Index: cfe/trunk/include/clang/Driver/CLCompatOptions.td
===================================================================
--- cfe/trunk/include/clang/Driver/CLCompatOptions.td
+++ cfe/trunk/include/clang/Driver/CLCompatOptions.td
@@ -182,6 +182,9 @@
   Flags<[CLOption, DriverOption]>, HelpText<"Use static run-time">;
 def _SLASH_MTd : Option<["/", "-"], "MTd", KIND_FLAG>, Group<_SLASH_M_Group>,
   Flags<[CLOption, DriverOption]>, HelpText<"Use static debug run-time">;
+def _SLASH_o : CLJoinedOrSeparate<"o">,
+  HelpText<"Set output file or directory (ends in / or \\)">,
+  MetaVarName<"<file or directory>">;
 def _SLASH_P : CLFlag<"P">, HelpText<"Preprocess to file">;
 def _SLASH_Tc : CLCompileJoinedOrSeparate<"Tc">,
   HelpText<"Specify a C source file">, MetaVarName<"<filename>">;
@@ -256,7 +259,6 @@
 def _SLASH_kernel : CLFlag<"kernel">;
 def _SLASH_LN : CLFlag<"LN">;
 def _SLASH_MP : CLJoined<"MP">;
-def _SLASH_o : CLJoinedOrSeparate<"o">;
 def _SLASH_openmp : CLFlag<"openmp">;
 def _SLASH_Qfast_transcendentals : CLFlag<"Qfast_transcendentals">;
 def _SLASH_QIfist : CLFlag<"QIfist">;
Index: cfe/trunk/lib/Driver/Driver.cpp
===================================================================
--- cfe/trunk/lib/Driver/Driver.cpp
+++ cfe/trunk/lib/Driver/Driver.cpp
@@ -1180,6 +1180,15 @@
     }
   }
 
+  // Diagnose misuse of /o.
+  if (Arg *A = Args.getLastArg(options::OPT__SLASH_o)) {
+    if (A->getValue()[0] == '\0') {
+      // It has to have a value.
+      Diag(clang::diag::err_drv_missing_argument) << A->getSpelling() << 1;
+      Args.eraseArg(options::OPT__SLASH_o);
+    }
+  }
+
   // Construct the actions to perform.
   ActionList LinkerInputs;
 
@@ -1651,7 +1660,8 @@
     assert(AtTopLevel && isa<PreprocessJobAction>(JA));
     StringRef BaseName = llvm::sys::path::filename(BaseInput);
     StringRef NameArg;
-    if (Arg *A = C.getArgs().getLastArg(options::OPT__SLASH_Fi))
+    if (Arg *A = C.getArgs().getLastArg(options::OPT__SLASH_Fi,
+                                        options::OPT__SLASH_o))
       NameArg = A->getValue();
     return C.addResultFile(MakeCLOutputFilename(C.getArgs(), NameArg, BaseName,
                                                 types::TY_PP_C), &JA);
@@ -1698,15 +1708,17 @@
   const char *NamedOutput;
 
   if (JA.getType() == types::TY_Object &&
-      C.getArgs().hasArg(options::OPT__SLASH_Fo)) {
-    // The /Fo flag decides the object filename.
-    StringRef Val = C.getArgs().getLastArg(options::OPT__SLASH_Fo)->getValue();
+      C.getArgs().hasArg(options::OPT__SLASH_Fo, options::OPT__SLASH_o)) {
+    // The /Fo or /o flag decides the object filename.
+    StringRef Val = C.getArgs().getLastArg(options::OPT__SLASH_Fo,
+                                           options::OPT__SLASH_o)->getValue();
     NamedOutput = MakeCLOutputFilename(C.getArgs(), Val, BaseName,
                                        types::TY_Object);
   } else if (JA.getType() == types::TY_Image &&
-             C.getArgs().hasArg(options::OPT__SLASH_Fe)) {
-    // The /Fe flag names the linked file.
-    StringRef Val = C.getArgs().getLastArg(options::OPT__SLASH_Fe)->getValue();
+             C.getArgs().hasArg(options::OPT__SLASH_Fe, options::OPT__SLASH_o)) {
+    // The /Fe or /o flag names the linked file.
+    StringRef Val = C.getArgs().getLastArg(options::OPT__SLASH_Fe,
+                                           options::OPT__SLASH_o)->getValue();
     NamedOutput = MakeCLOutputFilename(C.getArgs(), Val, BaseName,
                                        types::TY_Image);
   } else if (JA.getType() == types::TY_Image) {
Index: cfe/trunk/test/Driver/cl-outputs.c
===================================================================
--- cfe/trunk/test/Driver/cl-outputs.c
+++ cfe/trunk/test/Driver/cl-outputs.c
@@ -35,6 +35,59 @@
 // CHECK-MULTIPLESOURCEOK: "-o" "mydir{{[/\\]+}}cl-outputs.obj"
 
 
+// RUN: %clang_cl /c /oa -### -- %s 2>&1 | FileCheck -check-prefix=FooNAME1 %s
+// FooNAME1:  "-o" "a.obj"
+
+// RUN: %clang_cl /c /o a -### -- %s 2>&1 | FileCheck -check-prefix=FooNAME2 %s
+// FooNAME2:  "-o" "a.obj"
+
+// RUN: %clang_cl /c /oa.ext /ob.ext -### -- %s 2>&1 | FileCheck -check-prefix=FooNAMEEXT1 %s
+// FooNAMEEXT1:  "-o" "b.ext"
+
+// RUN: %clang_cl /c /o a.ext /ob.ext -### -- %s 2>&1 | FileCheck -check-prefix=FooNAMEEXT2 %s
+// FooNAMEEXT2:  "-o" "b.ext"
+
+// RUN: %clang_cl /c /ofoo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FooDIR1 %s
+// FooDIR1:  "-o" "foo.dir{{[/\\]+}}cl-outputs.obj"
+
+// RUN: %clang_cl /c /o foo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FooDIR2 %s
+// FooDIR2:  "-o" "foo.dir{{[/\\]+}}cl-outputs.obj"
+
+// RUN: %clang_cl /c /ofoo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FooDIRNAME1 %s
+// FooDIRNAME1:  "-o" "foo.dir{{[/\\]+}}a.obj"
+
+// RUN: %clang_cl /c /o foo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FooDIRNAME2 %s
+// FooDIRNAME2:  "-o" "foo.dir{{[/\\]+}}a.obj"
+
+// RUN: %clang_cl /c /ofoo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FooDIRNAMEEXT1 %s
+// FooDIRNAMEEXT1:  "-o" "foo.dir{{[/\\]+}}a.ext"
+
+// RUN: %clang_cl /c /o foo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FooDIRNAMEEXT2 %s
+// FooDIRNAMEEXT2:  "-o" "foo.dir{{[/\\]+}}a.ext"
+
+// RUN: %clang_cl /c /o.. -### -- %s 2>&1 | FileCheck -check-prefix=FooCRAZY1 %s
+// FooCRAZY1:  "-o" "..obj"
+
+// RUN: %clang_cl /c /o .. -### -- %s 2>&1 | FileCheck -check-prefix=FooCRAZY2 %s
+// FooCRAZY2:  "-o" "..obj"
+
+// RUN: %clang_cl /c %s -### /o 2>&1 | FileCheck -check-prefix=FooMISSINGARG %s
+// FooMISSINGARG: error: argument to '/o' is missing (expected 1 value)
+
+// RUN: %clang_cl /c /omydir/ -### -- %s %s 2>&1 | FileCheck -check-prefix=CHECK-oMULTIPLESOURCEOK1 %s
+// CHECK-oMULTIPLESOURCEOK1: "-o" "mydir{{[/\\]+}}cl-outputs.obj"
+
+// RUN: %clang_cl /c /o mydir/ -### -- %s %s 2>&1 | FileCheck -check-prefix=CHECK-oMULTIPLESOURCEOK2 %s
+// CHECK-oMULTIPLESOURCEOK2: "-o" "mydir{{[/\\]+}}cl-outputs.obj"
+
+
+// RUN: %clang_cl /c /obar /Fofoo -### -- %s 2>&1 | FileCheck -check-prefix=FooRACE1 %s
+// FooRACE1: "-o" "foo.obj"
+
+// RUN: %clang_cl /c /Fofoo /obar -### -- %s 2>&1 | FileCheck -check-prefix=FooRACE2 %s
+// FooRACE2: "-o" "bar.obj"
+
+
 // RUN: %clang_cl -### -- %s 2>&1 | FileCheck -check-prefix=DEFAULTEXE %s
 // DEFAULTEXE: cl-outputs.exe
 
@@ -90,6 +143,75 @@
 // FeOVERRIDE: "-out:bar.exe"
 
 
+// RUN: %clang_cl /obar /Fefoo -### -- %s 2>&1 | FileCheck -check-prefix=FeoRACE1 %s
+// FeoRACE1: "-out:foo.exe"
+
+// RUN: %clang_cl /Fefoo /obar -### -- %s 2>&1 | FileCheck -check-prefix=FeoRACE2 %s
+// FeoRACE2: "-out:bar.exe"
+
+
+// RUN: %clang_cl /ofoo -### -- %s 2>&1 | FileCheck -check-prefix=FeoNOEXT1 %s
+// FeoNOEXT1: "-out:foo.exe"
+
+// RUN: %clang_cl /o foo -### -- %s 2>&1 | FileCheck -check-prefix=FeoNOEXT2 %s
+// FeoNOEXT2: "-out:foo.exe"
+
+// RUN: %clang_cl /o foo /LD -### -- %s 2>&1 | FileCheck -check-prefix=FeoNOEXTDLL %s
+// RUN: %clang_cl /ofoo /LDd -### -- %s 2>&1 | FileCheck -check-prefix=FeoNOEXTDLL %s
+// FeoNOEXTDLL: "-out:foo.dll"
+// FeoNOEXTDLL: "-implib:foo.lib"
+
+// RUN: %clang_cl /ofoo.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoEXT1 %s
+// FeoEXT1: "-out:foo.ext"
+
+// RUN: %clang_cl /o foo.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoEXT2 %s
+// FeoEXT2: "-out:foo.ext"
+
+// RUN: %clang_cl /LD /o foo.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoEXTDLL %s
+// RUN: %clang_cl /LDd /ofoo.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoEXTDLL %s
+// FeoEXTDLL: "-out:foo.ext"
+// FeoEXTDLL: "-implib:foo.lib"
+
+// RUN: %clang_cl /ofoo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIR1 %s
+// FeoDIR1: "-out:foo.dir{{[/\\]+}}cl-outputs.exe"
+
+// RUN: %clang_cl /o foo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIR2 %s
+// FeoDIR2: "-out:foo.dir{{[/\\]+}}cl-outputs.exe"
+
+// RUN: %clang_cl /LD /o foo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRDLL %s
+// RUN: %clang_cl /LDd /ofoo.dir/ -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRDLL %s
+// FeoDIRDLL: "-out:foo.dir{{[/\\]+}}cl-outputs.dll"
+// FeoDIRDLL: "-implib:foo.dir{{[/\\]+}}cl-outputs.lib"
+
+// RUN: %clang_cl /ofoo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAME1 %s
+// FeoDIRNAME1: "-out:foo.dir{{[/\\]+}}a.exe"
+
+// RUN: %clang_cl /o foo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAME2 %s
+// FeoDIRNAME2: "-out:foo.dir{{[/\\]+}}a.exe"
+
+// RUN: %clang_cl /LD /o foo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEDLL %s
+// RUN: %clang_cl /LDd /ofoo.dir/a -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEDLL %s
+// FeoDIRNAMEDLL: "-out:foo.dir{{[/\\]+}}a.dll"
+// FeoDIRNAMEDLL: "-implib:foo.dir{{[/\\]+}}a.lib"
+
+// RUN: %clang_cl /ofoo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEEXT1 %s
+// FeoDIRNAMEEXT1: "-out:foo.dir{{[/\\]+}}a.ext"
+
+// RUN: %clang_cl /o foo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEEXT2 %s
+// FeoDIRNAMEEXT2: "-out:foo.dir{{[/\\]+}}a.ext"
+
+// RUN: %clang_cl /LD /o foo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEEXTDLL %s
+// RUN: %clang_cl /LDd /ofoo.dir/a.ext -### -- %s 2>&1 | FileCheck -check-prefix=FeoDIRNAMEEXTDLL %s
+// FeoDIRNAMEEXTDLL: "-out:foo.dir{{[/\\]+}}a.ext"
+// FeoDIRNAMEEXTDLL: "-implib:foo.dir{{[/\\]+}}a.lib"
+
+// RUN: %clang_cl -### /o 2>&1 | FileCheck -check-prefix=FeoMISSINGARG %s
+// FeoMISSINGARG: error: argument to '/o' is missing (expected 1 value)
+
+// RUN: %clang_cl /ofoo /o bar -### -- %s 2>&1 | FileCheck -check-prefix=FeoOVERRIDE %s
+// FeoOVERRIDE: "-out:bar.exe"
+
+
 // RUN: %clang_cl /FA -### -- %s 2>&1 | FileCheck -check-prefix=FA %s
 // FA: "-o" "cl-outputs.asm"
 // RUN: %clang_cl /FA /Fafoo -### -- %s 2>&1 | FileCheck -check-prefix=FaNAME %s
@@ -117,3 +239,28 @@
 // RUN: %clang_cl /P /Fifoo.x -### -- %s 2>&1 | FileCheck -check-prefix=Fi2 %s
 // Fi2: "-E"
 // Fi2: "-o" "foo.x"
+
+// RUN: %clang_cl /P /ofoo -### -- %s 2>&1 | FileCheck -check-prefix=Fio1 %s
+// Fio1: "-E"
+// Fio1: "-o" "foo.i"
+
+// RUN: %clang_cl /P /o foo -### -- %s 2>&1 | FileCheck -check-prefix=Fio2 %s
+// Fio2: "-E"
+// Fio2: "-o" "foo.i"
+
+// RUN: %clang_cl /P /ofoo.x -### -- %s 2>&1 | FileCheck -check-prefix=Fio3 %s
+// Fio3: "-E"
+// Fio3: "-o" "foo.x"
+
+// RUN: %clang_cl /P /o foo.x -### -- %s 2>&1 | FileCheck -check-prefix=Fio4 %s
+// Fio4: "-E"
+// Fio4: "-o" "foo.x"
+
+
+// RUN: %clang_cl /P /obar.x /Fifoo.x -### -- %s 2>&1 | FileCheck -check-prefix=FioRACE1 %s
+// FioRACE1: "-E"
+// FioRACE1: "-o" "foo.x"
+
+// RUN: %clang_cl /P /Fifoo.x /obar.x -### -- %s 2>&1 | FileCheck -check-prefix=FioRACE2 %s
+// FioRACE2: "-E"
+// FioRACE2: "-o" "bar.x"