2 files changed, 28 insertions, 2 deletions

diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index aa3c3f67ec1..f815628f30c 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -656,13 +656,22 @@ public:
 
   /// @brief Move assignment operator.
   APInt &operator=(APInt &&that) {
-    if (!isSingleWord())
+    if (!isSingleWord()) {
+      // The MSVC STL shipped in 2013 requires that self move assignment be a
+      // no-op.  Otherwise algorithms like stable_sort will produce answers
+      // where half of the output is left in a moved-from state.
+      if (this == &that)
+        return *this;
       delete[] pVal;
+    }
 
-    BitWidth = that.BitWidth;
     VAL = that.VAL;
 
+    // If 'this == &that', avoid zeroing our own bitwidth by storing to 'that'
+    // first.
+    unsigned ThatBitWidth = that.BitWidth;
     that.BitWidth = 0;
+    BitWidth = ThatBitWidth;
 
     return *this;
   }
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 19c47ab13bf..7da3fb25548 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -678,4 +678,21 @@ TEST(APIntTest, nearestLogBase2) {
   EXPECT_EQ(A9.nearestLogBase2(), UINT32_MAX);
 }
 
+TEST(APIntTest, SelfMoveAssignment) {
+  APInt X(32, 0xdeadbeef);
+  X = std::move(X);
+  EXPECT_EQ(32, X.getBitWidth());
+  EXPECT_EQ(0xdeadbeefULL, X.getLimitedValue());
+
+  uint64_t Bits[] = {0xdeadbeefdeadbeefULL, 0xdeadbeefdeadbeefULL};
+  APInt Y(128, Bits);
+  Y = std::move(Y);
+  EXPECT_EQ(128, Y.getBitWidth());
+  EXPECT_EQ(~0ULL, Y.getLimitedValue());
+  const uint64_t *Raw = Y.getRawData();
+  EXPECT_EQ(2, Y.getNumWords());
+  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[0]);
+  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[1]);
+}
+
 }