Simple cut of the kernel in place

csrc/amp_C_frontend.cpp

@@ -14,9 +14,16 @@ void multi_tensor_axpby_cuda(
   float b,
   int arg_to_check);
 
+at::Tensor multi_tensor_l2norm_cuda(
+  int chunk_size,
+  at::Tensor noop_flag,
+  std::vector<std::vector<at::Tensor>> tensor_lists);
+
 PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
   m.def("multi_tensor_scale", &multi_tensor_scale_cuda,
         "Fused overflow check + scale for a list of contiguous tensors");
   m.def("multi_tensor_axpby", &multi_tensor_axpby_cuda,
         "out = a*x + b*y for a list of contiguous tensors");
+  m.def("multi_tensor_l2norm", &multi_tensor_l2norm_cuda,
+        "Computes L2 norm for a list of contiguous tensors");
 }

csrc/type_shim.h

@@ -31,3 +31,54 @@ struct TypeShim
     default: \
       AT_ERROR(#NAME, " not implemented for '", toString(TYPE), "'");  \
   }
+
+
+template<typename T, typename ReduceOp>
+__device__ __forceinline__ T reduce_block_into_lanes
+  (T *x,
+   T val,
+   int lanes,
+   bool share_result) // lanes is intended to be <= 32.
+{
+  int tid = threadIdx.x + threadIdx.y*blockDim.x;
+  int blockSize = blockDim.x*blockDim.y; // blockSize is intended to be a multiple of 32.
+
+  if(blockSize >= 64)
+  {
+    x[tid] = val;
+    __syncthreads();
+  }
+
+  #pragma unroll
+  for(int i = (blockSize >> 1); i >= 64; i >>= 1)
+  {
+    if(tid < i)
+      x[tid] = x[tid] + x[tid+i];
+    __syncthreads();
+  }
+
+  T final;
+
+  if(tid < 32)
+  {
+    if(blockSize >= 64)
+      final = x[tid] + x[tid+32];
+    else
+      final = val;
+    // __SYNCWARP();
+
+    #pragma unroll
+    for(int i = 16; i >= lanes; i >>= 1)
+      final = final + __shfl_down_sync(0xffffffff, final, i);
+  }
+
+  if(share_result)
+  {
+    if(tid < lanes)
+      x[tid] = final; // EpilogueOp
+    // Make sure the smem result is visible to all warps.
+    __syncthreads();
+  }
+
+  return final;
+}

setup.py

@@ -71,7 +71,8 @@ if "--cuda_ext" in sys.argv:
             CUDAExtension(name='amp_C',
                           sources=['csrc/amp_C_frontend.cpp',
                                    'csrc/multi_tensor_scale_kernel.cu',
-                                   'csrc/multi_tensor_axpby_kernel.cu'],
+                                   'csrc/multi_tensor_axpby_kernel.cu',
+                                   'csrc/multi_tensor_l2norm_kernel.cu'],
                           extra_compile_args={'cxx': ['-O3'],
                                               'nvcc':['-lineinfo',
                                                       '-O3',