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Fix ROCm __syncthreads deadlock in compute_amax_and_quantize_kernel #5894

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Fix ROCm __syncthreads deadlock in compute_amax_and_quantize_kernel #5894

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13 changes: 9 additions & 4 deletions fbgemm_gpu/experimental/gen_ai/src/quantize/quantize.cu
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Original file line number Diff line number Diff line change
Expand Up @@ -1955,12 +1955,15 @@ __global__ void compute_amax_and_quantize_kernel(
const auto warp_id = threadIdx.y;
const int64_t idx = (block_idx + warp_id) * blocksize + thread_id;

if (idx >= n)
return;
// Do NOT early-return for out-of-range threads: compute_max() below uses a
// block-wide cub::BlockReduce + __syncthreads() that every thread must reach,
// otherwise the block deadlocks on ROCm/AMD. Mask the work with `active`.
const bool active = idx < n;

float scale = 1.f, unscale = 1.f;

float thread_val = x[idx];
// Inactive lanes feed 0 into the fabsf-max reduction (neutral).
float thread_val = active ? static_cast<float>(x[idx]) : 0.0f;
float block_amax = compute_max<THREAD_X, THREAD_Y>(thread_val, blocksize);

if (global_amax) {
Expand All @@ -1970,7 +1973,9 @@ __global__ void compute_amax_and_quantize_kernel(
compute_scale(scale, unscale, block_amax);
}

y[idx] = quantize(thread_val, scale, unscale);
if (active) {
y[idx] = quantize(thread_val, scale, unscale);
}
}

int ceil_div(const int a, const int b) {
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