMD64

It should alias to Xchg instead of Swap. Found when testing #16985.

Change-Id: If9fd734a1f89b8b2656f421eb31b9d1b0d95a49f
Reviewed-on: https://go-review.googlesource.com/28512
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -2706,17 +2706,17 @@ func intrinsicInit() {
 		i.std[intrinsicKey{"runtime/internal/atomic", "Store64"}]
 
 	i.std[intrinsicKey{"sync/atomic", "SwapInt32"}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg"}]
 	i.std[intrinsicKey{"sync/atomic", "SwapInt64"}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap64"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg64"}]
 	i.std[intrinsicKey{"sync/atomic", "SwapUint32"}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg"}]
 	i.std[intrinsicKey{"sync/atomic", "SwapUint64"}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap64"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg64"}]
 	i.ptrSized[sizedIntrinsicKey{"sync/atomic", "SwapUintptr", 4}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg"}]
 	i.ptrSized[sizedIntrinsicKey{"sync/atomic", "SwapUintptr", 8}] =
-		i.std[intrinsicKey{"runtime/internal/atomic", "Swap64"}]
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xchg64"}]
 
 	i.std[intrinsicKey{"sync/atomic", "CompareAndSwapInt32"}] =
 		i.std[intrinsicKey{"runtime/internal/atomic", "Cas"}]

test/intrinsic.dir/atomic.go

+// +build amd64
+
+// Copyright 2016 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 main
+
+import "sync/atomic"
+
+var x uint32
+
+func atomics() {
+	_ = atomic.LoadUint32(&x)             // ERROR "intrinsic substitution for LoadUint32"
+	atomic.StoreUint32(&x, 1)             // ERROR "intrinsic substitution for StoreUint32"
+	atomic.AddUint32(&x, 1)               // ERROR "intrinsic substitution for AddUint32"
+	atomic.SwapUint32(&x, 1)              // ERROR "intrinsic substitution for SwapUint32"
+	atomic.CompareAndSwapUint32(&x, 1, 2) // ERROR "intrinsic substitution for CompareAndSwapUint32"
+}