Index: llvm/lib/Support/Threading.cpp
===================================================================
--- llvm/lib/Support/Threading.cpp
+++ llvm/lib/Support/Threading.cpp
@@ -83,6 +83,8 @@
   // the same interface as std::thread but requests the same stack size as the
   // main thread (8MB) before creation.
 const std::optional<unsigned> llvm::thread::DefaultStackSize = 8 * 1024 * 1024;
+#elif defined(_AIX)
+const std::optional<unsigned> llvm::thread::DefaultStackSize = 4 * 1024 * 1024;
 #else
 const std::optional<unsigned> llvm::thread::DefaultStackSize;
 #endif