diff --git a/include/jemalloc/internal/arena.h b/include/jemalloc/internal/arena.h
index f2c18f43..bbcfedac 100644
--- a/include/jemalloc/internal/arena.h
+++ b/include/jemalloc/internal/arena.h
@@ -441,6 +441,7 @@ void	arena_postfork_child(arena_t *arena);
 #ifndef JEMALLOC_ENABLE_INLINE
 arena_chunk_map_t	*arena_mapp_get(arena_chunk_t *chunk, size_t pageind);
 size_t	*arena_mapbitsp_get(arena_chunk_t *chunk, size_t pageind);
+size_t	arena_mapbitsp_read(size_t *mapbitsp);
 size_t	arena_mapbits_get(arena_chunk_t *chunk, size_t pageind);
 size_t	arena_mapbits_unallocated_size_get(arena_chunk_t *chunk,
     size_t pageind);
@@ -451,6 +452,7 @@ size_t	arena_mapbits_dirty_get(arena_chunk_t *chunk, size_t pageind);
 size_t	arena_mapbits_unzeroed_get(arena_chunk_t *chunk, size_t pageind);
 size_t	arena_mapbits_large_get(arena_chunk_t *chunk, size_t pageind);
 size_t	arena_mapbits_allocated_get(arena_chunk_t *chunk, size_t pageind);
+void	arena_mapbitsp_write(size_t *mapbitsp, size_t mapbits);
 void	arena_mapbits_unallocated_set(arena_chunk_t *chunk, size_t pageind,
     size_t size, size_t flags);
 void	arena_mapbits_unallocated_size_set(arena_chunk_t *chunk, size_t pageind,
@@ -497,11 +499,18 @@ arena_mapbitsp_get(arena_chunk_t *chunk, size_t pageind)
 	return (&arena_mapp_get(chunk, pageind)->bits);
 }
 
+JEMALLOC_ALWAYS_INLINE size_t
+arena_mapbitsp_read(size_t *mapbitsp)
+{
+
+	return (*mapbitsp);
+}
+
 JEMALLOC_ALWAYS_INLINE size_t
 arena_mapbits_get(arena_chunk_t *chunk, size_t pageind)
 {
 
-	return (*arena_mapbitsp_get(chunk, pageind));
+	return (arena_mapbitsp_read(arena_mapbitsp_get(chunk, pageind)));
 }
 
 JEMALLOC_ALWAYS_INLINE size_t
@@ -584,83 +593,90 @@ arena_mapbits_allocated_get(arena_chunk_t *chunk, size_t pageind)
 	return (mapbits & CHUNK_MAP_ALLOCATED);
 }
 
+JEMALLOC_ALWAYS_INLINE void
+arena_mapbitsp_write(size_t *mapbitsp, size_t mapbits)
+{
+
+	*mapbitsp = mapbits;
+}
+
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_unallocated_set(arena_chunk_t *chunk, size_t pageind, size_t size,
     size_t flags)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
 
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
 	assert((size & PAGE_MASK) == 0);
 	assert((flags & ~CHUNK_MAP_FLAGS_MASK) == 0);
 	assert((flags & (CHUNK_MAP_DIRTY|CHUNK_MAP_UNZEROED)) == flags);
-	*mapbitsp = size | CHUNK_MAP_BININD_INVALID | flags;
+	arena_mapbitsp_write(mapbitsp, size | CHUNK_MAP_BININD_INVALID | flags);
 }
 
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_unallocated_size_set(arena_chunk_t *chunk, size_t pageind,
     size_t size)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
+	size_t mapbits = arena_mapbitsp_read(mapbitsp);
 
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
 	assert((size & PAGE_MASK) == 0);
-	assert((*mapbitsp & (CHUNK_MAP_LARGE|CHUNK_MAP_ALLOCATED)) == 0);
-	*mapbitsp = size | (*mapbitsp & PAGE_MASK);
+	assert((mapbits & (CHUNK_MAP_LARGE|CHUNK_MAP_ALLOCATED)) == 0);
+	arena_mapbitsp_write(mapbitsp, size | (mapbits & PAGE_MASK));
 }
 
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_large_set(arena_chunk_t *chunk, size_t pageind, size_t size,
     size_t flags)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
+	size_t mapbits = arena_mapbitsp_read(mapbitsp);
 	size_t unzeroed;
 
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
 	assert((size & PAGE_MASK) == 0);
 	assert((flags & CHUNK_MAP_DIRTY) == flags);
-	unzeroed = *mapbitsp & CHUNK_MAP_UNZEROED; /* Preserve unzeroed. */
-	*mapbitsp = size | CHUNK_MAP_BININD_INVALID | flags | unzeroed |
-	    CHUNK_MAP_LARGE | CHUNK_MAP_ALLOCATED;
+	unzeroed = mapbits & CHUNK_MAP_UNZEROED; /* Preserve unzeroed. */
+	arena_mapbitsp_write(mapbitsp, size | CHUNK_MAP_BININD_INVALID | flags
+	    | unzeroed | CHUNK_MAP_LARGE | CHUNK_MAP_ALLOCATED);
 }
 
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_large_binind_set(arena_chunk_t *chunk, size_t pageind,
     size_t binind)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
+	size_t mapbits = arena_mapbitsp_read(mapbitsp);
 
 	assert(binind <= BININD_INVALID);
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
 	assert(arena_mapbits_large_size_get(chunk, pageind) == PAGE);
-	*mapbitsp = (*mapbitsp & ~CHUNK_MAP_BININD_MASK) | (binind <<
-	    CHUNK_MAP_BININD_SHIFT);
+	arena_mapbitsp_write(mapbitsp, (mapbits & ~CHUNK_MAP_BININD_MASK) |
+	    (binind << CHUNK_MAP_BININD_SHIFT));
 }
 
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_small_set(arena_chunk_t *chunk, size_t pageind, size_t runind,
     size_t binind, size_t flags)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
+	size_t mapbits = arena_mapbitsp_read(mapbitsp);
 	size_t unzeroed;
 
 	assert(binind < BININD_INVALID);
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
 	assert(pageind - runind >= map_bias);
 	assert((flags & CHUNK_MAP_DIRTY) == flags);
-	unzeroed = *mapbitsp & CHUNK_MAP_UNZEROED; /* Preserve unzeroed. */
-	*mapbitsp = (runind << LG_PAGE) | (binind << CHUNK_MAP_BININD_SHIFT) |
-	    flags | unzeroed | CHUNK_MAP_ALLOCATED;
+	unzeroed = mapbits & CHUNK_MAP_UNZEROED; /* Preserve unzeroed. */
+	arena_mapbitsp_write(mapbitsp, (runind << LG_PAGE) | (binind <<
+	    CHUNK_MAP_BININD_SHIFT) | flags | unzeroed | CHUNK_MAP_ALLOCATED);
 }
 
 JEMALLOC_ALWAYS_INLINE void
 arena_mapbits_unzeroed_set(arena_chunk_t *chunk, size_t pageind,
     size_t unzeroed)
 {
-	size_t *mapbitsp;
+	size_t *mapbitsp = arena_mapbitsp_get(chunk, pageind);
+	size_t mapbits = arena_mapbitsp_read(mapbitsp);
 
-	mapbitsp = arena_mapbitsp_get(chunk, pageind);
-	*mapbitsp = (*mapbitsp & ~CHUNK_MAP_UNZEROED) | unzeroed;
+	arena_mapbitsp_write(mapbitsp, (mapbits & ~CHUNK_MAP_UNZEROED) |
+	    unzeroed);
 }
 
 JEMALLOC_INLINE bool
diff --git a/include/jemalloc/internal/private_namespace.h b/include/jemalloc/internal/private_namespace.h
index 65de3163..cdb0b0eb 100644
--- a/include/jemalloc/internal/private_namespace.h
+++ b/include/jemalloc/internal/private_namespace.h
@@ -33,6 +33,8 @@
 #define	arena_mapbits_unzeroed_get JEMALLOC_N(arena_mapbits_unzeroed_get)
 #define	arena_mapbits_unzeroed_set JEMALLOC_N(arena_mapbits_unzeroed_set)
 #define	arena_mapbitsp_get JEMALLOC_N(arena_mapbitsp_get)
+#define	arena_mapbitsp_read JEMALLOC_N(arena_mapbitsp_read)
+#define	arena_mapbitsp_write JEMALLOC_N(arena_mapbitsp_write)
 #define	arena_mapp_get JEMALLOC_N(arena_mapp_get)
 #define	arena_maxclass JEMALLOC_N(arena_maxclass)
 #define	arena_new JEMALLOC_N(arena_new)
diff --git a/src/arena.c b/src/arena.c
index 05a787f8..0a73be2a 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -569,17 +569,24 @@ arena_chunk_alloc(arena_t *arena)
 		 * unless the chunk is not zeroed.
 		 */
 		if (zero == false) {
+			VALGRIND_MAKE_MEM_UNDEFINED(
+			    (void *)arena_mapp_get(chunk, map_bias+1),
+			    (size_t)((uintptr_t) arena_mapp_get(chunk,
+			    chunk_npages-1) - (uintptr_t)arena_mapp_get(chunk,
+			    map_bias+1)));
 			for (i = map_bias+1; i < chunk_npages-1; i++)
 				arena_mapbits_unzeroed_set(chunk, i, unzeroed);
-		} else if (config_debug) {
+		} else {
 			VALGRIND_MAKE_MEM_DEFINED(
 			    (void *)arena_mapp_get(chunk, map_bias+1),
-			    (void *)((uintptr_t)
-			    arena_mapp_get(chunk, chunk_npages-1)
-			    - (uintptr_t)arena_mapp_get(chunk, map_bias+1)));
-			for (i = map_bias+1; i < chunk_npages-1; i++) {
-				assert(arena_mapbits_unzeroed_get(chunk, i) ==
-				    unzeroed);
+			    (size_t)((uintptr_t) arena_mapp_get(chunk,
+			    chunk_npages-1) - (uintptr_t)arena_mapp_get(chunk,
+			    map_bias+1)));
+			if (config_debug) {
+				for (i = map_bias+1; i < chunk_npages-1; i++) {
+					assert(arena_mapbits_unzeroed_get(chunk,
+					    i) == unzeroed);
+				}
 			}
 		}
 		arena_mapbits_unallocated_set(chunk, chunk_npages-1,