diff --git a/lib/Support/ConstantRange.cpp b/lib/Support/ConstantRange.cpp
index 04a1b68e072..8bab5377952 100644
--- a/lib/Support/ConstantRange.cpp
+++ b/lib/Support/ConstantRange.cpp
@@ -533,6 +533,8 @@ ConstantRange
 ConstantRange::add(const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())
     return ConstantRange(getBitWidth(), /*isFullSet=*/false);
+  if (isFullSet() || Other.isFullSet())
+    return ConstantRange(getBitWidth(), /*isFullSet=*/true);
 
   APInt Spread_X = getSetSize(), Spread_Y = Other.getSetSize();
   APInt NewLower = getLower() + Other.getLower();
diff --git a/unittests/Support/ConstantRangeTest.cpp b/unittests/Support/ConstantRangeTest.cpp
index 891c209f11c..cd91a9e103c 100644
--- a/unittests/Support/ConstantRangeTest.cpp
+++ b/unittests/Support/ConstantRangeTest.cpp
@@ -239,6 +239,7 @@ TEST_F(ConstantRangeTest, SubtractAPInt) {
 
 TEST_F(ConstantRangeTest, Add) {
   EXPECT_TRUE(Full.add(APInt(16, 4)).isFullSet());
+  EXPECT_EQ(Full.add(Full), Full);
   EXPECT_EQ(Full.add(Empty), Empty);
   EXPECT_EQ(Full.add(One), Full);
   EXPECT_EQ(Full.add(Some), Full);