From f284c8592b279d5e796842f662927c86c6bdc185 Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Wed, 24 Feb 2021 18:12:51 -0800
Subject: Don't crash when attempting to slice an empty tensor.

---
 .../Eigen/CXX11/src/Tensor/TensorMorphing.h        | 12 +++--
 unsupported/test/cxx11_tensor_morphing.cpp         | 60 ++++++++++++++++++++++
 2 files changed, 68 insertions(+), 4 deletions(-)

(limited to 'unsupported')

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
index ceecd54d0..ef79c8567 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
@@ -451,6 +451,7 @@ struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Devi
     }
 
     m_is_identity = true;
+    bool degenerate = false;
     for (int i = 0; i < internal::array_size<Dimensions>::value; ++i) {
       eigen_assert(m_impl.dimensions()[i] >=
                    op.sizes()[i] + op.startIndices()[i]);
@@ -458,6 +459,9 @@ struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Devi
           op.startIndices()[i] != 0) {
         m_is_identity = false;
       }
+      if (op.sizes()[i] == 0) { // we have an empty size
+        degenerate = true;
+      }
     }
 
     // No strides for scalars.
@@ -475,8 +479,8 @@ struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Devi
       m_outputStrides[0] = 1;
       for (int i = 1; i < NumDims; ++i) {
         m_outputStrides[i] = m_outputStrides[i-1] * output_dims[i-1];
-        m_fastOutputStrides[i] = internal::TensorIntDivisor<Index>(m_outputStrides[i]);
-      }
+        // NOTE: if tensor is degenerate, we send 1 to prevent TensorIntDivisor constructor crash
+        m_fastOutputStrides[i] = internal::TensorIntDivisor<Index>(degenerate ? 1 : m_outputStrides[i]);      }
     } else {
       m_inputStrides[NumDims-1] = 1;
       for (int i = NumDims - 2; i >= 0; --i) {
@@ -487,8 +491,8 @@ struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Devi
       m_outputStrides[NumDims-1] = 1;
       for (int i = NumDims - 2; i >= 0; --i) {
         m_outputStrides[i] = m_outputStrides[i+1] * output_dims[i+1];
-        m_fastOutputStrides[i] = internal::TensorIntDivisor<Index>(m_outputStrides[i]);
-      }
+        // NOTE: if tensor is degenerate, we send 1 to prevent TensorIntDivisor constructor crash
+        m_fastOutputStrides[i] = internal::TensorIntDivisor<Index>(degenerate ? 1 : m_outputStrides[i]);      }
     }
   }
 
diff --git a/unsupported/test/cxx11_tensor_morphing.cpp b/unsupported/test/cxx11_tensor_morphing.cpp
index e8c42a4cd..ed5d5ade3 100644
--- a/unsupported/test/cxx11_tensor_morphing.cpp
+++ b/unsupported/test/cxx11_tensor_morphing.cpp
@@ -479,6 +479,66 @@ static void test_composition()
   }
 }
 
+template<typename T, int DataLayout>
+static void test_empty_slice()
+{
+  Tensor<T, 3, DataLayout> tensor(2,3,5);
+  tensor.setRandom();
+  Tensor<T, 3, DataLayout> copy = tensor;
+
+  // empty size in first dimension
+  Eigen::DSizes<ptrdiff_t, 3> indices1(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes1(0,1,2);
+  Tensor<T, 3, DataLayout> slice1(0,1,2);
+  slice1.setRandom();
+  tensor.slice(indices1, sizes1) = slice1;
+
+  // empty size in second dimension
+  Eigen::DSizes<ptrdiff_t, 3> indices2(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes2(1,0,2);
+  Tensor<T, 3, DataLayout> slice2(1,0,2);
+  slice2.setRandom();
+  tensor.slice(indices2, sizes2) = slice2;
+
+  // empty size in third dimension
+  Eigen::DSizes<ptrdiff_t, 3> indices3(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes3(1,1,0);
+  Tensor<T, 3, DataLayout> slice3(1,1,0);
+  slice3.setRandom();
+  tensor.slice(indices3, sizes3) = slice3;
+
+  // empty size in first and second dimension
+  Eigen::DSizes<ptrdiff_t, 3> indices4(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes4(0,0,2);
+  Tensor<T, 3, DataLayout> slice4(0,0,2);
+  slice4.setRandom();
+  tensor.slice(indices4, sizes4) = slice4;
+
+  // empty size in second and third dimension
+  Eigen::DSizes<ptrdiff_t, 3> indices5(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes5(1,0,0);
+  Tensor<T, 3, DataLayout> slice5(1,0,0);
+  slice5.setRandom();
+  tensor.slice(indices5, sizes5) = slice5;
+
+  // empty size in all dimensions
+  Eigen::DSizes<ptrdiff_t, 3> indices6(1,2,3);
+  Eigen::DSizes<ptrdiff_t, 3> sizes6(0,0,0);
+  Tensor<T, 3, DataLayout> slice6(0,0,0);
+  slice6.setRandom();
+  tensor.slice(indices6, sizes6) = slice6;
+
+  // none of these operations should change the tensor's components
+  // because all of the rvalue slices have at least one zero dimension
+  for (int i = 0; i < 2; ++i) {
+    for (int j = 0; j < 3; ++j) {
+      for (int k = 0; k < 5; ++k) {
+          VERIFY_IS_EQUAL(tensor(i,j,k), copy(i,j,k));
+      }
+    }
+  }
+}
+
 #define CALL_SUBTEST_PART(PART) \
   CALL_SUBTEST_##PART
 
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