diff --git a/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp b/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
--- a/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
+++ b/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
@@ -246,9 +246,12 @@
 
 static const char *reverseOverSpaces(const char *First, const char *Last) {
   assert(First <= Last);
-  while (First != Last && isHorizontalWhitespace(Last[-1]))
+  const char *PrevLast = Last;
+  while (First != Last && isHorizontalWhitespace(Last[-1])) {
+    PrevLast = Last;
     --Last;
-  return Last;
+  }
+  return PrevLast;
 }
 
 static void skipLineComment(const char *&First, const char *const End) {
diff --git a/clang/unittests/Lex/DependencyDirectivesSourceMinimizerTest.cpp b/clang/unittests/Lex/DependencyDirectivesSourceMinimizerTest.cpp
--- a/clang/unittests/Lex/DependencyDirectivesSourceMinimizerTest.cpp
+++ b/clang/unittests/Lex/DependencyDirectivesSourceMinimizerTest.cpp
@@ -157,19 +157,19 @@
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives(
       "#define MACRO(\t)\tcon \t tent\t", Out));
-  EXPECT_STREQ("#define MACRO() con \t tent\n", Out.data());
+  EXPECT_STREQ("#define MACRO() con \t tent\t\n", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives(
       "#define MACRO(\f)\fcon \f tent\f", Out));
-  EXPECT_STREQ("#define MACRO() con \f tent\n", Out.data());
+  EXPECT_STREQ("#define MACRO() con \f tent\f\n", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives(
       "#define MACRO(\v)\vcon \v tent\v", Out));
-  EXPECT_STREQ("#define MACRO() con \v tent\n", Out.data());
+  EXPECT_STREQ("#define MACRO() con \v tent\v\n", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives(
       "#define MACRO \t\v\f\v\t con\f\t\vtent\v\f \v", Out));
-  EXPECT_STREQ("#define MACRO con\f\t\vtent\n", Out.data());
+  EXPECT_STREQ("#define MACRO con\f\t\vtent\v\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest, DefineMultilineArgs) {
@@ -187,7 +187,7 @@
                                            "        call((a),      \\\n"
                                            "             (b))",
                                            Out));
-  EXPECT_STREQ("#define MACRO(a,b) call((a),(b))\n", Out.data());
+  EXPECT_STREQ("#define MACRO(a,b) call((a), (b))\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest,
@@ -200,7 +200,19 @@
                                            "        call((a),      \\\r"
                                            "             (b))",
                                            Out));
-  EXPECT_STREQ("#define MACRO(a,b) call((a),(b))\n", Out.data());
+  EXPECT_STREQ("#define MACRO(a,b) call((a), (b))\n", Out.data());
+}
+
+TEST(MinimizeSourceToDependencyDirectivesTest,
+     DefineMultilineArgsStringize) {
+  SmallVector<char, 128> Out;
+
+  ASSERT_FALSE(
+      minimizeSourceToDependencyDirectives("#define MACRO(a,b) \\\n"
+                                           "                #a \\\n"
+                                           "                #b",
+                                           Out));
+  EXPECT_STREQ("#define MACRO(a,b) #a #b\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest,
@@ -213,7 +225,7 @@
                                            "        call((a),      \\\r\n"
                                            "             (b))",
                                            Out));
-  EXPECT_STREQ("#define MACRO(a,b) call((a),(b))\n", Out.data());
+  EXPECT_STREQ("#define MACRO(a,b) call((a), (b))\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest,
@@ -226,7 +238,7 @@
                                            "        call((a),      \\\n\r"
                                            "             (b))",
                                            Out));
-  EXPECT_STREQ("#define MACRO(a,b) call((a),(b))\n", Out.data());
+  EXPECT_STREQ("#define MACRO(a,b) call((a), (b))\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest, DefineNumber) {