cmd/compile: correct alignment of atomic.Int64

Same as CL 417555, but for cmd/compile.

Fixes #54220

Change-Id: I4cc6deaf0a87c952f636888b4ab73f81a44bfebd
Reviewed-on: https://go-review.googlesource.com/c/go/+/420975
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/noder/sizes.go

@@ -25,6 +25,17 @@ func (s *gcSizes) Alignof(T types2.Type) int64 {
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
 		return s.Alignof(t.Elem())
 	case *types2.Struct:
+		if t.NumFields() == 0 && types2.IsSyncAtomicAlign64(T) {
+			// Special case: sync/atomic.align64 is an
+			// empty struct we recognize as a signal that
+			// the struct it contains must be
+			// 64-bit-aligned.
+			//
+			// This logic is equivalent to the logic in
+			// cmd/compile/internal/types/size.go:calcStructOffset
+			return 8
+		}
+
 		// spec: "For a variable x of struct type: unsafe.Alignof(x)
 		// is the largest of the values unsafe.Alignof(x.f) for each
 		// field f of x, but at least 1."

src/cmd/compile/internal/types2/sizes.go

@@ -53,7 +53,7 @@ func (s *StdSizes) Alignof(T Type) int64 {
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
 		return s.Alignof(t.elem)
 	case *Struct:
-		if len(t.fields) == 0 && isSyncAtomicAlign64(T) {
+		if len(t.fields) == 0 && IsSyncAtomicAlign64(T) {
 			// Special case: sync/atomic.align64 is an
 			// empty struct we recognize as a signal that
 			// the struct it contains must be
@@ -104,7 +104,7 @@ func (s *StdSizes) Alignof(T Type) int64 {
 	return a
 }
 
-func isSyncAtomicAlign64(T Type) bool {
+func IsSyncAtomicAlign64(T Type) bool {
 	named, ok := T.(*Named)
 	if !ok {
 		return false

test/fixedbugs/issue54220.go

@@ -0,0 +1,26 @@
+// run
+
+// Copyright 2022 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 (
+	"strconv"
+	"sync/atomic"
+	"unsafe"
+)
+
+type t struct {
+	i1 atomic.Int32
+	i2 atomic.Int64
+}
+
+var v t
+
+func main() {
+	if o := unsafe.Offsetof(v.i2); o != 8 {
+		panic("unexpected offset, want: 8, got: " + strconv.Itoa(int(o)))
+	}
+}