Index: SingleSource/UnitTests/Vectorizer/common.h
===================================================================
--- SingleSource/UnitTests/Vectorizer/common.h
+++ SingleSource/UnitTests/Vectorizer/common.h
@@ -17,6 +17,24 @@
     _Pragma("clang loop vectorize(enable)") Loop                               \
   };
 
+#define DEFINE_SCALAR_AND_VECTOR_FN4(InnerLoopCode)                            \
+  auto ScalarFn = [](auto *A, auto *B, auto *C, unsigned TC) {                 \
+    for (unsigned long i = 0; i < TC; i++) {                                   \
+      _Pragma("clang loop vectorize(disable) interleave_count(1)")             \
+      for (unsigned long j = 0; j < TC; j++) {                                 \
+        InnerLoopCode                                                          \
+      }                                                                        \
+    }                                                                          \
+  };                                                                           \
+  auto VectorFn = [](auto *A, auto *B, auto *C, unsigned TC) {                 \
+    for (unsigned long i = 0; i < TC; i++) {                                   \
+      _Pragma("clang loop vectorize(enable)")                                  \
+      for (unsigned long j = 0; j < TC; j++) {                                 \
+        InnerLoopCode                                                          \
+      }                                                                        \
+    }                                                                          \
+  };
+
 static std::mt19937 rng;
 
 // Initialize arrays A with random numbers.
Index: SingleSource/UnitTests/Vectorizer/runtime-checks.cpp
===================================================================
--- SingleSource/UnitTests/Vectorizer/runtime-checks.cpp
+++ SingleSource/UnitTests/Vectorizer/runtime-checks.cpp
@@ -7,6 +7,7 @@
 
 #include "common.h"
 
+
 // Tests for memory runtime checks generated by the vectorizer. Runs scalar and
 // vectorized versions of a loop requiring runtime checks on the same inputs
 // with pointers to the same buffer using various offsets between reads and
@@ -108,6 +109,52 @@
     CheckWithOffsetSecond(i);
 }
 
+
+
+template <typename Ty>
+using Fn3Ty = std::function<void(Ty *, Ty *, Ty *, unsigned)>;
+template <typename Ty>
+static void checkOverlappingMemoryTwoRuntimeChecksNested(Fn3Ty<Ty> ScalarFn,
+                                                         Fn3Ty<Ty> VectorFn,
+                                                         const char *Name) {
+  std::cout << "Checking " << Name << "\n";
+
+  const int N = 100;
+  // Make sure we have enough extra elements so we can be liberal with offsets.
+  const unsigned NumArrayElements = (N * (N + 1)) * 8;
+  std::unique_ptr<Ty[]> Input1(new Ty[NumArrayElements]);
+  std::unique_ptr<Ty[]> Input2(new Ty[NumArrayElements]);
+  std::unique_ptr<Ty[]> Reference(new Ty[NumArrayElements]);
+  std::unique_ptr<Ty[]> ToCheck(new Ty[NumArrayElements]);
+
+  auto CheckWithOffsetSecond = [&](int Offset) {
+    init_data(Input1, NumArrayElements);
+    init_data(Input2, NumArrayElements);
+    for (unsigned i = 0; i < NumArrayElements; i++) {
+      Reference[i] = Input1[i];
+      ToCheck[i] = Input1[i];
+    }
+
+    // Run scalar function to generate reference output.
+    Ty *ReferenceStart = &Reference[NumArrayElements / 2];
+    ScalarFn(ReferenceStart + Offset, &Input2[0], ReferenceStart, N);
+
+    // Run vector function to generate output to check.
+    Ty *StartPtr = &ToCheck[NumArrayElements / 2];
+    callThroughOptnone(VectorFn, StartPtr + Offset, &Input2[0], StartPtr, N);
+
+    // Compare scalar and vector output.
+    check(Reference, ToCheck, NumArrayElements, Offset);
+  };
+
+  // With a nested loop, sometimes the runtime checks will fail and sometimes
+  // succeed. For example, with large offsets you'd expect for the first and
+  // last one or two executions of the inner loop there is no overlap.
+  for (int i = -(2 * (N + 1)); i <= (2 * (N + 1)); i++)
+    CheckWithOffsetSecond(i);
+}
+
+
 int main(void) {
   rng = std::mt19937(15);
 
@@ -261,5 +308,33 @@
         ScalarFn, VectorFn, "1 read, 2 writes, simple indices, uint64_t");
   }
 
+  {
+    DEFINE_SCALAR_AND_VECTOR_FN4(
+        auto X = C[(i * TC) + j];
+        A[(i * (TC + 1)) + j] = X;
+    );
+
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint8_t>(
+        ScalarFn, VectorFn, "1 read, 1 write, nested loop, uint8_t");
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint32_t>(
+        ScalarFn, VectorFn, "1 read, 1 write, nested loop, uint32_t");
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint64_t>(
+        ScalarFn, VectorFn, "1 read, 1 write, nested loop, uint64_t");
+  }
+
+  {
+    DEFINE_SCALAR_AND_VECTOR_FN4(
+        auto X = C[(i * TC) + j];
+        A[(i * (TC + 1)) + j] += X;
+    );
+
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint8_t>(
+        ScalarFn, VectorFn, "2 reads, 1 write, nested loop, uint8_t");
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint32_t>(
+        ScalarFn, VectorFn, "2 reads, 1 write, nested loop, uint32_t");
+    checkOverlappingMemoryTwoRuntimeChecksNested<uint64_t>(
+        ScalarFn, VectorFn, "2 reads, 1 write, nested loop, uint64_t");
+  }
+
   return 0;
 }
Index: SingleSource/UnitTests/Vectorizer/runtime-checks.reference_output
===================================================================
--- SingleSource/UnitTests/Vectorizer/runtime-checks.reference_output
+++ SingleSource/UnitTests/Vectorizer/runtime-checks.reference_output
@@ -28,4 +28,10 @@
 Checking 1 read, 2 writes, simple indices, uint8_t
 Checking 1 read, 2 writes, simple indices, uint32_t
 Checking 1 read, 2 writes, simple indices, uint64_t
+Checking 1 read, 1 write, nested loop, uint8_t
+Checking 1 read, 1 write, nested loop, uint32_t
+Checking 1 read, 1 write, nested loop, uint64_t
+Checking 2 reads, 1 write, nested loop, uint8_t
+Checking 2 reads, 1 write, nested loop, uint32_t
+Checking 2 reads, 1 write, nested loop, uint64_t
 exit 0