[mlir] Do not start threads in AsyncRuntime

[ezhulenev]

Summary

pthreads is not enabled for all builds by default

[jpienaar]

Thanks

[mehdi_amini]

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

62

Can we do:

#if LLVM_ENABLE_THREADS != 0
  std::thread thread([handle, resume]() { (*resume)(handle); });
#else
  (*resume)(handle);
#endif

[ezhulenev]

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

62

Did have much time to fix this issue, but it seems that LLVM_ENABLE_THREADS != enabled pthreads for some reason. Also internally we have LLVM_ENABLE_THREADS=0 (or undefined) but pthreads are enabled.

[mehdi_amini]

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

62

LLVM is using std::thread in other places guarded this way I believe, can you elaborate on what was the reason you removed it here?
(the revision does not provide the info right now)

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

mlirAsyncRuntimeAwaitToken(AsyncToken *token) {
   std::unique_lock<std::mutex> lock(token->mu);
   if (!token->ready)
     token->cv.wait(lock, [token] { return token->ready; });
   delete token;
 }
 
 extern "C" MLIR_ASYNCRUNTIME_EXPORT void
 mlirAsyncRuntimeExecute(CoroHandle handle, CoroResume resume) {
-  std::thread thread([handle, resume]() { (*resume)(handle); });
-  thread.detach();
+  (*resume)(handle);
 }
 
 extern "C" MLIR_ASYNCRUNTIME_EXPORT void
 mlirAsyncRuntimeAwaitTokenAndExecute(AsyncToken *token, CoroHandle handle,
                                      CoroResume resume) {
   std::unique_lock<std::mutex> lock(token->mu);
 
   auto execute = [token, handle, resume]() {