diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -143,8 +143,8 @@
   "missing argument to '%0'">;
 def err_drv_invalid_libcxx_deployment : Error<
   "invalid deployment target for -stdlib=libc++ (requires %0 or later)">;
-def err_drv_invalid_argument_to_fdebug_prefix_map : Error<
-  "invalid argument '%0' to -fdebug-prefix-map">;
+def err_drv_invalid_argument_to_option : Error<
+  "invalid argument '%0' to -%1">;
 def err_drv_malformed_sanitizer_blacklist : Error<
   "malformed sanitizer blacklist: '%0'">;
 def err_drv_duplicate_config : Error<
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1870,10 +1870,16 @@
   Flags<[CC1Option]>, HelpText<"Provide minimal debug info in the object/executable to facilitate online symbolication/stack traces in the absence of .dwo/.dwp files when using Split DWARF">;
 def fno_split_dwarf_inlining: Flag<["-"], "fno-split-dwarf-inlining">, Group<f_Group>,
   Flags<[CC1Option]>;
+def ffile_prefix_map_EQ
+  : Joined<["-"], "ffile-prefix-map=">, Group<f_Group>, Flags<[CC1Option]>,
+    HelpText<"remap file source paths in debug info and predefined preprocessor macros">;
 def fdebug_prefix_map_EQ
   : Joined<["-"], "fdebug-prefix-map=">, Group<f_Group>,
     Flags<[CC1Option,CC1AsOption]>,
     HelpText<"remap file source paths in debug info">;
+def fmacro_prefix_map_EQ
+  : Joined<["-"], "fmacro-prefix-map=">, Group<Preprocessor_Group>, Flags<[CC1Option]>,
+    HelpText<"remap file source paths in predefined preprocessor macros">;
 def g_Flag : Flag<["-"], "g">, Group<g_Group>,
   HelpText<"Generate source-level debug information">;
 def gline_tables_only : Flag<["-"], "gline-tables-only">, Group<gN_Group>,
diff --git a/clang/include/clang/Lex/PreprocessorOptions.h b/clang/include/clang/Lex/PreprocessorOptions.h
--- a/clang/include/clang/Lex/PreprocessorOptions.h
+++ b/clang/include/clang/Lex/PreprocessorOptions.h
@@ -169,6 +169,9 @@
   /// build it again.
   std::shared_ptr<FailedModulesSet> FailedModules;
 
+  /// A prefix map for __FILE__ and __BASE_FILE__
+  std::map<std::string, std::string, std::greater<std::string>> MacroPrefixMap;
+
 public:
   PreprocessorOptions() : PrecompiledPreambleBytes(0, false) {}
 
diff --git a/clang/lib/CodeGen/CGDebugInfo.h b/clang/lib/CodeGen/CGDebugInfo.h
--- a/clang/lib/CodeGen/CGDebugInfo.h
+++ b/clang/lib/CodeGen/CGDebugInfo.h
@@ -83,7 +83,7 @@
   /// Cache of previously constructed Types.
   llvm::DenseMap<const void *, llvm::TrackingMDRef> TypeCache;
 
-  llvm::SmallDenseMap<llvm::StringRef, llvm::StringRef> DebugPrefixMap;
+  std::map<llvm::StringRef, llvm::StringRef, std::greater<llvm::StringRef>> DebugPrefixMap;
 
   /// Cache that maps VLA types to size expressions for that type,
   /// represented by instantiated Metadata nodes.
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -475,10 +475,13 @@
 }
 
 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, true))
+      break;
+  while (llvm::sys::path::is_separator(p.back()))
+    p.set_size(p.size() - 1);
+  return p.str();
 }
 
 unsigned CGDebugInfo::getLineNumber(SourceLocation Loc) {
@@ -607,7 +610,7 @@
   if (CSKind)
     CSInfo.emplace(*CSKind, Checksum);
   llvm::DIFile *CUFile = DBuilder.createFile(
-      remapDIPath(MainFileName), remapDIPath(getCurrentDirname()), CSInfo,
+      MainFileName, remapDIPath(getCurrentDirname()), CSInfo,
       getSource(SM, SM.getMainFileID()));
 
   // Create new compile unit.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -611,16 +611,30 @@
 
 /// Add a CC1 and CC1AS option to specify the debug file path prefix map.
 static void addDebugPrefixMapArg(const Driver &D, const ArgList &Args, ArgStringList &CmdArgs) {
-  for (const Arg *A : Args.filtered(options::OPT_fdebug_prefix_map_EQ)) {
+  for (const Arg *A : Args.filtered(options::OPT_ffile_prefix_map_EQ,
+                                    options::OPT_fdebug_prefix_map_EQ)) {
     StringRef Map = A->getValue();
     if (Map.find('=') == StringRef::npos)
-      D.Diag(diag::err_drv_invalid_argument_to_fdebug_prefix_map) << Map;
+      D.Diag(diag::err_drv_invalid_argument_to_option) << Map << A->getOption().getName();
     else
       CmdArgs.push_back(Args.MakeArgString("-fdebug-prefix-map=" + Map));
     A->claim();
   }
 }
 
+/// Add a CC1 and CC1AS option to specify the macro file path prefix map.
+static void addMacroPrefixMapArg(const Driver &D, const ArgList &Args, ArgStringList &CmdArgs) {
+  for (const Arg *A : Args.filtered(options::OPT_ffile_prefix_map_EQ,
+                                    options::OPT_fmacro_prefix_map_EQ)) {
+    StringRef Map = A->getValue();
+    if (Map.find('=') == StringRef::npos)
+      D.Diag(diag::err_drv_invalid_argument_to_option) << Map << A->getOption().getName();
+    else
+      CmdArgs.push_back(Args.MakeArgString("-fmacro-prefix-map=" + Map));
+    A->claim();
+  }
+}
+
 /// Vectorize at all optimization levels greater than 1 except for -Oz.
 /// For -Oz the loop vectorizer is disabled, while the slp vectorizer is
 /// enabled.
@@ -1305,6 +1319,8 @@
     // For IAMCU add special include arguments.
     getToolChain().AddIAMCUIncludeArgs(Args, CmdArgs);
   }
+
+  addMacroPrefixMapArg(D, Args, CmdArgs);
 }
 
 // FIXME: Move to target hook.
diff --git a/clang/lib/Driver/ToolChains/FreeBSD.cpp b/clang/lib/Driver/ToolChains/FreeBSD.cpp
--- a/clang/lib/Driver/ToolChains/FreeBSD.cpp
+++ b/clang/lib/Driver/ToolChains/FreeBSD.cpp
@@ -12,6 +12,7 @@
 #include "Arch/Sparc.h"
 #include "CommonArgs.h"
 #include "clang/Driver/Compilation.h"
+#include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/SanitizerArgs.h"
 #include "llvm/Option/ArgList.h"
@@ -30,6 +31,7 @@
                                       const char *LinkingOutput) const {
   claimNoWarnArgs(Args);
   ArgStringList CmdArgs;
+  const auto &D = getToolChain().getDriver();
 
   // When building 32-bit code on FreeBSD/amd64, we have to explicitly
   // instruct as in the base system to assemble 32-bit code.
@@ -103,6 +105,18 @@
   }
   }
 
+  for (const Arg *A : Args.filtered(options::OPT_ffile_prefix_map_EQ,
+                                    options::OPT_fdebug_prefix_map_EQ)) {
+    StringRef Map = A->getValue();
+    if (Map.find('=') == StringRef::npos)
+      D.Diag(diag::err_drv_invalid_argument_to_option) << Map << A->getOption().getName();
+    else {
+      CmdArgs.push_back(Args.MakeArgString("--debug-prefix-map"));
+      CmdArgs.push_back(Args.MakeArgString(Map));
+    }
+    A->claim();
+  }
+
   Args.AddAllArgValues(CmdArgs, options::OPT_Wa_COMMA, options::OPT_Xassembler);
 
   CmdArgs.push_back("-o");
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -868,6 +868,18 @@
   }
   }
 
+  for (const Arg *A : Args.filtered(options::OPT_ffile_prefix_map_EQ,
+                                    options::OPT_fdebug_prefix_map_EQ)) {
+    StringRef Map = A->getValue();
+    if (Map.find('=') == StringRef::npos)
+      D.Diag(diag::err_drv_invalid_argument_to_option) << Map << A->getOption().getName();
+    else {
+      CmdArgs.push_back(Args.MakeArgString("--debug-prefix-map"));
+      CmdArgs.push_back(Args.MakeArgString(Map));
+    }
+    A->claim();
+  }
+
   Args.AddAllArgs(CmdArgs, options::OPT_I);
   Args.AddAllArgValues(CmdArgs, options::OPT_Wa_COMMA, options::OPT_Xassembler);
 
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3205,6 +3205,9 @@
   for (const auto *A : Args.filtered(OPT_error_on_deserialized_pch_decl))
     Opts.DeserializedPCHDeclsToErrorOn.insert(A->getValue());
 
+  for (const auto &A : Args.getAllArgValues(OPT_fmacro_prefix_map_EQ))
+    Opts.MacroPrefixMap.insert(StringRef(A).split('='));
+
   if (const Arg *A = Args.getLastArg(OPT_preamble_bytes_EQ)) {
     StringRef Value(A->getValue());
     size_t Comma = Value.find(',');
diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -28,6 +28,7 @@
 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/PreprocessorLexer.h"
+#include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Lex/Token.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
@@ -1436,6 +1437,15 @@
   return TI.getTriple().getEnvironment() == Env.getEnvironment();
 }
 
+static void remapMacroPath(
+    SmallString<256> &Path,
+    const std::map<std::string, std::string, std::greater<std::string>>
+        &MacroPrefixMap) {
+  for (const auto &Entry : MacroPrefixMap)
+    if (llvm::sys::path::replace_path_prefix(Path, Entry.first, Entry.second, true))
+      break;
+}
+
 /// ExpandBuiltinMacro - If an identifier token is read that is to be expanded
 /// as a builtin macro, handle it and return the next token as 'Tok'.
 void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
@@ -1502,7 +1512,7 @@
     }
 
     // Escape this filename.  Turn '\' -> '\\' '"' -> '\"'
-    SmallString<128> FN;
+    SmallString<256> FN;
     if (PLoc.isValid()) {
       // __FILE_NAME__ is a Clang-specific extension that expands to the
       // the last part of __FILE__.
@@ -1518,6 +1528,7 @@
         FN += PLoc.getFilename();
       }
       Lexer::Stringify(FN);
+      remapMacroPath(FN, PPOpts->MacroPrefixMap);
       OS << '"' << FN << '"';
     }
     Tok.setKind(tok::string_literal);
diff --git a/clang/test/CodeGen/debug-prefix-map.c b/clang/test/CodeGen/debug-prefix-map.c
--- a/clang/test/CodeGen/debug-prefix-map.c
+++ b/clang/test/CodeGen/debug-prefix-map.c
@@ -2,6 +2,8 @@
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH=empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-EVIL
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -main-file-name debug-prefix-map.c | FileCheck %s
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -fdebug-compilation-dir %p | FileCheck %s -check-prefix CHECK-COMPILATION-DIR
+// RUN: %clang -g -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang -g -ffile-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
 
 #include "Inputs/stdio.h"
 
diff --git a/clang/test/Driver/debug-prefix-map.S b/clang/test/Driver/debug-prefix-map.S
--- a/clang/test/Driver/debug-prefix-map.S
+++ b/clang/test/Driver/debug-prefix-map.S
@@ -1,4 +1,5 @@
 // RUN: %clang -### -g -fdebug-prefix-map=old=new %s 2>&1 | FileCheck %s
+// RUN: %clang -### -g -ffile-prefix-map=old=new %s 2>&1 | FileCheck %s
 
 // CHECK: cc1as
 // CHECK-SAME: -fdebug-prefix-map=old=new
diff --git a/clang/test/Driver/debug-prefix-map.c b/clang/test/Driver/debug-prefix-map.c
--- a/clang/test/Driver/debug-prefix-map.c
+++ b/clang/test/Driver/debug-prefix-map.c
@@ -1,9 +1,28 @@
-// RUN: %clang -### -fdebug-prefix-map=old %s 2>&1 | FileCheck %s -check-prefix CHECK-INVALID
-// RUN: %clang -### -fdebug-prefix-map=old=new %s 2>&1 | FileCheck %s -check-prefix CHECK-SIMPLE
-// RUN: %clang -### -fdebug-prefix-map=old=n=ew %s 2>&1 | FileCheck %s -check-prefix CHECK-COMPLEX
-// RUN: %clang -### -fdebug-prefix-map=old= %s 2>&1 | FileCheck %s -check-prefix CHECK-EMPTY
-
-// CHECK-INVALID: error: invalid argument 'old' to -fdebug-prefix-map
-// CHECK-SIMPLE: fdebug-prefix-map=old=new
-// CHECK-COMPLEX: fdebug-prefix-map=old=n=ew
-// CHECK-EMPTY: fdebug-prefix-map=old=
+// RUN: %clang -### -fdebug-prefix-map=old %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-INVALID
+// RUN: %clang -### -fmacro-prefix-map=old %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-INVALID
+// RUN: %clang -### -ffile-prefix-map=old %s 2>&1 | FileCheck %s -check-prefix CHECK-FILE-INVALID
+
+// RUN: %clang -### -fdebug-prefix-map=old=new %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-SIMPLE
+// RUN: %clang -### -fmacro-prefix-map=old=new %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-SIMPLE
+// RUN: %clang -### -ffile-prefix-map=old=new %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-SIMPLE
+// RUN: %clang -### -ffile-prefix-map=old=new %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-SIMPLE
+
+// RUN: %clang -### -fdebug-prefix-map=old=n=ew %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-COMPLEX
+// RUN: %clang -### -fmacro-prefix-map=old=n=ew %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-COMPLEX
+// RUN: %clang -### -ffile-prefix-map=old=n=ew %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-COMPLEX
+// RUN: %clang -### -ffile-prefix-map=old=n=ew %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-COMPLEX
+
+// RUN: %clang -### -fdebug-prefix-map=old= %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-EMPTY
+// RUN: %clang -### -fmacro-prefix-map=old= %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-EMPTY
+// RUN: %clang -### -ffile-prefix-map=old= %s 2>&1 | FileCheck %s -check-prefix CHECK-DEBUG-EMPTY
+// RUN: %clang -### -ffile-prefix-map=old= %s 2>&1 | FileCheck %s -check-prefix CHECK-MACRO-EMPTY
+
+// CHECK-DEBUG-INVALID: error: invalid argument 'old' to -fdebug-prefix-map
+// CHECK-MACRO-INVALID: error: invalid argument 'old' to -fmacro-prefix-map
+// CHECK-FILE-INVALID: error: invalid argument 'old' to -ffile-prefix-map
+// CHECK-DEBUG-SIMPLE: fdebug-prefix-map=old=new
+// CHECK-MACRO-SIMPLE: fmacro-prefix-map=old=new
+// CHECK-DEBUG-COMPLEX: fdebug-prefix-map=old=n=ew
+// CHECK-MACRO-COMPLEX: fmacro-prefix-map=old=n=ew
+// CHECK-DEBUG-EMPTY: fdebug-prefix-map=old=
+// CHECK-MACRO-EMPTY: fmacro-prefix-map=old=
diff --git a/clang/test/Preprocessor/file_test.h b/clang/test/Preprocessor/file_test.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/file_test.h
@@ -0,0 +1,2 @@
+filename: __FILE__
+basefile: __BASE_FILE__
diff --git a/clang/test/Preprocessor/file_test.c b/clang/test/Preprocessor/file_test.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/file_test.c
@@ -0,0 +1,22 @@
+// 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
+// RUN: %clang -E -fmacro-prefix-map=%p/= -c -o - %s | FileCheck %s --check-prefix CHECK-REMOVE
+
+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-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-NOT: filename:
+
+// CHECK-REMOVE: filename: "file_test.c"
+// CHECK-REMOVE: filename: "file_test.h"
+// CHECK-REMOVE: basefile: "file_test.c"
+// CHECK-REMOVE-NOT: filename:
diff --git a/llvm/include/llvm/Support/Path.h b/llvm/include/llvm/Support/Path.h
--- a/llvm/include/llvm/Support/Path.h
+++ b/llvm/include/llvm/Support/Path.h
@@ -150,18 +150,35 @@
 ///
 /// @code
 ///   /foo, /old, /new => /foo
+///   /old, /old, /new => /new
+///   /old, /old/, /new, false => /old
+///   /old, /old/, /new, true => /new
 ///   /old/foo, /old, /new => /new/foo
+///   /old/foo, /old/, /new => /new/foo
+///   /old/foo, /old/, /new/ => /new/foo
+///   /oldfoo, /old, /new => /oldfoo
 ///   /foo, <empty>, /new => /new/foo
-///   /old/foo, /old, <empty> => /foo
+///   /foo, <empty>, new => new/foo
+///   /old/foo, /old, <empty>, false => /foo
+///   /old/foo, /old, <empty>, true => foo
 /// @endcode
 ///
 /// @param Path If \a Path starts with \a OldPrefix modify to instead
 ///        start with \a NewPrefix.
-/// @param OldPrefix The path prefix to strip from \a Path.
+/// @param OldPrefix The path prefix to strip from \a Path. Any trailing
+///        path separator is ignored if strict is true.
 /// @param NewPrefix The path prefix to replace \a NewPrefix with.
-void replace_path_prefix(SmallVectorImpl<char> &Path,
+/// @param style The path separator style
+/// @param strict Strict prefix path checking
+/// @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);
+                         Style style = Style::native, bool strict = false);
+static inline bool replace_path_prefix(SmallVectorImpl<char> &Path,
+                         const StringRef &OldPrefix, const StringRef &NewPrefix,
+                         bool strict, Style style = Style::native) {
+  return replace_path_prefix(Path, OldPrefix, NewPrefix, style, strict);
+}
 
 /// Append to path.
 ///
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -496,27 +496,53 @@
   path.append(ext.begin(), ext.end());
 }
 
-void replace_path_prefix(SmallVectorImpl<char> &Path,
+bool replace_path_prefix(SmallVectorImpl<char> &Path,
                          const StringRef &OldPrefix, const StringRef &NewPrefix,
-                         Style style) {
+                         Style style, bool strict) {
   if (OldPrefix.empty() && NewPrefix.empty())
-    return;
+    return false;
 
   StringRef OrigPath(Path.begin(), Path.size());
-  if (!OrigPath.startswith(OldPrefix))
-    return;
+  StringRef OldPrefixDir;
+
+  if (!strict && OldPrefix.size() > OrigPath.size())
+    return false;
+
+  if (!strict && OldPrefix.size() > OrigPath.size())
+    return false;
+
+  // Ensure OldPrefixDir does not have a trailing separator.
+  if (!OldPrefix.empty() && is_separator(OldPrefix.back()))
+    OldPrefixDir = parent_path(OldPrefix, style);
+  else
+    OldPrefixDir = OldPrefix;
+
+  if (!OrigPath.startswith(OldPrefixDir))
+    return false;
+
+  if (OrigPath.size() > OldPrefixDir.size())
+    if (!is_separator(OrigPath[OldPrefixDir.size()], style) && strict)
+      return false;
 
   // If prefixes have the same size we can simply copy the new one over.
-  if (OldPrefix.size() == NewPrefix.size()) {
+  if (OldPrefixDir.size() == NewPrefix.size() && !strict) {
     llvm::copy(NewPrefix, Path.begin());
-    return;
+    return true;
   }
 
-  StringRef RelPath = OrigPath.substr(OldPrefix.size());
+  StringRef RelPath = OrigPath.substr(OldPrefixDir.size());
   SmallString<256> NewPath;
   path::append(NewPath, style, NewPrefix);
-  path::append(NewPath, style, RelPath);
+  if (!RelPath.empty()) {
+    if (!is_separator(RelPath[0], style) || !strict)
+      path::append(NewPath, style, RelPath);
+    else
+      path::append(NewPath, style, relative_path(RelPath, style));
+  }
+
   Path.swap(NewPath);
+
+  return true;
 }
 
 void native(const Twine &path, SmallVectorImpl<char> &result, Style style) {
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -1219,7 +1219,9 @@
 TEST(Support, ReplacePathPrefix) {
   SmallString<64> Path1("/foo");
   SmallString<64> Path2("/old/foo");
+  SmallString<64> Path3("/oldnew/foo");
   SmallString<64> OldPrefix("/old");
+  SmallString<64> OldPrefixSep("/old/");
   SmallString<64> NewPrefix("/new");
   SmallString<64> NewPrefix2("/longernew");
   SmallString<64> EmptyPrefix("");
@@ -1239,6 +1241,33 @@
   Path = Path2;
   path::replace_path_prefix(Path, OldPrefix, EmptyPrefix);
   EXPECT_EQ(Path, "/foo");
+  Path = Path2;
+  path::replace_path_prefix(Path, OldPrefix, EmptyPrefix, true);
+  EXPECT_EQ(Path, "foo");
+  Path = Path3;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix, false);
+  EXPECT_EQ(Path, "/newnew/foo");
+  Path = Path3;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix, true);
+  EXPECT_EQ(Path, "/oldnew/foo");
+  Path = Path3;
+  path::replace_path_prefix(Path, OldPrefixSep, NewPrefix, true);
+  EXPECT_EQ(Path, "/oldnew/foo");
+  Path = Path1;
+  path::replace_path_prefix(Path, EmptyPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new/foo");
+  Path = OldPrefix;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new");
+  Path = OldPrefixSep;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new/");
+  Path = OldPrefix;
+  path::replace_path_prefix(Path, OldPrefixSep, NewPrefix, false);
+  EXPECT_EQ(Path, "/old");
+  Path = OldPrefix;
+  path::replace_path_prefix(Path, OldPrefixSep, NewPrefix, true);
+  EXPECT_EQ(Path, "/new");
 }
 
 TEST_F(FileSystemTest, OpenFileForRead) {