semaphore.h

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/*! \file
    \brief Implementation of a CTA-wide semaphore for inter-CTA synchronization.
*/

#pragma once

/////////////////////////////////////////////////////////////////////////////////////////////////

// namespace cutlass {

/////////////////////////////////////////////////////////////////////////////////////////////////

/// CTA-wide semaphore for inter-CTA synchronization.
class Semaphore {
public:
    int *lock;
    bool wait_thread;
    int state;

public:
    /// Implements a semaphore to wait for a flag to reach a given value
    __host__ __device__ Semaphore(int *lock_, int thread_id)
        : lock(lock_), wait_thread(thread_id < 0 || thread_id == 0), state(-1) {}

    /// Permit fetching the synchronization mechanism early
    __device__ void fetch() {
        if (wait_thread) {
#if defined(__CUDA_ARCH__) && __CUDA_ARCH__ >= 700
            asm volatile("ld.global.acquire.gpu.b32 %0, [%1];\n" : "=r"(state) : "l"(lock));
#else
            asm volatile("ld.global.cg.b32 %0, [%1];\n" : "=r"(state) : "l"(lock));
#endif
        }
    }

    /// Gets the internal state
    __device__ int get_state() const {
        return state;
    }

    /// Waits until the semaphore is equal to the given value
    __device__ void wait(int status = 0) {
        while (__syncthreads_and(state != status)) {
            fetch();
        }

        __syncthreads();
    }

    /// Updates the lock with the given result
    __device__ void release(int status = 0) {
        __syncthreads();

        if (wait_thread) {
#if defined(__CUDA_ARCH__) && __CUDA_ARCH__ >= 700
            asm volatile("st.global.release.gpu.b32 [%0], %1;\n" : : "l"(lock), "r"(status));
#else
            asm volatile("st.global.cg.b32 [%0], %1;\n" : : "l"(lock), "r"(status));
#endif
        }
    }
};

/////////////////////////////////////////////////////////////////////////////////////////////////

// } // namespace cutlass

/////////////////////////////////////////////////////////////////////////////////////////////////