Cuda kernels for fast min/max reductions (#203)

* Add the min/max cuda kernels.

* Better integration of the cuda kernels.

candle-kernels/src/reduce.cu

@@ -2,6 +2,7 @@
 // https://people.maths.ox.ac.uk/gilesm/cuda/prac4/reduction.pdf
 #include "cuda_utils.cuh"
 #include<stdint.h>
+#include<cmath>
 
 const int BLOCK_SIZE = 1024;
 
@@ -27,7 +28,7 @@ __device__ void fast_sum(
   size_t tid = threadIdx.x;
   size_t dst_id = blockIdx.x;
 
-  shr[tid] = 0.0;
+  shr[tid] = 0;
   // Elements summed in this block range from dst_id * el_to_sum_per_block
   // to (dst_id + 1) * el_to_sum_per_block.
   size_t start_idx = dst_id * el_to_sum_per_block;
@@ -49,11 +50,113 @@ __device__ void fast_sum(
     if (tid < s) shr[tid] += shr[tid + s];
   }
 
-  if (tid == 0) atomicAdd(dst + dst_id, shr[0]);
+  if (tid == 0) dst[dst_id] = shr[0];
 }
 
-#define FAST_SUM_OP(TYPENAME, FN_NAME) \
-extern "C" __global__ void FN_NAME(  \
+template <typename T>
+__device__ void fast_max(
+    const size_t src_numel,
+    const size_t el_to_sum_per_block,
+    const size_t num_dims, 
+    const size_t *info,
+    const T *src,
+    T *dst
+) {
+  const size_t *dims = info;
+  const size_t *strides = info + num_dims;
+
+  __shared__ T shr[BLOCK_SIZE];
+  size_t tid = threadIdx.x;
+  size_t dst_id = blockIdx.x;
+
+  shr[tid] = -INFINITY;
+  // Elements summed in this block range from dst_id * el_to_sum_per_block
+  // to (dst_id + 1) * el_to_sum_per_block.
+  size_t start_idx = dst_id * el_to_sum_per_block;
+  size_t stop_idx = min(start_idx + el_to_sum_per_block, src_numel);
+  size_t idx = start_idx + tid;
+
+  while (idx < stop_idx) {
+    // TODO: Fast version for the contiguous case.
+    size_t strided_i = get_strided_index(idx, num_dims, dims, strides);
+    shr[tid] = maxg(shr[tid], src[strided_i]);
+    idx += blockDim.x;
+  }
+
+  // Parallel reduction, see the slides:
+  // https://www.olcf.ornl.gov/wp-content/uploads/2019/12/05_Atomics_Reductions_Warp_Shuffle.pdf
+  // https://stackoverflow.com/questions/66078814/is-cuda-atomicadd-operation-faster-than-launch-another-kernel-when-we-do-reduce
+  for (int s = blockDim.x / 2; s > 0; s >>= 1) {
+    __syncthreads();
+    if (tid < s) shr[tid] = maxg(shr[tid], shr[tid + s]);
+  }
+
+  if (tid == 0) dst[dst_id] = shr[0];
+}
+
+template <typename T>
+__device__ void fast_min(
+    const size_t src_numel,
+    const size_t el_to_sum_per_block,
+    const size_t num_dims, 
+    const size_t *info,
+    const T *src,
+    T *dst
+) {
+  const size_t *dims = info;
+  const size_t *strides = info + num_dims;
+
+  __shared__ T shr[BLOCK_SIZE];
+  size_t tid = threadIdx.x;
+  size_t dst_id = blockIdx.x;
+
+  shr[tid] = INFINITY;
+  // Elements summed in this block range from dst_id * el_to_sum_per_block
+  // to (dst_id + 1) * el_to_sum_per_block.
+  size_t start_idx = dst_id * el_to_sum_per_block;
+  size_t stop_idx = min(start_idx + el_to_sum_per_block, src_numel);
+  size_t idx = start_idx + tid;
+
+  while (idx < stop_idx) {
+    // TODO: Fast version for the contiguous case.
+    size_t strided_i = get_strided_index(idx, num_dims, dims, strides);
+    shr[tid] = ming(shr[tid], src[strided_i]);
+    idx += blockDim.x;
+  }
+
+  // Parallel reduction, see the slides:
+  // https://www.olcf.ornl.gov/wp-content/uploads/2019/12/05_Atomics_Reductions_Warp_Shuffle.pdf
+  // https://stackoverflow.com/questions/66078814/is-cuda-atomicadd-operation-faster-than-launch-another-kernel-when-we-do-reduce
+  for (int s = blockDim.x / 2; s > 0; s >>= 1) {
+    __syncthreads();
+    if (tid < s) shr[tid] = ming(shr[tid], shr[tid + s]);
+  }
+
+  if (tid == 0) dst[dst_id] = shr[0];
+}
+
+#define FAST_OP(TYPENAME, MIN_NAME, MAX_NAME, SUM_NAME) \
+extern "C" __global__ void MIN_NAME(  \
+    const size_t src_numel, \
+    const size_t el_to_sum_per_block, \
+    const size_t num_dims,  \
+    const size_t *info, \
+    const TYPENAME *src, \
+    TYPENAME *dst \
+) {  \
+  fast_min(src_numel, el_to_sum_per_block, num_dims, info, src, dst); \
+} \
+extern "C" __global__ void MAX_NAME(  \
+    const size_t src_numel, \
+    const size_t el_to_sum_per_block, \
+    const size_t num_dims,  \
+    const size_t *info, \
+    const TYPENAME *src, \
+    TYPENAME *dst \
+) {  \
+  fast_max(src_numel, el_to_sum_per_block, num_dims, info, src, dst); \
+} \
+extern "C" __global__ void SUM_NAME(  \
     const size_t src_numel, \
     const size_t el_to_sum_per_block, \
     const size_t num_dims,  \
@@ -106,18 +209,18 @@ extern "C" __global__ void FN_NAME(  \
 
 #if __CUDA_ARCH__ >= 800
 SUM_OP(__nv_bfloat16, sum_bf16)
-FAST_SUM_OP(__nv_bfloat16, fast_sum_bf16)
+FAST_OP(__nv_bfloat16, fast_min_bf16, fast_max_bf16, fast_sum_bf16)
 #endif
 
 #if __CUDA_ARCH__ >= 530
 SUM_OP(__half, sum_f16)
-FAST_SUM_OP(__half, fast_sum_f16)
+FAST_OP(__half, fast_min_f16, fast_max_f16, fast_sum_f16)
 #endif
 
 SUM_OP(float, sum_f32)
 SUM_OP(double, sum_f64)
 SUM_OP(uint32_t, sum_u32)
 
-FAST_SUM_OP(float, fast_sum_f32)
-FAST_SUM_OP(double, fast_sum_f64)
-FAST_SUM_OP(uint32_t, fast_sum_u32)
+FAST_OP(float, fast_min_f32, fast_max_f32, fast_sum_f32)
+FAST_OP(double, fast_min_f64, fast_max_f64, fast_sum_f64)
+FAST_OP(uint32_t, fast_min_u32, fast_max_u32, fast_sum_u32)