Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -1010,6 +1010,8 @@
                     .Case("obj", SaveTempsObj)
                     .Default(SaveTempsCwd);
   }
+  for (auto *Str : {&Dir, &InstalledDir, &SysRoot, &DyldPrefix, &ResourceDir})
+    *Str = llvm::sys::path::convert_to_slash(*Str);
 
   setLTOMode(Args);
 
Index: lib/Driver/ToolChain.cpp
===================================================================
--- lib/Driver/ToolChain.cpp
+++ lib/Driver/ToolChain.cpp
@@ -378,7 +378,7 @@
     SmallString<128> P(LibPath);
     llvm::sys::path::append(P, Prefix + Twine("clang_rt.") + Component + Suffix);
     if (getVFS().exists(P))
-      return P.str();
+      return llvm::sys::path::convert_to_slash(P);
   }
 
   StringRef Arch = getArchNameForCompilerRTLib(*this, Args);
@@ -386,7 +386,7 @@
   SmallString<128> Path(getCompilerRTPath());
   llvm::sys::path::append(Path, Prefix + Twine("clang_rt.") + Component + "-" +
                                     Arch + Env + Suffix);
-  return Path.str();
+  return llvm::sys::path::convert_to_slash(Path);
 }
 
 const char *ToolChain::getCompilerRTArgString(const llvm::opt::ArgList &Args,
@@ -425,7 +425,7 @@
 }
 
 std::string ToolChain::GetFilePath(const char *Name) const {
-  return D.GetFilePath(Name, *this);
+  return llvm::sys::path::convert_to_slash(D.GetFilePath(Name, *this));
 }
 
 std::string ToolChain::GetProgramPath(const char *Name) const {
@@ -774,12 +774,14 @@
 void ToolChain::AddFilePathLibArgs(const ArgList &Args,
                                    ArgStringList &CmdArgs) const {
   for (const auto &LibPath : getLibraryPaths())
-    if(LibPath.length() > 0)
-      CmdArgs.push_back(Args.MakeArgString(StringRef("-L") + LibPath));
+    if (LibPath.length() > 0)
+      CmdArgs.push_back(Args.MakeArgString(
+          StringRef("-L") + llvm::sys::path::convert_to_slash(LibPath)));
 
   for (const auto &LibPath : getFilePaths())
-    if(LibPath.length() > 0)
-      CmdArgs.push_back(Args.MakeArgString(StringRef("-L") + LibPath));
+    if (LibPath.length() > 0)
+      CmdArgs.push_back(Args.MakeArgString(
+          StringRef("-L") + llvm::sys::path::convert_to_slash(LibPath)));
 }
 
 void ToolChain::AddCCKextLibArgs(const ArgList &Args,
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -3488,7 +3488,8 @@
       // Handled with other dependency code.
     } else if (Output.isFilename()) {
       CmdArgs.push_back("-o");
-      CmdArgs.push_back(Output.getFilename());
+      CmdArgs.push_back(Args.MakeArgString(
+          llvm::sys::path::convert_to_slash(Output.getFilename())));
     } else {
       assert(Output.isNothing() && "Input output.");
     }
@@ -3496,7 +3497,8 @@
     for (const auto &II : Inputs) {
       addDashXForInput(Args, II, CmdArgs);
       if (II.isFilename())
-        CmdArgs.push_back(II.getFilename());
+        CmdArgs.push_back(Args.MakeArgString(
+            llvm::sys::path::convert_to_slash(II.getFilename())));
       else
         II.getInputArg().renderAsInput(Args, CmdArgs);
     }
@@ -4893,7 +4895,8 @@
 
   for (const InputInfo &Input : FrontendInputs) {
     if (Input.isFilename())
-      CmdArgs.push_back(Input.getFilename());
+      CmdArgs.push_back(Args.MakeArgString(
+          llvm::sys::path::convert_to_slash(Input.getFilename())));
     else
       Input.getInputArg().renderAsInput(Args, CmdArgs);
   }
Index: lib/Driver/ToolChains/Gnu.cpp
===================================================================
--- lib/Driver/ToolChains/Gnu.cpp
+++ lib/Driver/ToolChains/Gnu.cpp
@@ -1655,7 +1655,7 @@
                                           llvm::StringRef SysRoot) {
   const Arg *A = Args.getLastArg(clang::driver::options::OPT_gcc_toolchain);
   if (A)
-    return A->getValue();
+    return llvm::sys::path::convert_to_slash(A->getValue());
 
   // If we have a SysRoot, ignore GCC_INSTALL_PREFIX.
   // GCC_INSTALL_PREFIX specifies the gcc installation for the default