diff --git a/llvm/lib/Support/NativeFormatting.cpp b/llvm/lib/Support/NativeFormatting.cpp
--- a/llvm/lib/Support/NativeFormatting.cpp
+++ b/llvm/lib/Support/NativeFormatting.cpp
@@ -168,7 +168,7 @@
     S << "nan";
     return;
   } else if (std::isinf(N)) {
-    S << "INF";
+    S << (std::signbit(N) ? "-INF" : "INF");
     return;
   }
 
diff --git a/llvm/unittests/Support/NativeFormatTests.cpp b/llvm/unittests/Support/NativeFormatTests.cpp
--- a/llvm/unittests/Support/NativeFormatTests.cpp
+++ b/llvm/unittests/Support/NativeFormatTests.cpp
@@ -125,6 +125,8 @@
                                  FloatStyle::Fixed));
   EXPECT_EQ("INF", format_number(std::numeric_limits<double>::infinity(),
                                  FloatStyle::Fixed));
+  EXPECT_EQ("-INF", format_number(-std::numeric_limits<double>::infinity(),
+                                  FloatStyle::Fixed));
 }
 
 TEST(NativeFormatTest, HexTests) {