[Openmp-commits] [openmp] [Offload][OpenMP][libdevice] Make check to enter state machine architecture dependent (PR #188144)

[Kevin Sala Penades via Openmp-commits]

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@@ -62,6 +62,45 @@ bool mayUseThreadStates();
 /// parallelism, or if it was explicitly disabled by the user.
 bool mayUseNestedParallelism();
 
+/// Returns true if the current thread should enter the generic state machine.
+/// On some architectures, some threads should not enter the state machine to
+/// avoid warp-level barrier forwarding issues during initialization.
+/// On other architectures, all threads must enter the state machine to satisfy
+/// the requirements of workgroup synchronization.
+static inline bool shouldEnterStateMachine(bool IsSPMD);
+
+} // namespace config
+} // namespace ompx
+
+#include "Mapping.h"
+
+namespace ompx {
+namespace config {
+
+static inline bool shouldEnterStateMachine(bool IsSPMD) {
+#if defined(__NVPTX__) || defined(__AMDGPU__)
+  // This check is important for NVIDIA Pascal (but not Volta) and AMD
+  // GPU. In those cases, a single thread can apparently satisfy a barrier on
+  // behalf of all threads in the same warp. Thus, it would not be safe for
+  // other threads in the main thread's warp to reach the first
+  // synchronize::threads call in genericStateMachine before the main thread
+  // reaches its corresponding synchronize::threads call: that would permit all
+  // active worker threads to proceed before the main thread has actually set
+  // state::ParallelRegionFn, and then they would immediately quit without
+  // doing any work.  mapping::getMaxTeamThreads() does not include any of the
+  // main thread's warp, so none of its threads can ever be active worker
+  // threads.
+  return mapping::getThreadIdInBlock() < mapping::getMaxTeamThreads(IsSPMD);
+#else
+  // On other architectures (e.g., Intel GPUs) all threads must enter the state
+  // machine to satisfy the requirements of workgroup of synchronize::threads
+  // call in genericStateMachine. Otherwise, the workers will wait on the
+  // call to synchronize::threads forever and never proceed.
+  (void)IsSPMD;
+  return true;
+#endif
+}
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kevinsala wrote:

I don't think this is the proper file for this function. I would directly define it in Kernel.cpp, near the `genericStateMachine()` function.

https://github.com/llvm/llvm-project/pull/188144