[AIX] Define llvm::thread::DefaultStackSize to 4 megabytes on AIX

[w2yehia]

Summary

Link time thinLTO spawns pthreads to parallelize optimization and codegen of the input bitcode files.
On AIX, the pthread's stack size limit is ~192k for 64-bit programs.
If std::optional<unsigned> llvm::thread::DefaultStackSize is initialized, then its value is used on a call to pthread_attr_setstacksize before pthread_create occurs.

We think increasing the default thread stack size limit in llvm on AIX to 4mb make sense.

[ZarkoCA]

llvm/lib/Support/Threading.cpp

86–87

Minor nit to have the comment explaining why like Darwin does above.

llvm/lib/Support/Threading.cpp

 #include "llvm/Support/thread.h"
 
 #if defined(__APPLE__)
   // Darwin's default stack size for threads except the main one is only 512KB,
   // which is not enough for some/many normal LLVM compilations. This implements
   // 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
 
 
 #endif
 
 std::optional<ThreadPoolStrategy>