diff --git a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
--- a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
@@ -1585,6 +1585,9 @@
   Hasher.final(Hash);
   uint8_t Tag = Hash[0];
 
+  // TagMaskByte must be one less than a power of two for the logic below to
+  // work reliably.
+  assert((TagMaskByte & (TagMaskByte + 1)) == 0);
   for (GlobalVariable *GV : Globals) {
     Tag &= TagMaskByte;
     // Skip tag 0 in order to avoid collisions with untagged memory.