diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_allocator_combined.h b/compiler-rt/lib/sanitizer_common/sanitizer_allocator_combined.h
index de96e27686ac..a6d0864193fe 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_allocator_combined.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_allocator_combined.h
@@ -49,16 +49,29 @@ class CombinedAllocator {
       size = 1;
     if (size + alignment < size) return ReturnNullOrDieOnBadRequest();
     if (check_rss_limit && RssLimitIsExceeded()) return ReturnNullOrDieOnOOM();
+    uptr original_size = size;
+    // If alignment requirements are to be fulfilled by the frontend allocator
+    // rather than by the primary or secondary, passing an alignment lower than
+    // or equal to 8 will prevent any further rounding up, as well as the later
+    // alignment check.
     if (alignment > 8)
       size = RoundUpTo(size, alignment);
     void *res;
     bool from_primary = primary_.CanAllocate(size, alignment);
+    // The primary allocator should return a 2^x aligned allocation when
+    // requested 2^x bytes, hence using the rounded up 'size' when being
+    // serviced by the primary. The secondary takes care of the alignment
+    // without such requirement, and allocating 'size' would use extraneous
+    // memory, so we employ 'original_size'.
     if (from_primary)
       res = cache->Allocate(&primary_, primary_.ClassID(size));
     else
-      res = secondary_.Allocate(&stats_, size, alignment);
+      res = secondary_.Allocate(&stats_, original_size, alignment);
     if (alignment > 8)
       CHECK_EQ(reinterpret_cast<uptr>(res) & (alignment - 1), 0);
+    // When serviced by the secondary, the chunk comes from a mmap allocation
+    // and will be zero'd out anyway. We only need to clear our the chunk if
+    // it was serviced by the primary, hence using the rounded up 'size'.
     if (cleared && res && from_primary)
       internal_bzero_aligned16(res, RoundUpTo(size, 16));
     return res;
diff --git a/compiler-rt/lib/scudo/scudo_allocator_secondary.h b/compiler-rt/lib/scudo/scudo_allocator_secondary.h
index 4b62b8a359fd..3b247bfb3134 100644
--- a/compiler-rt/lib/scudo/scudo_allocator_secondary.h
+++ b/compiler-rt/lib/scudo/scudo_allocator_secondary.h
@@ -46,7 +46,7 @@ class ScudoLargeMmapAllocator {
     uptr UserBeg = MapBeg + PageSize + HeadersSize;
     // In the event of larger alignments, we will attempt to fit the mmap area
     // better and unmap extraneous memory. This will also ensure that the
-    // offset field of the header stays small (it will always be 0).
+    // offset and unused bytes field of the header stay small.
     if (Alignment > MinAlignment) {
       if (UserBeg & (Alignment - 1))
         UserBeg += Alignment - (UserBeg & (Alignment - 1));
@@ -54,8 +54,9 @@ class ScudoLargeMmapAllocator {
       uptr NewMapBeg = UserBeg - HeadersSize;
       NewMapBeg = RoundDownTo(NewMapBeg, PageSize) - PageSize;
       CHECK_GE(NewMapBeg, MapBeg);
-      uptr NewMapSize = RoundUpTo(MapSize - Alignment, PageSize);
-      uptr NewMapEnd = NewMapBeg + NewMapSize;
+      uptr NewMapEnd =
+          RoundUpTo(UserBeg + Size - Alignment - AlignedChunkHeaderSize,
+                    PageSize) + PageSize;
       CHECK_LE(NewMapEnd, MapEnd);
       // Unmap the extra memory if it's large enough.
       uptr Diff = NewMapBeg - MapBeg;
@@ -65,8 +66,8 @@ class ScudoLargeMmapAllocator {
       if (Diff > PageSize)
         UnmapOrDie(reinterpret_cast<void *>(NewMapEnd), Diff);
       MapBeg = NewMapBeg;
-      MapSize = NewMapSize;
       MapEnd = NewMapEnd;
+      MapSize = NewMapEnd - NewMapBeg;
     }
     uptr UserEnd = UserBeg - AlignedChunkHeaderSize + Size;
     // For larger alignments, Alignment was added by the frontend to Size.
diff --git a/compiler-rt/lib/scudo/scudo_utils.cpp b/compiler-rt/lib/scudo/scudo_utils.cpp
index 2d66865b926b..c0269ec11efb 100644
--- a/compiler-rt/lib/scudo/scudo_utils.cpp
+++ b/compiler-rt/lib/scudo/scudo_utils.cpp
@@ -17,7 +17,9 @@
 #include <fcntl.h>
 #include <stdarg.h>
 #include <unistd.h>
-#include <cpuid.h>
+#if defined(__x86_64__) || defined(__i386__)
+# include <cpuid.h>
+#endif
 
 #include <cstring>