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minimize gpuAtomicAdd overhead in bounds_check_indices_kernel_v2 (#5124) #5171

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minimize gpuAtomicAdd overhead in bounds_check_indices_kernel_v2 (#5124) #5171

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31 changes: 31 additions & 0 deletions fbgemm_gpu/codegen/utils/embedding_bounds_check_v2.cu
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,12 @@ __global__ __launch_bounds__(kMaxThreads) void bounds_check_indices_kernel_v2(
index_t invalid_i = -1, invalid_idx = -1;
int32_t invalid_b_t = -1;
int64_t warning_inc = 0;
#ifdef USE_ROCM
__shared__ int64_t block_warning_buffer[kMaxThreads];
const int linear_tid = threadIdx.z * (blockDim.y * blockDim.x) +
threadIdx.y * blockDim.x + threadIdx.x;
const int active_threads = blockDim.x * blockDim.y * blockDim.z;
#endif

// Check the last element
if (b_t_start == 0 && threadIdx.x == 0) {
Expand Down Expand Up @@ -142,9 +148,34 @@ __global__ __launch_bounds__(kMaxThreads) void bounds_check_indices_kernel_v2(
}
} // for b_t

#ifdef USE_ROCM
// Accumulate per-thread warning counts in shared memory and reduce once per
// block.
block_warning_buffer[linear_tid] = warning_inc;
__syncthreads();

// Parallel tree reduction
for (int stride = active_threads / 2; stride > 0; stride >>= 1) {
if (linear_tid < stride) {
block_warning_buffer[linear_tid] +=
block_warning_buffer[linear_tid + stride];
}
__syncthreads();
}

// Thread 0 has the final sum
if (linear_tid == 0) {
int64_t block_warning_sum = block_warning_buffer[0];
if (block_warning_sum > 0) {
gpuAtomicAdd(&warning[0], block_warning_sum);
}
}
__syncthreads();
#else
if (warning_inc > 0) {
gpuAtomicAdd(&warning[0], warning_inc);
}
#endif
if (bounds_check_mode == BoundsCheckMode::WARNING && invalid_i != -1 &&
static_cast<int64_t>(atomicAdd(
reinterpret_cast<unsigned long long int*>(&warning[0]), 0)) == 0) {
Expand Down
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