From 8e1b020488a8b9abd116fa0458a1bec0221ace43 Mon Sep 17 00:00:00 2001
From: Zhang Zheng <32410583+ZzSean@users.noreply.github.com>
Date: Wed, 9 Feb 2022 16:54:39 +0800
Subject: [PATCH] Optimize performance of softmax_fwd when axis!=-1 (#38602)

* Optimize performence of softmax_fwd when axis!=-1

* use functor

* support hip

* fix functor
---
 paddle/fluid/operators/softmax_cudnn_op.cu.h | 165 ++++++++++++++++++-
 1 file changed, 164 insertions(+), 1 deletion(-)

diff --git a/paddle/fluid/operators/softmax_cudnn_op.cu.h b/paddle/fluid/operators/softmax_cudnn_op.cu.h
index 236ea448f30..85bc3946bae 100644
--- a/paddle/fluid/operators/softmax_cudnn_op.cu.h
+++ b/paddle/fluid/operators/softmax_cudnn_op.cu.h
@@ -186,6 +186,58 @@ struct UnaryDivFunctor {
   Tx n_inv;
 };
 
+template <typename Tx, typename Ty = Tx>
+struct SoftmaxForwardFunctor {
+  HOSTDEVICE inline SoftmaxForwardFunctor(Tx max, Tx sum)
+      : max(max), sum(sum) {}
+
+  HOSTDEVICE inline Ty operator()(const Tx& x) const {
+    return static_cast<Ty>(std::exp(x - max) / sum);
+  }
+
+ private:
+  Tx max;
+  Tx sum;
+};
+
+template <typename Tx, typename Ty = Tx>
+struct SoftmaxBackwardFunctor {
+  HOSTDEVICE inline SoftmaxBackwardFunctor(Tx sum) : sum(sum) {}
+
+  HOSTDEVICE inline Ty operator()(const Tx& grad_out, const Tx& out) const {
+    return static_cast<Ty>(out * (grad_out - sum));
+  }
+
+ private:
+  Tx sum;
+};
+
+template <typename Tx, typename Ty = Tx>
+struct LogSoftmaxForwardFunctor {
+  HOSTDEVICE inline LogSoftmaxForwardFunctor(Tx max, Tx sum)
+      : max(max), log_sum(std::log(sum)) {}
+
+  HOSTDEVICE inline Ty operator()(const Tx& x) const {
+    return static_cast<Ty>(x - max - log_sum);
+  }
+
+ private:
+  Tx max;
+  Tx log_sum;
+};
+
+template <typename Tx, typename Ty = Tx>
+struct LogSoftmaxBackwardFunctor {
+  HOSTDEVICE inline LogSoftmaxBackwardFunctor(Tx sum) : sum(sum) {}
+
+  HOSTDEVICE inline Ty operator()(const Tx& grad_out, const Tx& out) const {
+    return static_cast<Ty>(grad_out - std::exp(out) * sum);
+  }
+
+ private:
+  Tx sum;
+};
+
 /*
 Core function of computing softmax forward for axis=-1.
 The computation includes
@@ -255,7 +307,8 @@ __global__ void WarpSoftmaxForward(T* softmax, const T* src,
                                  ReduceMaxFunctor<AccT>(), true);
   WarpReduceMax<AccT, kBatchSize, kWarpSize>(max);
 
-  // compute sum
+// compute sum
+#pragma unroll
   for (int i = 0; i < kBatchSize; ++i) {
     kps::ElementwiseUnary<AccT, AccT, kVItem, 1, 1, ExpSubFunctor<AccT>>(
         &srcdata[i][0][0], &srcdata[i][0][0], ExpSubFunctor<AccT>(max[i]));
@@ -443,6 +496,113 @@ void SwitchWarpSoftmaxBackward(const int blocks, const dim3 threads,
 #undef SOFTMAX_WARP_FORWARD_CASE
 #undef SOFTMAX_WARP_BACKWARD_CASE
 
+/**
+ * <NormalSoftmaxKernel>
+ * Better performence when axis != -1
+ */
+
+static void GetGridDim(int high_dim, int mid_dim, int low_dim,
+                       const dim3& block, dim3* grid) {
+  int device_id = paddle::platform::GetCurrentDeviceId();
+  int max_mp = paddle::platform::GetGPUMultiProcessors(device_id);
+  int max_threads_per_mp =
+      paddle::platform::GetGPUMaxThreadsPerMultiProcessor(device_id);
+  int max_threads = max_threads_per_mp * max_mp;
+  int num_threads = block.x * block.y;
+  int max_num_blocks = max_threads / num_threads;
+
+  int grid_x = (low_dim + block.x - 1) / block.x;
+  grid_x = std::min(grid_x, max_num_blocks);
+  int grid_y = (max_num_blocks + grid_x - 1) / grid_x;
+  grid_y = std::min(grid_y, high_dim);
+  grid->x = grid_x;
+  grid->y = grid_y;
+}
+
+static void GetBlockDim(int mid_dim, int low_dim, dim3* block) {
+#ifdef __HIPCC__
+  constexpr int max_num_threads = 256;
+#else
+  constexpr int max_num_threads = 1024;
+#endif
+  int block_x = 1 << log2_ceil(low_dim);
+  int block_y = 1 << log2_ceil(mid_dim);
+  block->x = std::min(block_x, 32);
+  block->y = std::min(block_y, static_cast<int>(max_num_threads / block->x));
+  block->x = std::min(block_x, static_cast<int>(max_num_threads / block->y));
+}
+
+static void GetLaunchConfig(int high_dim, int mid_dim, int low_dim, dim3* grid,
+                            dim3* block) {
+  GetBlockDim(mid_dim, low_dim, block);
+  GetGridDim(high_dim, mid_dim, low_dim, *block, grid);
+}
+
+template <typename T, typename AccT,
+          template <typename, typename> class Functor>
+__global__ void NormalSoftmaxForward(T* output, const T* input, int high_dim,
+                                     int mid_dim, int low_dim) {
+  using kMode = kps::details::ReduceMode;
+  const int high_stride = mid_dim * low_dim;
+  const int mid_stride = low_dim;
+  for (int high_id = blockIdx.y; high_id < high_dim; high_id += gridDim.y) {
+    for (int low_id = blockIdx.x * blockDim.x + threadIdx.x; low_id < low_dim;
+         low_id += blockDim.x * gridDim.x) {
+      const int input_offset = high_id * high_stride + low_id;
+
+      // 1. reduce max
+      AccT max_value = -std::numeric_limits<AccT>::infinity();
+      AccT value = -std::numeric_limits<AccT>::infinity();
+      for (int mid_id = threadIdx.y; mid_id < mid_dim; mid_id += blockDim.y) {
+        value = static_cast<AccT>(input[input_offset + mid_id * mid_stride]);
+        max_value = kps::MaxFunctor<AccT>()(max_value, value);
+      }
+
+      if (blockDim.y > 1) {
+        kps::Reduce<AccT, 1, 1, 1, kps::MaxFunctor<AccT>, kMode::kGlobalMode>(
+            &max_value, &max_value, kps::MaxFunctor<AccT>(), false);
+      }
+
+      // 2. reduce sum
+      AccT sum = 0;
+      for (int mid_id = threadIdx.y; mid_id < mid_dim; mid_id += blockDim.y) {
+        value = static_cast<AccT>(input[input_offset + mid_id * mid_stride]);
+        sum += std::exp(value - max_value);
+      }
+      if (blockDim.y > 1) {
+        kps::Reduce<AccT, 1, 1, 1, kps::AddFunctor<AccT>, kMode::kGlobalMode>(
+            &sum, &sum, kps::AddFunctor<AccT>(), false);
+      }
+
+      // 3. (log)softmax
+      Functor<AccT, T> functor(max_value, sum);
+      for (int mid_id = threadIdx.y; mid_id < mid_dim; mid_id += blockDim.y) {
+        int data_offset = input_offset + mid_id * mid_stride;
+        output[data_offset] = functor(static_cast<AccT>(input[data_offset]));
+      }
+    }
+  }
+}
+
+template <typename T, bool LogMode = false>
+void LaunchNormalSoftmaxForward(const platform::CUDADeviceContext& dev_ctx,
+                                T* output_data, const T* input_data,
+                                int high_dim, int mid_dim, int low_dim) {
+  using AccT = typename details::MPTypeTrait<T>::Type;
+  dim3 grid, block;
+  GetLaunchConfig(high_dim, mid_dim, low_dim, &grid, &block);
+  if (LogMode) {
+    NormalSoftmaxForward<
+        T, AccT,
+        LogSoftmaxForwardFunctor><<<grid, block, 0, dev_ctx.stream()>>>(
+        output_data, input_data, high_dim, mid_dim, low_dim);
+  } else {
+    NormalSoftmaxForward<
+        T, AccT, SoftmaxForwardFunctor><<<grid, block, 0, dev_ctx.stream()>>>(
+        output_data, input_data, high_dim, mid_dim, low_dim);
+  }
+}
+
 template <typename T, bool LogMode = false>
 void SoftmaxForwardCUDAKernelDriver(const platform::CUDADeviceContext& dev_ctx,
                                     const Tensor& x, const int input_axis,
@@ -490,6 +650,9 @@ void SoftmaxForwardCUDAKernelDriver(const platform::CUDADeviceContext& dev_ctx,
                                               out_data, x.data<T>(), N, dim,
                                               dim, kDimLog2);
     }
+  } else if (D > 1) {
+    LaunchNormalSoftmaxForward<T, LogMode>(dev_ctx, out_data, x.data<T>(), N,
+                                           dim, D);
   } else {
     ScopedTensorDescriptor desc;
     std::vector<int> tensor_dims = {N, dim, D, 1};
-- 
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