From 31518ca4e4ddbd6df02dc4c4c4c2836ec6508eaf Mon Sep 17 00:00:00 2001
From: fkunstner <frederik.kunstner@gmail.com>
Date: Sun, 19 Jan 2020 18:44:11 -0800
Subject: [PATCH 1/2] Add tests for Issue #30

---
 test/bugfixes_test.py | 57 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 test/bugfixes_test.py

diff --git a/test/bugfixes_test.py b/test/bugfixes_test.py
new file mode 100644
index 000000000..de0b2d824
--- /dev/null
+++ b/test/bugfixes_test.py
@@ -0,0 +1,57 @@
+import itertools
+import pytest
+import torch
+import backpack
+
+
+def parameters_issue_30():
+    possible_values = {
+        "N": [4],
+        "C_in": [4],
+        "C_out": [6],
+        "H": [6],
+        "W": [6],
+        "K": [3],
+        "S": [1, 3],
+        "pad": [0, 2],
+        "dil": [1, 2],
+    }
+
+    configs = [
+        dict(zip(possible_values.keys(), config_tuple))
+        for config_tuple in itertools.product(*possible_values.values())
+    ]
+
+    return {
+        "argvalues": configs,
+        "ids": [str(config) for config in configs],
+    }
+
+
+@pytest.mark.parametrize("params", **parameters_issue_30())
+def test_convolutions_stride_issue_30(params):
+    """
+    https://github.com/f-dangel/backpack/issues/30
+
+    The gradient for the convolution is wrong when `stride` is not a multiple of
+    `D + 2*padding - dilation*(kernel-1) - 1`.
+    """
+    torch.manual_seed(0)
+
+    mod = torch.nn.Conv2d(
+        in_channels=params["C_in"],
+        out_channels=params["C_out"],
+        kernel_size=params["K"],
+        stride=params["S"],
+        padding=params["pad"],
+        dilation=params["dil"],
+    )
+    backpack.extend(mod)
+    x = torch.randn(size=(params["N"], params["C_in"], params["W"], params["H"]))
+
+    with backpack.backpack(backpack.extensions.BatchGrad()):
+        loss = torch.sum(mod(x))
+        loss.backward()
+
+        for p in mod.parameters():
+            assert torch.allclose(p.grad, p.grad_batch.sum(0))

From 368786c3de77826bb3e6db976a9ff07ab207d2ca Mon Sep 17 00:00:00 2001
From: fkunstner <frederik.kunstner@gmail.com>
Date: Sun, 19 Jan 2020 18:55:15 -0800
Subject: [PATCH 2/2] Add correct stride support - Issue #30

---
 backpack/core/derivatives/conv2d.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/backpack/core/derivatives/conv2d.py b/backpack/core/derivatives/conv2d.py
index 5bc813b1e..f936469cc 100644
--- a/backpack/core/derivatives/conv2d.py
+++ b/backpack/core/derivatives/conv2d.py
@@ -205,8 +205,10 @@ def weight_jac_t_mat_prod(self, module, g_inp, g_out, mat, sum_batch=True):
         )
 
         grad_weight = grad_weight.view(
-            num_cols, batch, out_channels * in_channels, k_x, k_y
+            num_cols, batch, -1, grad_weight.shape[2], grad_weight.shape[3]
         )
+        grad_weight = grad_weight.narrow(3, 0, k_x).narrow(4, 0, k_y)
+
         if sum_batch is True:
             grad_weight = grad_weight.sum(1)
             batch = 1