diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index cb66240..3265a13 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -30,9 +30,11 @@
 
 namespace art {
 
+#if defined(__APPLE__)
+
 // This works on Mac OS 10.6 but hasn't been tested on older releases.
 struct __attribute__((__may_alias__)) darwin_pthread_mutex_t {
-  long padding0;  // NOLINT(runtime/int)
+  long padding0;  // NOLINT(runtime/int) exact match to darwin type
   int padding1;
   uint32_t padding2;
   int16_t padding3;
@@ -43,7 +45,7 @@
 };
 
 struct __attribute__((__may_alias__)) darwin_pthread_rwlock_t {
-  long padding0;  // NOLINT(runtime/int)
+  long padding0;  // NOLINT(runtime/int) exact match to darwin type
   pthread_mutex_t padding1;
   int padding2;
   pthread_cond_t padding3;
@@ -54,6 +56,10 @@
   // ...other stuff we don't care about.
 };
 
+#endif  // __APPLE__
+
+#if defined(__GLIBC__)
+
 struct __attribute__((__may_alias__)) glibc_pthread_mutex_t {
   int32_t padding0[2];
   int owner;
@@ -70,6 +76,8 @@
   // ...other stuff we don't care about.
 };
 
+#endif  // __GLIBC__
+
 #if ART_USE_FUTEXES
 static bool ComputeRelativeTimeSpec(timespec* result_ts, const timespec& lhs, const timespec& rhs) {
   const int32_t one_sec = 1000 * 1000 * 1000;  // one second in nanoseconds.