diff --git a/src/pkg/runtime/atomic_arm.c b/src/pkg/runtime/atomic_arm.c
index 9193d599d36..a78b1dfe2ce 100644
--- a/src/pkg/runtime/atomic_arm.c
+++ b/src/pkg/runtime/atomic_arm.c
@@ -5,9 +5,9 @@
 #include "runtime.h"
 #include "arch_GOARCH.h"
 
-static union {
+static struct {
 	Lock l;
-	byte pad [CacheLineSize];
+	byte pad[CacheLineSize-sizeof(Lock)];
 } locktab[57];
 
 #define LOCK(addr) (&locktab[((uintptr)(addr)>>3)%nelem(locktab)].l)
diff --git a/src/pkg/runtime/sema.goc b/src/pkg/runtime/sema.goc
index c4b5247b3da..4df01fc4e49 100644
--- a/src/pkg/runtime/sema.goc
+++ b/src/pkg/runtime/sema.goc
@@ -44,13 +44,13 @@ struct SemaRoot
 // Prime to not correlate with any user patterns.
 #define SEMTABLESZ 251
 
-union semtable
+struct semtable
 {
 	SemaRoot;
-	uint8 pad[CacheLineSize];
+	uint8 pad[CacheLineSize-sizeof(SemaRoot)];
 };
 #pragma dataflag 16 /* mark semtable as 'no pointers', hiding from garbage collector */
-static union semtable semtable[SEMTABLESZ];
+static struct semtable semtable[SEMTABLESZ];
 
 static SemaRoot*
 semroot(uint32 *addr)