xref: /llvm-project-15.0.7/openmp/libomptarget/DeviceRTL/src/Kernel.cpp (revision 6ce43697)

//===--- Kernel.cpp - OpenMP device kernel interface -------------- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the kernel entry points for the device.
//
//===----------------------------------------------------------------------===//

#include "Debug.h"
#include "Interface.h"
#include "Mapping.h"
#include "State.h"
#include "Synchronization.h"
#include "Types.h"

using namespace _OMP;

#pragma omp begin declare target device_type(nohost)

static void inititializeRuntime(bool IsSPMD) {
  // Order is important here.
  synchronize::init(IsSPMD);
  mapping::init(IsSPMD);
  state::init(IsSPMD);
}

/// Simple generic state machine for worker threads.
static void genericStateMachine(IdentTy *Ident) {
  FunctionTracingRAII();

  uint32_t TId = mapping::getThreadIdInBlock();

  do {
    ParallelRegionFnTy WorkFn = nullptr;

    // Wait for the signal that we have a new work function.
    synchronize::threads();

    // Retrieve the work function from the runtime.
    bool IsActive = __kmpc_kernel_parallel(&WorkFn);

    // If there is nothing more to do, break out of the state machine by
    // returning to the caller.
    if (!WorkFn)
      return;

    if (IsActive) {
      ASSERT(!mapping::isSPMDMode());
      ((void (*)(uint32_t, uint32_t))WorkFn)(0, TId);
      __kmpc_kernel_end_parallel();
    }

    synchronize::threads();

  } while (true);
}

extern "C" {

/// Initialization
///
/// \param Ident               Source location identification, can be NULL.
///
int32_t __kmpc_target_init(IdentTy *Ident, int8_t Mode,
                           bool UseGenericStateMachine, bool) {
  FunctionTracingRAII();
  const bool IsSPMD = Mode & OMP_TGT_EXEC_MODE_SPMD;
  if (IsSPMD) {
    inititializeRuntime(/* IsSPMD */ true);
    synchronize::threadsAligned();
  } else {
    inititializeRuntime(/* IsSPMD */ false);
    // No need to wait since only the main threads will execute user
    // code and workers will run into a barrier right away.
  }

  if (IsSPMD) {
    state::assumeInitialState(IsSPMD);
    return -1;
  }

  if (mapping::isInitialThreadInLevel0(IsSPMD))
    return -1;

  // Enter the generic state machine if enabled and if this thread can possibly
  // be an active worker thread.
  //
  // The latter 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::getBlockSize() does not include any of the main
  // thread's warp, so none of its threads can ever be active worker threads.
  if (UseGenericStateMachine &&
      mapping::getThreadIdInBlock() < mapping::getBlockSize(IsSPMD)) {
    genericStateMachine(Ident);
  } else {
    // Retrieve the work function just to ensure we always call
    // __kmpc_kernel_parallel even if a custom state machine is used.
    // TODO: this is not super pretty. The problem is we create the call to
    // __kmpc_kernel_parallel in the openmp-opt pass but while we optimize it is
    // not there yet. Thus, we assume we never reach it from
    // __kmpc_target_deinit. That allows us to remove the store in there to
    // ParallelRegionFn, which leads to bad results later on.
    ParallelRegionFnTy WorkFn = nullptr;
    __kmpc_kernel_parallel(&WorkFn);
    ASSERT(WorkFn == nullptr);
  }

  return mapping::getThreadIdInBlock();
}

/// De-Initialization
///
/// In non-SPMD, this function releases the workers trapped in a state machine
/// and also any memory dynamically allocated by the runtime.
///
/// \param Ident Source location identification, can be NULL.
///
void __kmpc_target_deinit(IdentTy *Ident, int8_t Mode, bool) {
  FunctionTracingRAII();
  const bool IsSPMD = Mode & OMP_TGT_EXEC_MODE_SPMD;
  state::assumeInitialState(IsSPMD);
  if (IsSPMD)
    return;

  // Signal the workers to exit the state machine and exit the kernel.
  state::ParallelRegionFn = nullptr;
}

int8_t __kmpc_is_spmd_exec_mode() {
  FunctionTracingRAII();
  return mapping::isSPMDMode();
}
}

#pragma omp end declare target