Index: libcxxabi/src/demangle/Utility.h
===================================================================
--- libcxxabi/src/demangle/Utility.h
+++ libcxxabi/src/demangle/Utility.h
@@ -33,12 +33,18 @@
   size_t CurrentPosition = 0;
   size_t BufferCapacity = 0;
 
-  // Ensure there is at least n more positions in buffer.
+  // Ensure there is at least N more positions in the buffer.
   void grow(size_t N) {
-    if (N + CurrentPosition >= BufferCapacity) {
+    size_t Need = N + CurrentPosition;
+    if (Need > BufferCapacity) {
       BufferCapacity *= 2;
-      if (BufferCapacity < N + CurrentPosition)
-        BufferCapacity = N + CurrentPosition;
+      if (Need > BufferCapacity) {
+        // Avoid many reallocations during startup
+        constexpr size_t MinAlloc = 1024;
+        if (!BufferCapacity && Need < MinAlloc)
+          Need = MinAlloc;
+        BufferCapacity = Need;
+      }
       Buffer = static_cast<char *>(std::realloc(Buffer, BufferCapacity));
       if (Buffer == nullptr)
         std::terminate();
Index: llvm/include/llvm/Demangle/Utility.h
===================================================================
--- llvm/include/llvm/Demangle/Utility.h
+++ llvm/include/llvm/Demangle/Utility.h
@@ -33,12 +33,18 @@
   size_t CurrentPosition = 0;
   size_t BufferCapacity = 0;
 
-  // Ensure there is at least n more positions in buffer.
+  // Ensure there is at least N more positions in the buffer.
   void grow(size_t N) {
-    if (N + CurrentPosition >= BufferCapacity) {
+    size_t Need = N + CurrentPosition;
+    if (Need > BufferCapacity) {
       BufferCapacity *= 2;
-      if (BufferCapacity < N + CurrentPosition)
-        BufferCapacity = N + CurrentPosition;
+      if (Need > BufferCapacity) {
+        // Avoid many reallocations during startup
+        constexpr size_t MinAlloc = 1024;
+        if (!BufferCapacity && Need < MinAlloc)
+          Need = MinAlloc;
+        BufferCapacity = Need;
+      }
       Buffer = static_cast<char *>(std::realloc(Buffer, BufferCapacity));
       if (Buffer == nullptr)
         std::terminate();