Align Group.Stats properly for 32-bit platforms.

Fixes golang/go#18334

Author: bradfitz

groupcache.go

@@ -166,6 +166,8 @@ type Group struct {
 	// concurrent callers.
 	loadGroup flightGroup
 
+	_ int32 // force Stats to be 8-byte aligned on 32-bit platforms
+
 	// Stats are statistics on the group.
 	Stats Stats
 }

groupcache_test.go

@@ -27,6 +27,7 @@ import (
 	"sync"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/golang/protobuf/proto"
 
@@ -443,5 +444,13 @@ func TestNoDedup(t *testing.T) {
 	}
 }
 
+func TestGroupStatsAlignment(t *testing.T) {
+	var g Group
+	off := unsafe.Offsetof(g.Stats)
+	if off%8 != 0 {
+		t.Fatal("Stats structure is not 8-byte aligned.")
+	}
+}
+
 // TODO(bradfitz): port the Google-internal full integration test into here,
 // using HTTP requests instead of our RPC system.