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fix(warp_reduce): add explicit add_op overload to resolve CUB template ambiguity on CUDA 13.2+ #3050

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fix(warp_reduce): add explicit add_op overload to resolve CUB template ambiguity on CUDA 13.2+ #3050

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fix(warp_reduce): add explicit add_op overload to resolve CUB templat…
zbrad Jun 8, 2026
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zbrad Jun 8, 2026
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Merge branch 'main' into fix/warp-reduce-cub-ambiguity
zbrad Jul 25, 2026
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7 changes: 7 additions & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
@@ -1,3 +1,10 @@
## build logs
build_*.log

@coderabbitai coderabbitai Bot Jul 25, 2026

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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Match the build log generated by CI.

.github/workflows/pr.yaml:401-402 writes telemetry-artifacts/build.log, but build_*.log only matches filenames beginning with build_. Use build*.log or the more explicit telemetry-artifacts/*.log.

Proposed fix 
-build_*.log
+build*.log
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
build_*.log
build*.log
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In @.gitignore at line 2, Update the build log ignore pattern in .gitignore from
build_*.log so it matches the CI-generated telemetry-artifacts/build.log, using
either build*.log or the more explicit telemetry-artifacts/*.log pattern.


## aarch64 build artifacts
cpp/build-aarch64/
*.tar.bz2

## common
__pycache__
*.pyc
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15 changes: 15 additions & 0 deletions cpp/include/raft/util/reduction.cuh
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Expand Up @@ -52,6 +52,21 @@ DI T warpReduce(T val, ReduceLambda reduce_op)
return logicalWarpReduce<WarpSize>(val, reduce_op);
}

/**
* @brief Warp-level reduction with raft::add_op
* @tparam T Value type to be reduced
* @param val input value
* @param reduce_op raft::add_op instance
* @return Reduction result. All lanes will have the valid result.
* @note Explicit overload to disambiguate from cub::detail::scan::warpReduce when
* called with raft::add_op; avoids ambiguous template instantiation on CUDA 13.2+
*/
template <typename T>
DI T warpReduce(T val, raft::add_op reduce_op)
{
return logicalWarpReduce<WarpSize>(val, reduce_op);
}

/**
* @brief Warp-level reduction
* @tparam T Value type to be reduced
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38 changes: 36 additions & 2 deletions cpp/tests/util/reduction.cu
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Expand Up @@ -31,6 +31,15 @@ RAFT_KERNEL test_reduction_kernel(const int* input, int* reduction_res, ReduceLa
if (threadIdx.x == 0) { reduction_res[0] = th_val; }
}

// regression: warpReduce(val, raft::add_op{}) must not cause ambiguity with CUB on CUDA 13.2+
RAFT_KERNEL test_warp_reduce_with_add_op_kernel(const int* input, int* reduction_res)
{
assert(gridDim.x == 1);
int th_val = input[threadIdx.x];
th_val = raft::warpReduce(th_val, raft::add_op{});

@achirkin achirkin Jun 8, 2026

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Does this really cause an ambiguity without the extra overload in util/reduction.cuh? It's called here with raft namespace, so I doubt CUB overload is ever picked up here.

@zbrad zbrad Jun 8, 2026

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It showed up for me when trying to do a full source rebuild of cuvs on the dgx spark. When finally debugging my build failure, it traced down to raft, but only showed up when building for arm64.

@zbrad zbrad Jun 8, 2026 •
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Oh, I have the cuvs regression test for it that I'm submitting to cuvs, it's at cpp/tests/regression/warp_reduce_add_op.cu

if (threadIdx.x % 32 == 0) { atomicAdd(reduction_res, th_val); }
}

template <typename ReduceLambda>
RAFT_KERNEL test_ranked_reduction_kernel(const int* input,
int* reduction_res,
Expand Down Expand Up @@ -131,6 +140,20 @@ struct reduction_launch {
RAFT_CUDA_TRY(cudaPeekAtLastError());
ASSERT_EQ(ref_d.value(stream), ref_val);
}

static void run_warp_reduce_with_add_op(const rmm::device_uvector<int>& arr_d,
int ref_val,
rmm::cuda_stream_view stream)
{
rmm::device_scalar<int> ref_d(0, stream);
const int block_dim = 64;
const int grid_dim = 1;
test_warp_reduce_with_add_op_kernel<<<grid_dim, block_dim, 0, stream>>>(arr_d.data(),
ref_d.data());
stream.synchronize();
RAFT_CUDA_TRY(cudaPeekAtLastError());
ASSERT_EQ(ref_d.value(stream), ref_val);
}
};

template <typename T>
Expand Down Expand Up @@ -162,6 +185,12 @@ class ReductionTest : public testing::TestWithParam<std::vector<int>> { // NOLI
}

void run_binary_reduction() { reduction_launch::run_binary(arr_d, 24, stream); }

void run_warp_reduce_with_add_op()
{
// two warps of 32 threads; warp 0 sums elements 0-31 (=78), warp 1 sums 32-63 (=80)
reduction_launch::run_warp_reduce_with_add_op(arr_d, 158, stream);
}
};

const std::vector<int> test_vector{1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 5, 1, 2, 3, 4, 1, 2,
Expand All @@ -173,13 +202,18 @@ const std::vector<int> binary_test_vector{
auto reduction_input = ::testing::Values(test_vector);
auto binary_reduction_input = ::testing::Values(binary_test_vector);

using ReductionTestInt = ReductionTest<int>; // NOLINT
using BinaryReductionTestInt = ReductionTest<int>; // NOLINT
using ReductionTestInt = ReductionTest<int>; // NOLINT
using BinaryReductionTestInt = ReductionTest<int>; // NOLINT
using WarpReduceAddOpTestInt = ReductionTest<int>; // NOLINT
TEST_P(ReductionTestInt, REDUCTIONS) { run_reduction(); }
INSTANTIATE_TEST_CASE_P(ReductionTest, ReductionTestInt, reduction_input); // NOLINT
TEST_P(BinaryReductionTestInt, BINARY_REDUCTION) { run_binary_reduction(); } // NOLINT
INSTANTIATE_TEST_CASE_P(BinaryReductionTest,
BinaryReductionTestInt,
binary_reduction_input); // NOLINT
TEST_P(WarpReduceAddOpTestInt, WARP_REDUCE_WITH_ADD_OP) { run_warp_reduce_with_add_op(); } // NOLINT
INSTANTIATE_TEST_CASE_P(WarpReduceAddOpTest,
WarpReduceAddOpTestInt,
reduction_input); // NOLINT

} // namespace raft::util
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