Enable seq_pool op to accept len 0 input (#17284)

* Enable seq_pool op to accept len 0 input

test=develop

* Update sequence_pool's api

test=develop

* Add more unittest cases for seq_pool op

test=develop

* Remove legacy comments

test=develop

* Don't use template in op maker

test=develop

paddle/fluid/API.spec

@@ -87,7 +87,7 @@ paddle.fluid.layers.chunk_eval (ArgSpec(args=['input', 'label', 'chunk_scheme',
 paddle.fluid.layers.sequence_conv (ArgSpec(args=['input', 'num_filters', 'filter_size', 'filter_stride', 'padding', 'bias_attr', 'param_attr', 'act', 'name'], varargs=None, keywords=None, defaults=(3, 1, None, None, None, None, None)), ('document', '3d8e8f3e0e1cf520156be37605e83ccd'))
 paddle.fluid.layers.conv2d (ArgSpec(args=['input', 'num_filters', 'filter_size', 'stride', 'padding', 'dilation', 'groups', 'param_attr', 'bias_attr', 'use_cudnn', 'act', 'name'], varargs=None, keywords=None, defaults=(1, 0, 1, None, None, None, True, None, None)), ('document', '8ca6121acd6d23cd8806a93f493c2e17'))
 paddle.fluid.layers.conv3d (ArgSpec(args=['input', 'num_filters', 'filter_size', 'stride', 'padding', 'dilation', 'groups', 'param_attr', 'bias_attr', 'use_cudnn', 'act', 'name'], varargs=None, keywords=None, defaults=(1, 0, 1, None, None, None, True, None, None)), ('document', '37042620f9bd3a2da6e5d3138b2f724b'))
-paddle.fluid.layers.sequence_pool (ArgSpec(args=['input', 'pool_type', 'is_test'], varargs=None, keywords=None, defaults=(False,)), ('document', 'a194fb80614023f543df3949fbd0d0b8'))
+paddle.fluid.layers.sequence_pool (ArgSpec(args=['input', 'pool_type', 'is_test', 'pad_value'], varargs=None, keywords=None, defaults=(False, 0.0)), ('document', 'e90a93251c52dc4e6fb34fb3991b3f82'))
 paddle.fluid.layers.sequence_softmax (ArgSpec(args=['input', 'use_cudnn', 'name'], varargs=None, keywords=None, defaults=(False, None)), ('document', '19ef6f9cdd27feac8a1ae060f19c10b4'))
 paddle.fluid.layers.softmax (ArgSpec(args=['input', 'use_cudnn', 'name', 'axis'], varargs=None, keywords=None, defaults=(False, None, -1)), ('document', 'cee673c79e3ff4582656a24e04f841e5'))
 paddle.fluid.layers.pool2d (ArgSpec(args=['input', 'pool_size', 'pool_type', 'pool_stride', 'pool_padding', 'global_pooling', 'use_cudnn', 'ceil_mode', 'name', 'exclusive'], varargs=None, keywords=None, defaults=(-1, 'max', 1, 0, False, True, False, None, True)), ('document', 'bbd84e855e660cd1084bb71a2fd0cdaa'))

paddle/fluid/operators/math/sequence_pooling.cc

@@ -36,8 +36,8 @@ template <typename T, bool is_test>
 class MaxSeqPoolFunctor {
  public:
   void operator()(const platform::CPUDeviceContext& context,
-                  const framework::LoDTensor& input, framework::Tensor* output,
-                  framework::Tensor* index) {
+                  const framework::LoDTensor& input, T pad_value,
+                  framework::Tensor* output, framework::Tensor* index) {
     auto in_dims = input.dims();
     auto out_dims = output->dims();
     auto idx_dims = index->dims();
@@ -56,6 +56,13 @@ class MaxSeqPoolFunctor {
     int64_t num_seq = out_dims[0];
     int64_t dim = output->numel() / num_seq;
     for (int64_t i = 0; i < num_seq; ++i) {
+      if (starts[i] == starts[i + 1]) {
+        for (int64_t k = 0; k < dim; ++k) {
+          out_data[i * dim + k] = pad_value;
+          max_index[i * dim + k] = -1;
+        }
+        continue;
+      }
       for (int64_t k = 0; k < dim; ++k) {
         out_data[i * dim + k] = in_data[starts[i] * dim + k];
         max_index[i * dim + k] = starts[i];
@@ -77,8 +84,8 @@ template <typename T>
 class MaxSeqPoolFunctor<T, true> {
  public:
   void operator()(const platform::CPUDeviceContext& context,
-                  const framework::LoDTensor& input, framework::Tensor* output,
-                  framework::Tensor* index) {
+                  const framework::LoDTensor& input, T pad_value,
+                  framework::Tensor* output, framework::Tensor* index) {
     auto in_dims = input.dims();
     auto out_dims = output->dims();
     PADDLE_ENFORCE_GT(in_dims.size(), 1);
@@ -94,6 +101,12 @@ class MaxSeqPoolFunctor<T, true> {
     int64_t num_seq = out_dims[0];
     int64_t dim = output->numel() / num_seq;
     for (int64_t i = 0; i < num_seq; ++i) {
+      if (starts[i] == starts[i + 1]) {
+        for (int64_t k = 0; k < dim; ++k) {
+          out_data[i * dim + k] = pad_value;
+        }
+        continue;
+      }
       std::memcpy(&out_data[i * dim], &in_data[starts[i] * dim],
                   dim * sizeof(T));
       for (size_t j = starts[i] + 1; j < starts[i + 1]; ++j) {
@@ -134,6 +147,7 @@ class MaxSeqPoolGradFunctor {
     for (int64_t i = 0; i < num_seq; ++i) {
       for (int64_t j = 0; j < dim; ++j) {
         int step_id = max_index[i * dim + j];
+        if (step_id == -1) continue;
         ig_data[step_id * dim + j] = og_data[i * dim + j];
       }
     }
@@ -144,7 +158,7 @@ template <typename T>
 class LastSeqPoolFunctor {
  public:
   void operator()(const platform::CPUDeviceContext& context,
-                  const framework::LoDTensor& input,
+                  const framework::LoDTensor& input, T pad_value,
                   framework::Tensor* output) {
     // Create pointers to input and output data
     auto* in_data = input.data<T>();
@@ -157,10 +171,16 @@ class LastSeqPoolFunctor {
     for (int i = 0; i < seq_num; ++i) {
       // Calculate the length of each sequence
       int64_t seq_len = static_cast<int64_t>(lod[i + 1] - lod[i]);
+      if (seq_len == 0) {
+        for (int j = 0; j < item_size; ++j) {
+          out_data[j] = pad_value;
+        }
+      } else {
         // Point to the begin of next sequence
         in_data += seq_len * item_size;
         // Copy the last item of sequence to output
         std::memcpy(out_data, (in_data - item_size), item_size * sizeof(T));
+      }
       out_data += item_size;
     }
   }
@@ -170,7 +190,7 @@ template <typename T>
 class FirstSeqPoolFunctor {
  public:
   void operator()(const platform::CPUDeviceContext& context,
-                  const framework::LoDTensor& input,
+                  const framework::LoDTensor& input, T pad_value,
                   framework::Tensor* output) {
     // Create pointers to input and output data
     auto* in_data = input.data<T>();
@@ -183,10 +203,16 @@ class FirstSeqPoolFunctor {
     for (int i = 0; i < seq_num; ++i) {
       // Calculate the length of each sequence
       int64_t seq_len = static_cast<int64_t>(lod[i + 1] - lod[i]);
+      if (seq_len == 0) {
+        for (int j = 0; j < item_size; ++j) {
+          out_data[j] = pad_value;
+        }
+      } else {
         // Copy the first item of sequence to output
         std::memcpy(out_data, in_data, item_size * sizeof(T));
         // Point to the next sequence
         in_data += seq_len * item_size;
+      }
       out_data += item_size;
     }
   }
@@ -207,6 +233,7 @@ class SumSeqPoolGradFunctor {
     auto blas = math::GetBlas<platform::CPUDeviceContext, T>(context);
     for (int i = 0; i < static_cast<int>(lod.size()) - 1; ++i) {
       int64_t h = static_cast<int64_t>(lod[i + 1] - lod[i]);
+      if (h == 0) continue;
       int64_t in_offset = lod[i] * in_w;
       const T* out_pos = out_g_data + i * out_w;
       T* in_pos = in_g_data + in_offset;
@@ -222,27 +249,27 @@ class SequencePoolFunctor<platform::CPUDeviceContext, T> {
  public:
   /* max pool has index output */
   void operator()(const platform::CPUDeviceContext& context,
-                  const std::string pooltype, const framework::LoDTensor& input,
-                  framework::Tensor* output, bool is_test,
-                  framework::Tensor* index = nullptr) {
+                  const std::string pooltype, T pad_value,
+                  const framework::LoDTensor& input, framework::Tensor* output,
+                  bool is_test, framework::Tensor* index = nullptr) {
     if (pooltype == "MAX") {
       if (is_test) {
         math::MaxSeqPoolFunctor<T, true> max_pool;
-        max_pool(context, input, output, index);
+        max_pool(context, input, pad_value, output, index);
       } else {
         math::MaxSeqPoolFunctor<T, false> max_pool;
-        max_pool(context, input, output, index);
+        max_pool(context, input, pad_value, output, index);
       }
       return;
     }
     if (pooltype == "LAST") {
       math::LastSeqPoolFunctor<T> last_pool;
-      last_pool(context, input, output);
+      last_pool(context, input, pad_value, output);
       return;
     }
     if (pooltype == "FIRST") {
       math::FirstSeqPoolFunctor<T> first_pool;
-      first_pool(context, input, output);
+      first_pool(context, input, pad_value, output);
       return;
     }
 
@@ -260,7 +287,13 @@ class SequencePoolFunctor<platform::CPUDeviceContext, T> {
               .At(attr);
       for (int i = 0; i < static_cast<int>(lod.size()) - 1; ++i) {
         attr.h = static_cast<int>(lod[i + 1] - lod[i]);
+        if (attr.h == 0) {
+          for (int j = 0; j < attr.w; ++j) {
+            dst[j] = pad_value;
+          }
+        } else {
           seqpool(src, dst, &attr);
+        }
         dst += attr.w;
         src += attr.h * attr.w;
       }
@@ -268,11 +301,17 @@ class SequencePoolFunctor<platform::CPUDeviceContext, T> {
     }
     auto& place = *context.eigen_device();
     for (int i = 0; i < static_cast<int>(lod.size()) - 1; ++i) {
+      Tensor out_t = output->Slice(i, i + 1);
+      int64_t w = input.numel() / input.dims()[0];
+      if (lod[i] == lod[i + 1]) {
+        for (int j = 0; j < w; ++j) {
+          out_t.data<T>()[j] = pad_value;
+        }
+        continue;
+      }
       Tensor in_t =
           input.Slice(static_cast<int>(lod[i]), static_cast<int>(lod[i + 1]));
-      Tensor out_t = output->Slice(i, i + 1);
       int64_t h = static_cast<int64_t>(lod[i + 1] - lod[i]);
-      int64_t w = input.numel() / input.dims()[0];
       auto in_e = EigenMatrix<T>::From(in_t, framework::make_ddim({h, w}));
       auto out_e = EigenVector<T>::Flatten(out_t);
       if (pooltype == "AVERAGE") {
@@ -316,6 +355,7 @@ class SequencePoolGradFunctor<platform::CPUDeviceContext, T> {
     auto lod = in_grad->lod()[0];
     auto& place = *context.eigen_device();
     for (int i = 0; i < static_cast<int>(lod.size()) - 1; ++i) {
+      if (lod[i] == lod[i + 1]) continue;
       auto in_g_t = in_grad->Slice(static_cast<int>(lod[i]),
                                    static_cast<int>(lod[i + 1]));
       auto out_g_t = out_grad.Slice(i, i + 1);

paddle/fluid/operators/math/sequence_pooling.cu

@@ -24,12 +24,16 @@ namespace math {
 
 template <typename T>
 struct MaxPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
       T max_val = static_cast<T>(-FLT_MAX);
       int max_index = -1;
+      if (start == end) {
+        output[tid] = pad_value;
+        index[tid] = -1;
+      } else {
         for (int i = start; i < end; ++i) {
           if (max_val < input[item_dim * i + tid]) {
             max_val = input[item_dim * i + tid];
@@ -40,14 +44,18 @@ struct MaxPoolFunctor {
         index[tid] = max_index;
       }
     }
+  }
 };
 
 template <typename T>
 struct AvgPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
+      if (start == end) {
+        output[tid] = pad_value;
+      } else {
         T val = static_cast<T>(0);
         for (int i = start; i < end; ++i) {
           val += input[item_dim * i + tid];
@@ -56,14 +64,18 @@ struct AvgPoolFunctor {
         output[tid] = val / static_cast<T>(end - start);
       }
     }
+  }
 };
 
 template <typename T>
 struct SumPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
+      if (start == end) {
+        output[tid] = pad_value;
+      } else {
         T val = static_cast<T>(0);
         for (int i = start; i < end; ++i) {
           val += input[item_dim * i + tid];
@@ -71,14 +83,18 @@ struct SumPoolFunctor {
         output[tid] = val;
       }
     }
+  }
 };
 
 template <typename T>
 struct SqrtPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
+      if (start == end) {
+        output[tid] = pad_value;
+      } else {
         T val = static_cast<T>(0);
         for (int i = start; i < end; ++i) {
           val += input[item_dim * i + tid];
@@ -87,33 +103,43 @@ struct SqrtPoolFunctor {
         output[tid] = val / sqrt(end - start);
       }
     }
+  }
 };
 
 template <typename T>
 struct LastPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
+      if (start == end) {
+        output[tid] = pad_value;
+      } else {
         output[tid] = input[item_dim * (end - 1) + tid];
       }
     }
+  }
 };
 
 template <typename T>
 struct FirstPoolFunctor {
-  HOSTDEVICE void operator()(const T* input, const size_t start,
-                             const size_t end, const size_t item_dim, T* output,
-                             int* index) {
+  HOSTDEVICE void operator()(const T* input, const T pad_value,
+                             const size_t start, const size_t end,
+                             const size_t item_dim, T* output, int* index) {
     for (int tid = threadIdx.x; tid < item_dim; tid += blockDim.x) {
+      if (start == end) {
+        output[tid] = pad_value;
+      } else {
         output[tid] = input[item_dim * start + tid];
       }
     }
+  }
 };
 
 template <typename T, typename Range_OP>
 __global__ void sequence_pool_kernel(Range_OP op, const T* input,
-                                     const size_t* lod, const size_t lod_size,
+                                     const T pad_value, const size_t* lod,
+                                     const size_t lod_size,
                                      const size_t item_dim, T* output,
                                      int* index) {
   int bid = blockIdx.x;
@@ -124,16 +150,17 @@ __global__ void sequence_pool_kernel(Range_OP op, const T* input,
   if (index != nullptr) {
     index_offset = &index[bid * item_dim];
   }
-  op(input, start, end, item_dim, &output[bid * item_dim], index_offset);
+  op(input, pad_value, start, end, item_dim, &output[bid * item_dim],
+     index_offset);
 }
 
 template <typename T>
 class SequencePoolFunctor<platform::CUDADeviceContext, T> {
  public:
   void operator()(const platform::CUDADeviceContext& context,
-                  const std::string pooltype, const framework::LoDTensor& input,
-                  framework::Tensor* output, bool is_test,
-                  framework::Tensor* index = nullptr) {
+                  const std::string pooltype, T pad_value,
+                  const framework::LoDTensor& input, framework::Tensor* output,
+                  bool is_test, framework::Tensor* index = nullptr) {
     auto& lod = input.lod()[0];
     const size_t item_dim = output->numel() / output->dims()[0];
     dim3 threads(1024, 1);
@@ -141,37 +168,37 @@ class SequencePoolFunctor<platform::CUDADeviceContext, T> {
     if (pooltype == "MAX") {
       sequence_pool_kernel<
           T, MaxPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          MaxPoolFunctor<T>(), input.data<T>(),
+          MaxPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), index->data<int>());
     } else if (pooltype == "AVERAGE") {
       sequence_pool_kernel<
           T, AvgPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          AvgPoolFunctor<T>(), input.data<T>(),
+          AvgPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), nullptr);
     } else if (pooltype == "SUM") {
       sequence_pool_kernel<
           T, SumPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          SumPoolFunctor<T>(), input.data<T>(),
+          SumPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), nullptr);
     } else if (pooltype == "SQRT") {
       sequence_pool_kernel<
           T, SqrtPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          SqrtPoolFunctor<T>(), input.data<T>(),
+          SqrtPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), nullptr);
     } else if (pooltype == "LAST") {
       sequence_pool_kernel<
           T, LastPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          LastPoolFunctor<T>(), input.data<T>(),
+          LastPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), nullptr);
     } else if (pooltype == "FIRST") {
       sequence_pool_kernel<
           T, FirstPoolFunctor<T>><<<grid, threads, 0, context.stream()>>>(
-          FirstPoolFunctor<T>(), input.data<T>(),
+          FirstPoolFunctor<T>(), input.data<T>(), pad_value,
           lod.CUDAData(context.GetPlace()), lod.size(), item_dim,
           output->mutable_data<T>(context.GetPlace()), nullptr);
     } else {

paddle/fluid/operators/math/sequence_pooling.h

@@ -27,8 +27,9 @@ class SequencePoolFunctor {
  public:
   /* max pool has index output */
   void operator()(const DeviceContext& context, const std::string pooltype,
-                  const framework::LoDTensor& input, framework::Tensor* output,
-                  bool is_test = false, framework::Tensor* index = nullptr);
+                  T pad_value, const framework::LoDTensor& input,
+                  framework::Tensor* output, bool is_test = false,
+                  framework::Tensor* index = nullptr);
 };
 
 template <typename DeviceContext, typename T>

paddle/fluid/operators/sequence_ops/sequence_pool_op.cc

@@ -57,6 +57,9 @@ class SequencePoolOpMaker : public framework::OpProtoAndCheckerMaker {
         "(string, default 'AVERAGE') the pooling pooltype of SequencePoolOp.")
         .SetDefault("AVERAGE")
         .InEnum({"AVERAGE", "SUM", "SQRT", "LAST", "FIRST", "MAX"});
+    AddAttr<float>("pad_value",
+                   "(float, default 0.0) The value to pad for empty sequence.")
+        .SetDefault(0.0);
     AddComment(R"DOC(
 Sequence Pool Operator.
 
@@ -69,6 +72,8 @@ It supports six pooling types:
 5. FIRST:   Out[i] = first instance in i-th sequence X[i]
 6. MAX:     $$Out[i] = max(X_i)$$
 
+and for the empty sequence Out[i] = attr(pad_value).
+
 The following example explains how this works:
 For a mini-batch of 3 variable-length sentences,
 containing 2, 3, and 2 time-steps:

paddle/fluid/operators/sequence_ops/sequence_pool_op.h

@@ -32,6 +32,7 @@ class SequencePoolKernel : public framework::OpKernel<T> {
     auto* in = context.Input<LoDTensor>("X");
     auto* out = context.Output<Tensor>("Out");
     std::string pooltype = context.Attr<std::string>("pooltype");
+    T pad_value = static_cast<T>(context.Attr<float>("pad_value"));
 
     auto dims = in->dims();
     auto lod = in->lod();
@@ -58,8 +59,8 @@ class SequencePoolKernel : public framework::OpKernel<T> {
       index->mutable_data<int>(context.GetPlace());
     }
     math::SequencePoolFunctor<DeviceContext, T> pool;
-    pool(context.template device_context<DeviceContext>(), pooltype, *in, out,
-         is_test, index);
+    pool(context.template device_context<DeviceContext>(), pooltype, pad_value,
+         *in, out, is_test, index);
   }
 };
 

python/paddle/fluid/layers/nn.py

@@ -2346,7 +2346,7 @@ def conv3d(input,
     return helper.append_activation(pre_act)
 
 
-def sequence_pool(input, pool_type, is_test=False):
+def sequence_pool(input, pool_type, is_test=False, pad_value=0.0):
     """
     This function add the operator for sequence pooling.
     It pools features of all time-steps of each instance, and is applied
@@ -2361,29 +2361,32 @@ def sequence_pool(input, pool_type, is_test=False):
 
     .. code-block:: text
 
-       x is a 1-level LoDTensor:
-         x.lod = [[2, 3, 2]]
+       x is a 1-level LoDTensor and **pad_value** = 0.0:
+         x.lod = [[2, 3, 2, 0]]
          x.data = [1, 3, 2, 4, 6, 5, 1]
          x.dims = [7, 1]
 
        then output is a Tensor:
-         out.dim = [3, 1]
+         out.dim = [4, 1]
          with condition len(x.lod[-1]) == out.dims[0]
 
        for different pool_type:
-         average: out.data = [2, 4, 3], where 2=(1+3)/2, 4=(2+4+6)/3, 3=(5+1)/2
-         sum    : out.data = [4, 12, 6], where 4=1+3, 12=2+4+6, 6=5+1
-         sqrt   : out.data = [2.82, 6.93, 4.24], where 2.82=(1+3)/sqrt(2),
+         average: out.data = [2, 4, 3, 0.0], where 2=(1+3)/2, 4=(2+4+6)/3, 3=(5+1)/2
+         sum    : out.data = [4, 12, 6, 0.0], where 4=1+3, 12=2+4+6, 6=5+1
+         sqrt   : out.data = [2.82, 6.93, 4.24, 0.0], where 2.82=(1+3)/sqrt(2),
                     6.93=(2+4+6)/sqrt(3), 4.24=(5+1)/sqrt(2)
-         max    : out.data = [3, 6, 5], where 3=max(1,3), 6=max(2,4,6), 5=max(5,1)
-         last   : out.data = [3, 6, 1], where 3=last(1,3), 6=last(2,4,6), 1=last(5,1)
-         first  : out.data = [1, 2, 5], where 1=first(1,3), 2=first(2,4,6), 5=first(5,1)
+         max    : out.data = [3, 6, 5, 0.0], where 3=max(1,3), 6=max(2,4,6), 5=max(5,1)
+         last   : out.data = [3, 6, 1, 0.0], where 3=last(1,3), 6=last(2,4,6), 1=last(5,1)
+         first  : out.data = [1, 2, 5, 0.0], where 1=first(1,3), 2=first(2,4,6), 5=first(5,1)
+
+         and all above 0.0 = **pad_value**.
 
     Args:
         input (variable): The input variable which is a LoDTensor.
         pool_type (string): The pooling type of sequence_pool.
             It supports average, sum, sqrt and max.
-        is_test(bool, Default False): Used distinguish training from scoring mode.
+        is_test (bool): Used to distinguish training from scoring mode. Default False.
+        pad_value (float): Used to pad the pooling result for empty input sequence.
 
     Returns:
         The sequence pooling variable which is a Tensor.
@@ -2392,6 +2395,8 @@ def sequence_pool(input, pool_type, is_test=False):
 
         .. code-block:: python
 
+             import paddle.fluid as fluid
+
              x = fluid.layers.data(name='x', shape=[7, 1],
                               dtype='float32', lod_level=1)
              avg_x = fluid.layers.sequence_pool(input=x, pool_type='average')
@@ -2413,8 +2418,11 @@ def sequence_pool(input, pool_type, is_test=False):
         inputs={"X": input},
         outputs={"Out": pool_out,
                  "MaxIndex": max_index},
-        attrs={"pooltype": pool_type.upper(),
-               "is_test": is_test})
+        attrs={
+            "pooltype": pool_type.upper(),
+            "is_test": is_test,
+            "pad_value": pad_value
+        })
 
     # when pool_type is max, variable max_index is initialized,
     # so we stop the gradient explicitly here