CUDA backend: reduce (#2269)

mlx/backend/cuda/reduce/reduce_ops.cuh

@@ -0,0 +1,144 @@
+// Copyright © 2025 Apple Inc.
+
+#pragma once
+
+#include "mlx/backend/cuda/kernels/utils.cuh"
+
+namespace mlx::core::cu {
+
+// Reduce ops.
+struct And {
+  __device__ bool operator()(bool a, bool b) {
+    return a && b;
+  }
+};
+
+struct Or {
+  __device__ bool operator()(bool a, bool b) {
+    return a || b;
+  }
+};
+
+struct Sum {
+  template <typename T>
+  __device__ T operator()(T a, T b) {
+    return a + b;
+  }
+};
+
+struct Prod {
+  template <typename T>
+  __device__ T operator()(T a, T b) {
+    return a * b;
+  }
+};
+
+struct Min {
+  template <typename T>
+  __device__ T operator()(T a, T b) {
+    return a < b ? a : b;
+  }
+};
+
+struct Max {
+  template <typename T>
+  __device__ T operator()(T a, T b) {
+    return a > b ? a : b;
+  }
+};
+
+// Traits to get the result type of reduce op.
+template <typename Op, typename T>
+struct ReduceResult;
+
+template <typename T>
+struct ReduceResult<And, T> {
+  using type = bool;
+};
+
+template <typename T>
+struct ReduceResult<Or, T> {
+  using type = bool;
+};
+
+template <typename T>
+struct ReduceResult<Sum, T> {
+  using type = cuda::std::conditional_t<
+      (cuda::std::is_integral_v<T> && sizeof(T) <= 4),
+      int32_t,
+      T>;
+};
+
+template <typename T>
+struct ReduceResult<Prod, T> {
+  using type = cuda::std::conditional_t<
+      (cuda::std::is_integral_v<T> && sizeof(T) <= 4),
+      int32_t,
+      T>;
+};
+
+template <typename T>
+struct ReduceResult<Min, T> {
+  using type = T;
+};
+
+template <typename T>
+struct ReduceResult<Max, T> {
+  using type = T;
+};
+
+// Traits to get the init value of reduce op.
+template <typename Op, typename T>
+struct ReduceInit;
+
+template <typename T>
+struct ReduceInit<And, T> {
+  static constexpr __host__ __device__ bool value() {
+    return true;
+  }
+};
+
+template <typename T>
+struct ReduceInit<Or, T> {
+  static constexpr __host__ __device__ bool value() {
+    return false;
+  }
+};
+
+template <typename T>
+struct ReduceInit<Sum, T> {
+  static constexpr __host__ __device__ auto value() {
+    if constexpr (cuda::std::is_same_v<T, cuComplex>) {
+      return T{0, 0};
+    } else {
+      return typename ReduceResult<Sum, T>::type{0};
+    }
+  }
+};
+
+template <typename T>
+struct ReduceInit<Prod, T> {
+  static constexpr __host__ __device__ auto value() {
+    if constexpr (cuda::std::is_same_v<T, cuComplex>) {
+      return T{1, 1};
+    } else {
+      return typename ReduceResult<Prod, T>::type{1};
+    }
+  }
+};
+
+template <typename T>
+struct ReduceInit<Min, T> {
+  static constexpr __host__ __device__ T value() {
+    return Limits<T>::max();
+  }
+};
+
+template <typename T>
+struct ReduceInit<Max, T> {
+  static constexpr __host__ __device__ T value() {
+    return Limits<T>::min();
+  }
+};
+
+} // namespace mlx::core::cu