Index: compiler-rt/trunk/lib/scudo/scudo_allocator.h
===================================================================
--- compiler-rt/trunk/lib/scudo/scudo_allocator.h
+++ compiler-rt/trunk/lib/scudo/scudo_allocator.h
@@ -44,10 +44,10 @@
 typedef u64 PackedHeader;
 struct UnpackedHeader {
   u64 Checksum    : 16;
-  u64 UnusedBytes : 24; // Needed for reallocation purposes.
+  u64 UnusedBytes : 20; // Needed for reallocation purposes.
   u64 State       : 2;  // available, allocated, or quarantined
   u64 AllocType   : 2;  // malloc, new, new[], or memalign
-  u64 Offset      : 12; // Offset from the beginning of the backend
+  u64 Offset      : 16; // Offset from the beginning of the backend
                         // allocation to the beginning of the chunk itself,
                         // in multiples of MinAlignment. See comment about
                         // its maximum value and test in init().
Index: compiler-rt/trunk/lib/scudo/scudo_allocator.cpp
===================================================================
--- compiler-rt/trunk/lib/scudo/scudo_allocator.cpp
+++ compiler-rt/trunk/lib/scudo/scudo_allocator.cpp
@@ -68,7 +68,7 @@
 # elif SANITIZER_WORDSIZE == 64
 typedef TwoLevelByteMap<(NumRegions >> 12), 1 << 12> ByteMap;
 # endif  // SANITIZER_WORDSIZE
-typedef SizeClassMap<3, 4, 8, 16, 64, 14> SizeClassMap;
+typedef DefaultSizeClassMap SizeClassMap;
 typedef SizeClassAllocator32<0, SANITIZER_MMAP_RANGE_SIZE, 0, SizeClassMap,
     RegionSizeLog, ByteMap> PrimaryAllocator;
 #endif  // SANITIZER_CAN_USE_ALLOCATOR64
@@ -354,11 +354,11 @@
                      "header\n");
     }
     // Verify that we can fit the maximum amount of unused bytes in the header.
-    // The worst case scenario would be when allocating 1 byte on a MaxAlignment
-    // alignment. Since the combined allocator currently rounds the size up to
-    // the alignment before passing it to the secondary, we end up with
-    // MaxAlignment - 1 extra bytes.
-    uptr MaxUnusedBytes = MaxAlignment - 1;
+    // Given that the Secondary fits the allocation to a page, the worst case
+    // scenario happens in the Primary. It will depend on the second to last
+    // and last class sizes, as well as the dynamic base for the Primary. The
+    // following is an over-approximation that works for our needs.
+    uptr MaxUnusedBytes = SizeClassMap::kMaxSize - 1 - AlignedChunkHeaderSize;
     Header.UnusedBytes = MaxUnusedBytes;
     if (Header.UnusedBytes != MaxUnusedBytes) {
       dieWithMessage("ERROR: the maximum possible unused bytes doesn't fit in "