[OpenMP][libomptarget] Simplify warp master selection for data sharing

[gtbercea]

Summary

There is currently no supported situation where the warp master is not the first thread in the warp.

This also avoids the device execution from hanging on Volta GPUs when ballot_sync is called by a number of threads that is less that the size of a warp.

[grokos]

I completely missed this one. The proposed change makes sense.

libomptarget/deviceRTLs/nvptx/src/data_sharing.cu

 // as long as the size requested fits the pre-allocated size.
 EXTERN void* __kmpc_data_sharing_push_stack(size_t DataSize,
     int16_t UseSharedMemory) {
   // Frame pointer must be visible to all workers in the same warp.
   unsigned WID = getWarpId();
   void *&FrameP = DataSharingState.FramePtr[WID];
 
   // Only warp active master threads manage the stack.
-  if (IsWarpMasterActiveThread()) {
+  if (getThreadId() % WARPSIZE == 0) {
     // SlotP will point to either the shared memory slot or an existing
     // global memory slot.
     __kmpc_data_sharing_slot *&SlotP = DataSharingState.SlotPtr[WID];
     void *&StackP = DataSharingState.StackPtr[WID];
 
     // Compute the total memory footprint of the requested data.
     // The master thread requires a stack only for itself. A worker
     // thread (which at this point is a warp master) will require

 }
 
 // Pop the stack and free any memory which can be reclaimed.
 //
 // When the pop operation removes the last global memory slot,
 // reclaim all outstanding global memory slots since it is
 // likely we have reached the end of the kernel.
 EXTERN void __kmpc_data_sharing_pop_stack(void *FrameStart) {
-  if (IsWarpMasterActiveThread()) {
+  if (getThreadId() % WARPSIZE == 0) {
     unsigned WID = getWarpId();
 
     // Current slot
     __kmpc_data_sharing_slot *&SlotP = DataSharingState.SlotPtr[WID];
 
     // Pointer to next available stack.
     void *&StackP = DataSharingState.StackPtr[WID];