Enable using threads on z/OS (#171847)

z/OS 3.1 enables TLS support (limited to compile time constant
initializers). To enable building with thread support, we need to update
the code to handle the difference in definition of pthread_t. It is a
struct on z/OS, not an integer. The existing code assumes that pthread_t
is an integer. This usually happens when checking to see if pthread_t is
null or not.

In Parallel.cpp, there was a variable `Backoff` defined as TLS. The
initializer for this requires C++ initialization which isn't supported
on z/OS. The variable isn't actually used (see declaration of local var
with same name inside the loop) so deleting it solved the build failure
this was causing.

llvm/CMakeLists.txt

@@ -597,12 +597,7 @@ option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
 option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON)
 
-# On z/OS, threads cannot be used because TLS is not supported.
-if (CMAKE_SYSTEM_NAME MATCHES "OS390")
-  option(LLVM_ENABLE_THREADS "Use threads if available." OFF)
-else()
-  option(LLVM_ENABLE_THREADS "Use threads if available." ON)
-endif()
+option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
 set(LLVM_ENABLE_ICU "OFF" CACHE STRING "Use ICU for text encoding conversion support if available. Can be ON, OFF, or FORCE_ON")
 

llvm/include/llvm/Support/thread.h

@@ -51,7 +51,11 @@ class thread {
 public:
 #ifdef LLVM_ON_UNIX
   using native_handle_type = pthread_t;
+#ifdef __MVS__
+  using id = unsigned long long;
+#else
   using id = pthread_t;
+#endif
   using start_routine_type = void *(*)(void *);
 
   template <typename CalleeTuple> static void *ThreadProxy(void *Ptr) {
@@ -97,7 +101,7 @@ public:
     return *this;
   }
 
-  bool joinable() const noexcept { return Thread != native_handle_type(); }
+  bool joinable() const noexcept { return get_id() != 0; }
 
   inline id get_id() const noexcept;
 
@@ -133,7 +137,7 @@ thread::thread(std::optional<unsigned> StackSizeInBytes, Function &&f,
 
   Thread = llvm_execute_on_thread_impl(ThreadProxy<CalleeTuple>, Callee.get(),
                                        StackSizeInBytes);
-  if (Thread != native_handle_type())
+  if (joinable())
     Callee.release();
 }
 

llvm/lib/Support/Parallel.cpp

@@ -129,8 +129,6 @@ private:
     // first successful tryAcquire() in a process. This guarantees forward
     // progress without requiring a dedicated "always-on" thread here.
 
-    static thread_local std::unique_ptr<ExponentialBackoff> Backoff;
-
     while (true) {
       if (TheJobserver) {
         // Jobserver-mode scheduling:

llvm/lib/Support/Unix/Threading.inc

@@ -119,9 +119,17 @@ void llvm_thread_join_impl(pthread_t Thread) {
   }
 }
 
-pthread_t llvm_thread_get_id_impl(pthread_t Thread) { return Thread; }
+llvm::thread::id llvm_thread_get_id_impl(pthread_t Thread) {
+#ifdef __MVS__
+  return Thread.__;
+#else
+  return Thread;
+#endif
+}
 
-pthread_t llvm_thread_get_current_id_impl() { return ::pthread_self(); }
+llvm::thread::id llvm_thread_get_current_id_impl() {
+  return llvm_thread_get_id_impl(::pthread_self());
+}
 
 } // namespace llvm
 
@@ -148,6 +156,8 @@ uint64_t llvm::get_threadid() {
   return uint64_t(syscall(__NR_gettid));
 #elif defined(_AIX)
   return uint64_t(thread_self());
+#elif defined(__MVS__)
+  return llvm_thread_get_id_impl(pthread_self());
 #else
   return uint64_t(pthread_self());
 #endif