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
@@ -93,6 +93,7 @@
   void printDirectiveBody(const char *&First, const char *const End);
   void printAdjacentMacroArgs(const char *&First, const char *const End);
   LLVM_NODISCARD bool printMacroArgs(const char *&First, const char *const End);
+  void maybePrintNewLine(const char *&Last, const char *const End);
 
   /// Reports a diagnostic if the diagnostic engine is provided. Always returns
   /// true at the end.
@@ -446,13 +447,15 @@
   }
 }
 
-static void skipWhitespace(const char *&First, const char *const End) {
+static size_t skipWhitespace(const char *&First, const char *const End) {
+  const char *FirstBefore = First;
+
   for (;;) {
     assert(First <= End);
     skipOverSpaces(First, End);
 
     if (End - First < 2)
-      return;
+      break;
 
     if (First[0] == '\\' && isVerticalWhitespace(First[1])) {
       skipNewline(++First, End);
@@ -461,21 +464,23 @@
 
     // Check for a non-comment character.
     if (First[0] != '/')
-      return;
+      break;
 
     // "// ...".
     if (First[1] == '/') {
       skipLineComment(First, End);
-      return;
+      break;
     }
 
     // Cannot be a comment.
     if (First[1] != '*')
-      return;
+      break;
 
     // "/*...*/".
     skipBlockComment(First, End);
   }
+
+  return First - FirstBefore;
 }
 
 void Minimizer::printAdjacentModuleNameParts(const char *&First,
@@ -519,7 +524,7 @@
   printToNewline(First, End);
   while (Out.back() == ' ')
     Out.pop_back();
-  put('\n');
+  maybePrintNewLine(First, End);
 }
 
 LLVM_NODISCARD static const char *lexRawIdentifier(const char *First,
@@ -595,6 +600,12 @@
   }
 }
 
+void Minimizer::maybePrintNewLine(const char *&Last, const char *const End) {
+  // Only print newline if doing so won't make the output larger than the input.
+  if (Last != End)
+    put('\n');
+}
+
 /// Looks for an identifier starting from Last.
 ///
 /// Updates "First" to just past the next identifier, if any.  Returns true iff
@@ -704,15 +715,17 @@
       // Be robust to bad macro arguments, since they can show up in disabled
       // code.
       Out.resize(Size);
-      append("(/* invalid */\n");
+      maybePrintNewLine(Last, End);
       skipLine(Last, End);
       return false;
     }
   }
-  skipWhitespace(Last, End);
+  size_t WhitespaceLength = skipWhitespace(Last, End);
   if (Last == End)
     return false;
-  if (!isVerticalWhitespace(*Last))
+  // Only print space if we actually skipped some whitespace. This prevents
+  // making the output larger than the input.
+  if (WhitespaceLength > 0 && !isVerticalWhitespace(*Last))
     put(' ');
   printDirectiveBody(Last, End);
   First = Last;
@@ -887,9 +900,11 @@
 bool Minimizer::minimize() {
   bool Error = minimizeImpl(Input.begin(), Input.end());
 
+  assert(Out.size() <= Input.size() && "Output is not larger than input");
+
   if (!Error) {
-    // Add a trailing newline and an EOF on success.
-    if (!Out.empty() && Out.back() != '\n')
+    // Add a trailing newline if it won't make the output larger than the input.
+    if (!Out.empty() && Out.back() != '\n' && Out.size() < Input.size())
       Out.push_back('\n');
     makeToken(pp_eof);
   }
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
@@ -94,7 +94,7 @@
 
   ASSERT_FALSE(
       minimizeSourceToDependencyDirectives("#define MACRO", Out, Tokens));
-  EXPECT_STREQ("#define MACRO\n", Out.data());
+  EXPECT_STREQ("#define MACRO", Out.data());
   ASSERT_EQ(2u, Tokens.size());
   ASSERT_EQ(pp_define, Tokens.front().K);
 }
@@ -123,7 +123,7 @@
   SmallVector<char, 128> Out;
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives("#define MACRO()", Out));
-  EXPECT_STREQ("#define MACRO()\n", Out.data());
+  EXPECT_STREQ("#define MACRO()", Out.data());
 
   ASSERT_FALSE(
       minimizeSourceToDependencyDirectives("#define MACRO(a, b...)", Out));
@@ -131,7 +131,7 @@
 
   ASSERT_FALSE(
       minimizeSourceToDependencyDirectives("#define MACRO content", Out));
-  EXPECT_STREQ("#define MACRO content\n", Out.data());
+  EXPECT_STREQ("#define MACRO content", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives(
       "#define MACRO   con  tent   ", Out));
@@ -146,14 +146,14 @@
   SmallVector<char, 128> Out;
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives("#define MACRO((a))", Out));
-  EXPECT_STREQ("#define MACRO(/* invalid */\n", Out.data());
+  EXPECT_STREQ("#define MACRO\n", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives("#define MACRO(", Out));
-  EXPECT_STREQ("#define MACRO(/* invalid */\n", Out.data());
+  EXPECT_STREQ("#define MACRO\n", Out.data());
 
   ASSERT_FALSE(
       minimizeSourceToDependencyDirectives("#define MACRO(a * b)", Out));
-  EXPECT_STREQ("#define MACRO(/* invalid */\n", Out.data());
+  EXPECT_STREQ("#define MACRO\n", Out.data());
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest, DefineHorizontalWhitespace) {
@@ -259,7 +259,7 @@
   SmallVector<char, 128> Out;
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives("#define AND&\n", Out));
-  EXPECT_STREQ("#define AND &\n", Out.data());
+  EXPECT_STREQ("#define AND&\n", Out.data());
 
   ASSERT_FALSE(minimizeSourceToDependencyDirectives("#define AND\\\n"
                                                     "&\n",