From 4bed0ac9bab45246183184d84ff1b742e60574c7 Mon Sep 17 00:00:00 2001
From: Jeff Donahue <jeff.donahue@gmail.com>
Date: Mon, 24 Aug 2015 19:44:18 -0700
Subject: [PATCH 1/2] TestConcatLayer: add gradient check for bottom[1] only
 (to verify propagate_down[0] == false correctness)

---
 src/caffe/test/test_concat_layer.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/caffe/test/test_concat_layer.cpp b/src/caffe/test/test_concat_layer.cpp
index 662a50fa23b..088e0a41685 100644
--- a/src/caffe/test/test_concat_layer.cpp
+++ b/src/caffe/test/test_concat_layer.cpp
@@ -173,4 +173,13 @@ TYPED_TEST(ConcatLayerTest, TestGradientChannels) {
     this->blob_top_vec_);
 }
 
+TYPED_TEST(ConcatLayerTest, TestGradientChannelsBottomOneOnly) {
+  typedef typename TypeParam::Dtype Dtype;
+  LayerParameter layer_param;
+  ConcatLayer<Dtype> layer(layer_param);
+  GradientChecker<Dtype> checker(1e-2, 1e-2);
+  checker.CheckGradient(&layer, this->blob_bottom_vec_0_,
+    this->blob_top_vec_, 1);
+}
+
 }  // namespace caffe

From 6a7d4d6652018245f7bde1499d5208996912f3fb Mon Sep 17 00:00:00 2001
From: Jeff Donahue <jeff.donahue@gmail.com>
Date: Mon, 24 Aug 2015 19:22:54 -0700
Subject: [PATCH 2/2] bugfix for ConcatLayer with propagate_down set

if propagate_down[i] was set, offset_concat_axis was not correctly
updated for subsequent bottoms i+1, i+2, ...
---
 src/caffe/layers/concat_layer.cpp | 13 +++++++------
 src/caffe/layers/concat_layer.cu  | 17 +++++++++--------
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/src/caffe/layers/concat_layer.cpp b/src/caffe/layers/concat_layer.cpp
index 1cac8fc3387..95fba105b9a 100644
--- a/src/caffe/layers/concat_layer.cpp
+++ b/src/caffe/layers/concat_layer.cpp
@@ -76,13 +76,14 @@ void ConcatLayer<Dtype>::Backward_cpu(const vector<Blob<Dtype>*>& top,
   int offset_concat_axis = 0;
   const int top_concat_axis = top[0]->shape(concat_axis_);
   for (int i = 0; i < bottom.size(); ++i) {
-    if (!propagate_down[i]) { continue; }
-    Dtype* bottom_diff = bottom[i]->mutable_cpu_diff();
     const int bottom_concat_axis = bottom[i]->shape(concat_axis_);
-    for (int n = 0; n < num_concats_; ++n) {
-      caffe_copy(bottom_concat_axis * concat_input_size_, top_diff +
-          (n * top_concat_axis + offset_concat_axis) * concat_input_size_,
-          bottom_diff + n * bottom_concat_axis * concat_input_size_);
+    if (propagate_down[i]) {
+      Dtype* bottom_diff = bottom[i]->mutable_cpu_diff();
+      for (int n = 0; n < num_concats_; ++n) {
+        caffe_copy(bottom_concat_axis * concat_input_size_, top_diff +
+            (n * top_concat_axis + offset_concat_axis) * concat_input_size_,
+            bottom_diff + n * bottom_concat_axis * concat_input_size_);
+      }
     }
     offset_concat_axis += bottom_concat_axis;
   }
diff --git a/src/caffe/layers/concat_layer.cu b/src/caffe/layers/concat_layer.cu
index 8f2e85d8f52..3c64c7ef224 100644
--- a/src/caffe/layers/concat_layer.cu
+++ b/src/caffe/layers/concat_layer.cu
@@ -53,15 +53,16 @@ void ConcatLayer<Dtype>::Backward_gpu(const vector<Blob<Dtype>*>& top,
   const int top_concat_axis = top[0]->shape(concat_axis_);
   const bool kForward = false;
   for (int i = 0; i < bottom.size(); ++i) {
-    if (!propagate_down[i]) { continue; }
-    Dtype* bottom_diff = bottom[i]->mutable_gpu_diff();
     const int bottom_concat_axis = bottom[i]->shape(concat_axis_);
-    const int bottom_concat_size = bottom_concat_axis * concat_input_size_;
-    const int nthreads = bottom_concat_size * num_concats_;
-    Concat<Dtype>  // NOLINT_NEXT_LINE(whitespace/operators)
-        <<<CAFFE_GET_BLOCKS(nthreads), CAFFE_CUDA_NUM_THREADS>>>(
-        nthreads, top_diff, kForward, num_concats_, concat_input_size_,
-        top_concat_axis, bottom_concat_axis, offset_concat_axis, bottom_diff);
+    if (propagate_down[i]) {
+      Dtype* bottom_diff = bottom[i]->mutable_gpu_diff();
+      const int bottom_concat_size = bottom_concat_axis * concat_input_size_;
+      const int nthreads = bottom_concat_size * num_concats_;
+      Concat<Dtype>  // NOLINT_NEXT_LINE(whitespace/operators)
+          <<<CAFFE_GET_BLOCKS(nthreads), CAFFE_CUDA_NUM_THREADS>>>(
+          nthreads, top_diff, kForward, num_concats_, concat_input_size_,
+          top_concat_axis, bottom_concat_axis, offset_concat_axis, bottom_diff);
+    }
     offset_concat_axis += bottom_concat_axis;
   }
 }