Index: clang/lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.cpp
+++ clang/lib/CodeGen/CGDebugInfo.cpp
@@ -470,10 +470,11 @@
 }
 
 std::string CGDebugInfo::remapDIPath(StringRef Path) const {
+  SmallString<256> P = Path;
   for (const auto &Entry : DebugPrefixMap)
-    if (Path.startswith(Entry.first))
-      return (Twine(Entry.second) + Path.substr(Entry.first.size())).str();
-  return Path.str();
+    if (llvm::sys::path::replace_path_prefix(P, Entry.first, Entry.second))
+      break;
+  return P.str().str();
 }
 
 unsigned CGDebugInfo::getLineNumber(SourceLocation Loc) {
Index: clang/lib/Lex/PPMacroExpansion.cpp
===================================================================
--- clang/lib/Lex/PPMacroExpansion.cpp
+++ clang/lib/Lex/PPMacroExpansion.cpp
@@ -1456,10 +1456,8 @@
     const std::map<std::string, std::string, std::greater<std::string>>
         &MacroPrefixMap) {
   for (const auto &Entry : MacroPrefixMap)
-    if (Path.startswith(Entry.first)) {
-      Path = (Twine(Entry.second) + Path.substr(Entry.first.size())).str();
+    if (llvm::sys::path::replace_path_prefix(Path, Entry.first, Entry.second))
       break;
-    }
 }
 
 /// ExpandBuiltinMacro - If an identifier token is read that is to be expanded
@@ -1543,8 +1541,8 @@
       } else {
         FN += PLoc.getFilename();
       }
-      Lexer::Stringify(FN);
       remapMacroPath(FN, PPOpts->MacroPrefixMap);
+      Lexer::Stringify(FN);
       OS << '"' << FN << '"';
     }
     Tok.setKind(tok::string_literal);
Index: clang/test/Preprocessor/file_test.c
===================================================================
--- clang/test/Preprocessor/file_test.c
+++ clang/test/Preprocessor/file_test.c
@@ -1,4 +1,4 @@
-// XFAIL: system-windows
+// UNSUPPORTED: system-windows
 // RUN: %clang -E -ffile-prefix-map=%p=/UNLIKELY_PATH/empty -c -o - %s | FileCheck %s
 // RUN: %clang -E -fmacro-prefix-map=%p=/UNLIKELY_PATH/empty -c -o - %s | FileCheck %s
 // RUN: %clang -E -fmacro-prefix-map=%p=/UNLIKELY_PATH=empty -c -o - %s | FileCheck %s -check-prefix CHECK-EVIL
@@ -7,14 +7,14 @@
 filename: __FILE__
 #include "file_test.h"
 
-// CHECK: filename: "/UNLIKELY_PATH/empty{{/|\\\\}}file_test.c"
-// CHECK: filename: "/UNLIKELY_PATH/empty{{/|\\\\}}file_test.h"
-// CHECK: basefile: "/UNLIKELY_PATH/empty{{/|\\\\}}file_test.c"
+// CHECK: filename: "/UNLIKELY_PATH/empty/file_test.c"
+// CHECK: filename: "/UNLIKELY_PATH/empty/file_test.h"
+// CHECK: basefile: "/UNLIKELY_PATH/empty/file_test.c"
 // CHECK-NOT: filename:
 
-// CHECK-EVIL: filename: "/UNLIKELY_PATH=empty{{/|\\\\}}file_test.c"
-// CHECK-EVIL: filename: "/UNLIKELY_PATH=empty{{/|\\\\}}file_test.h"
-// CHECK-EVIL: basefile: "/UNLIKELY_PATH=empty{{/|\\\\}}file_test.c"
+// CHECK-EVIL: filename: "/UNLIKELY_PATH=empty/file_test.c"
+// CHECK-EVIL: filename: "/UNLIKELY_PATH=empty/file_test.h"
+// CHECK-EVIL: basefile: "/UNLIKELY_PATH=empty/file_test.c"
 // CHECK-EVIL-NOT: filename:
 
 // CHECK-REMOVE: filename: "file_test.c"
Index: clang/test/Preprocessor/file_test_windows.c
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/file_test_windows.c
@@ -0,0 +1,23 @@
+// REQUIRES: system-windows
+// RUN: %clang -E -ffile-prefix-map=%p=A:\UNLIKELY_PATH\empty -c -o - %s | FileCheck %s
+// RUN: %clang -E -fmacro-prefix-map=%p=A:\UNLIKELY_PATH\empty -c -o - %s | FileCheck %s
+// RUN: %clang -E -fmacro-prefix-map=%p=A:\UNLIKELY_PATH=empty -c -o - %s | FileCheck %s -check-prefix CHECK-EVIL
+// RUN: %clang -E -fmacro-prefix-map=%p\= -c -o - %s | FileCheck %s --check-prefix CHECK-REMOVE
+
+filename: __FILE__
+#include "file_test.h"
+
+// CHECK: filename: "A:\\UNLIKELY_PATH\\empty{{/|\\\\}}file_test_windows.c"
+// CHECK: filename: "A:\\UNLIKELY_PATH\\empty{{/|\\\\}}file_test.h"
+// CHECK: basefile: "A:\\UNLIKELY_PATH\\empty{{/|\\\\}}file_test_windows.c"
+// CHECK-NOT: filename:
+
+// CHECK-EVIL: filename: "A:\\UNLIKELY_PATH=empty{{/|\\\\}}file_test_windows.c"
+// CHECK-EVIL: filename: "A:\\UNLIKELY_PATH=empty{{/|\\\\}}file_test.h"
+// CHECK-EVIL: basefile: "A:\\UNLIKELY_PATH=empty{{/|\\\\}}file_test_windows.c"
+// CHECK-EVIL-NOT: filename:
+
+// CHECK-REMOVE: filename: "file_test_windows.c"
+// CHECK-REMOVE: filename: "file_test.h"
+// CHECK-REMOVE: basefile: "file_test_windows.c"
+// CHECK-REMOVE-NOT: filename:
Index: llvm/include/llvm/Support/Path.h
===================================================================
--- llvm/include/llvm/Support/Path.h
+++ llvm/include/llvm/Support/Path.h
@@ -176,9 +176,9 @@
 ///        separators will only be matched and stripped when present
 ///        in \a OldPrefix.
 /// @result true if \a Path begins with OldPrefix
-bool replace_path_prefix(SmallVectorImpl<char> &Path,
-                         const StringRef &OldPrefix, const StringRef &NewPrefix,
-                         Style style = Style::native, bool strict = false);
+bool replace_path_prefix(SmallVectorImpl<char> &Path, StringRef OldPrefix,
+                         StringRef NewPrefix, Style style = Style::native,
+                         bool strict = false);
 
 /// Append to path.
 ///
Index: llvm/lib/DWARFLinker/DWARFLinker.cpp
===================================================================
--- llvm/lib/DWARFLinker/DWARFLinker.cpp
+++ llvm/lib/DWARFLinker/DWARFLinker.cpp
@@ -1920,10 +1920,12 @@
 
 static std::string remapPath(StringRef Path,
                              const objectPrefixMap &ObjectPrefixMap) {
+
+  SmallString<256> p = Path;
   for (const auto &Entry : ObjectPrefixMap)
-    if (Path.startswith(Entry.first))
-      return (Twine(Entry.second) + Path.substr(Entry.first.size())).str();
-  return Path.str();
+    if (llvm::sys::path::replace_path_prefix(p, Entry.first, Entry.second))
+      break;
+  return p.str().str();
 }
 
 bool DWARFLinker::registerModuleReference(
Index: llvm/lib/MC/MCContext.cpp
===================================================================
--- llvm/lib/MC/MCContext.cpp
+++ llvm/lib/MC/MCContext.cpp
@@ -589,13 +589,17 @@
 
 void MCContext::RemapDebugPaths() {
   const auto &DebugPrefixMap = this->DebugPrefixMap;
+  if (DebugPrefixMap.empty())
+    return;
+
   const auto RemapDebugPath = [&DebugPrefixMap](std::string &Path) {
-    for (const auto &Entry : DebugPrefixMap)
-      if (StringRef(Path).startswith(Entry.first)) {
-        std::string RemappedPath =
-            (Twine(Entry.second) + Path.substr(Entry.first.size())).str();
-        Path.swap(RemappedPath);
+    SmallString<256> P(Path);
+    for (const auto &Entry : DebugPrefixMap) {
+      if (llvm::sys::path::replace_path_prefix(P, Entry.first, Entry.second)) {
+        Path = P.str().str();
+        break;
       }
+    }
   };
 
   // Remap compilation directory.
Index: llvm/lib/Support/Path.cpp
===================================================================
--- llvm/lib/Support/Path.cpp
+++ llvm/lib/Support/Path.cpp
@@ -496,9 +496,8 @@
   path.append(ext.begin(), ext.end());
 }
 
-bool replace_path_prefix(SmallVectorImpl<char> &Path,
-                         const StringRef &OldPrefix, const StringRef &NewPrefix,
-                         Style style, bool strict) {
+bool replace_path_prefix(SmallVectorImpl<char> &Path, StringRef OldPrefix,
+                         StringRef NewPrefix, Style style, bool strict) {
   if (OldPrefix.empty() && NewPrefix.empty())
     return false;
 
@@ -509,11 +508,13 @@
     return false;
 
   // Ensure OldPrefixDir does not have a trailing separator.
-  if (!OldPrefix.empty() && is_separator(OldPrefix.back()))
+  if (!OldPrefix.empty() && is_separator(OldPrefix.back(), style))
     OldPrefixDir = parent_path(OldPrefix, style);
   else
     OldPrefixDir = OldPrefix;
 
+  StringRef OldPrefixSep = OldPrefix.substr(OldPrefixDir.size());
+
   if (!OrigPath.startswith(OldPrefixDir))
     return false;
 
@@ -528,13 +529,12 @@
   }
 
   StringRef RelPath = OrigPath.substr(OldPrefixDir.size());
-  SmallString<256> NewPath;
-  path::append(NewPath, style, NewPrefix);
+  SmallString<256> NewPath = NewPrefix;
   if (!RelPath.empty()) {
-    if (!is_separator(RelPath[0], style) || !strict)
+    if (!is_separator(RelPath[0], style) || (!strict && OldPrefixSep.empty()))
       path::append(NewPath, style, RelPath);
     else
-      path::append(NewPath, style, relative_path(RelPath, style));
+      path::append(NewPath, style, relative_path(RelPath, style));
   }
 
   Path.swap(NewPath);
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -1254,6 +1254,9 @@
   path::replace_path_prefix(Path, OldPrefix, EmptyPrefix);
   EXPECT_EQ(Path, "/foo");
   Path = Path2;
+  path::replace_path_prefix(Path, OldPrefixSep, EmptyPrefix);
+  EXPECT_EQ(Path, "foo");
+  Path = Path2;
   path::replace_path_prefix(Path, OldPrefix, EmptyPrefix, path::Style::native,
                             true);
   EXPECT_EQ(Path, "foo");