diff --git a/libcxxabi/src/cxa_demangle.cpp b/libcxxabi/src/cxa_demangle.cpp
index d70ffdf10651..1cdc29b811c3 100644
--- a/libcxxabi/src/cxa_demangle.cpp
+++ b/libcxxabi/src/cxa_demangle.cpp
@@ -19,6 +19,7 @@
 #include <algorithm>
 #include <numeric>
 #include <cassert>
+#include <cstddef>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -1890,7 +1891,7 @@ class BumpPointerAllocator {
   static constexpr size_t AllocSize = 4096;
   static constexpr size_t UsableAllocSize = AllocSize - sizeof(BlockMeta);
 
-  alignas(16) char InitialBuffer[AllocSize];
+  alignas(std::max_align_t) char InitialBuffer[AllocSize];
   BlockMeta* BlockList = nullptr;
 
   void grow() {
diff --git a/llvm/lib/Demangle/ItaniumDemangle.cpp b/llvm/lib/Demangle/ItaniumDemangle.cpp
index 8dd182fad150..c7bba85f5d88 100644
--- a/llvm/lib/Demangle/ItaniumDemangle.cpp
+++ b/llvm/lib/Demangle/ItaniumDemangle.cpp
@@ -17,6 +17,7 @@
 #include <cassert>
 #include <cctype>
 #include <cstdio>
+#include <cstddef>
 #include <cstdlib>
 #include <cstring>
 #include <numeric>
@@ -1950,7 +1951,7 @@ class BumpPointerAllocator {
   static constexpr size_t AllocSize = 4096;
   static constexpr size_t UsableAllocSize = AllocSize - sizeof(BlockMeta);
 
-  alignas(16) char InitialBuffer[AllocSize];
+  alignas(std::max_align_t) char InitialBuffer[AllocSize];
   BlockMeta* BlockList = nullptr;
 
   void grow() {