diff --git a/include/jemalloc/internal/mutex_structs.h b/include/jemalloc/internal/mutex_structs.h
index 4a18a075..c34c1d47 100644
--- a/include/jemalloc/internal/mutex_structs.h
+++ b/include/jemalloc/internal/mutex_structs.h
@@ -2,23 +2,39 @@
 #define JEMALLOC_INTERNAL_MUTEX_STRUCTS_H
 
 struct malloc_mutex_s {
+	union {
+		struct {
 #ifdef _WIN32
 #  if _WIN32_WINNT >= 0x0600
-	SRWLOCK         	lock;
+			SRWLOCK         	lock;
 #  else
-	CRITICAL_SECTION	lock;
+			CRITICAL_SECTION	lock;
 #  endif
 #elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
-	os_unfair_lock		lock;
+			os_unfair_lock		lock;
 #elif (defined(JEMALLOC_OSSPIN))
-	OSSpinLock		lock;
+			OSSpinLock		lock;
 #elif (defined(JEMALLOC_MUTEX_INIT_CB))
-	pthread_mutex_t		lock;
-	malloc_mutex_t		*postponed_next;
+			pthread_mutex_t		lock;
+			malloc_mutex_t		*postponed_next;
 #else
-	pthread_mutex_t		lock;
+			pthread_mutex_t		lock;
 #endif
+		};
+		/*
+		 * We only touch witness when configured w/ debug. However we
+		 * keep the field in a union when !debug so that we don't have
+		 * to pollute the code base with #ifdefs, while avoid paying the
+		 * memory cost.
+		 */
+#if !defined(JEMALLOC_DEBUG)
+		witness_t		witness;
+#endif
+	};
+
+#if defined(JEMALLOC_DEBUG)
 	witness_t		witness;
+#endif
 };
 
 #endif /* JEMALLOC_INTERNAL_MUTEX_STRUCTS_H */
diff --git a/include/jemalloc/internal/mutex_types.h b/include/jemalloc/internal/mutex_types.h
index 8c9f249d..b7e3a7a1 100644
--- a/include/jemalloc/internal/mutex_types.h
+++ b/include/jemalloc/internal/mutex_types.h
@@ -7,25 +7,25 @@ typedef struct malloc_mutex_s malloc_mutex_t;
 #  define MALLOC_MUTEX_INITIALIZER
 #elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
 #  define MALLOC_MUTEX_INITIALIZER					\
-     {OS_UNFAIR_LOCK_INIT, WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
+     {{{OS_UNFAIR_LOCK_INIT}}, WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
 #elif (defined(JEMALLOC_OSSPIN))
 #  define MALLOC_MUTEX_INITIALIZER					\
-     {0, WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
+     {{{0}}, WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
 #elif (defined(JEMALLOC_MUTEX_INIT_CB))
 #  define MALLOC_MUTEX_INITIALIZER					\
-    {PTHREAD_MUTEX_INITIALIZER, NULL,					\
-     WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
+     {{{PTHREAD_MUTEX_INITIALIZER, NULL}},				\
+      WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
 #else
 #  if (defined(JEMALLOC_HAVE_PTHREAD_MUTEX_ADAPTIVE_NP) &&		\
        defined(PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP))
 #    define MALLOC_MUTEX_TYPE PTHREAD_MUTEX_ADAPTIVE_NP
 #    define MALLOC_MUTEX_INITIALIZER					\
-       {PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP,				\
+       {{{PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP}},			\
         WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
 #  else
 #    define MALLOC_MUTEX_TYPE PTHREAD_MUTEX_DEFAULT
 #    define MALLOC_MUTEX_INITIALIZER					\
-       {PTHREAD_MUTEX_INITIALIZER,					\
+       {{{PTHREAD_MUTEX_INITIALIZER}},					\
         WITNESS_INITIALIZER("mutex", WITNESS_RANK_OMIT)}
 #  endif
 #endif
diff --git a/include/jemalloc/internal/witness_types.h b/include/jemalloc/internal/witness_types.h
index 0678b082..3efaad7e 100644
--- a/include/jemalloc/internal/witness_types.h
+++ b/include/jemalloc/internal/witness_types.h
@@ -55,6 +55,10 @@ typedef int witness_comp_t (const witness_t *, void *, const witness_t *,
 #define WITNESS_RANK_PROF_NEXT_THR_UID	WITNESS_RANK_LEAF
 #define WITNESS_RANK_PROF_THREAD_ACTIVE_INIT	WITNESS_RANK_LEAF
 
-#define WITNESS_INITIALIZER(name, rank) {name, rank, NULL, NULL, {NULL, NULL}}
+#if defined(JEMALLOC_DEBUG)
+#  define WITNESS_INITIALIZER(name, rank) {name, rank, NULL, NULL, {NULL, NULL}}
+#else
+#  define WITNESS_INITIALIZER(name, rank)
+#endif
 
 #endif /* JEMALLOC_INTERNAL_WITNESS_TYPES_H */