Index: clang/lib/Driver/ToolChains/MinGW.cpp
===================================================================
--- clang/lib/Driver/ToolChains/MinGW.cpp
+++ clang/lib/Driver/ToolChains/MinGW.cpp
@@ -160,7 +160,19 @@
   }
 
   CmdArgs.push_back("-o");
-  CmdArgs.push_back(Output.getFilename());
+  const char *OutputFile = Output.getFilename();
+  // GCC implicitly adds an .exe extension if it is given an output file name
+  // that lacks an extension. However, GCC only does this when actually
+  // running on windows, not when operating as a cross compiler. As some users
+  // have come to rely on this behaviour, try to replicate it.
+#ifdef _WIN32
+  if (!llvm::sys::path::has_extension(OutputFile))
+    CmdArgs.push_back(Args.MakeArgString(Twine(OutputFile) + ".exe"));
+  else
+    CmdArgs.push_back(OutputFile);
+#else
+  CmdArgs.push_back(OutputFile);
+#endif
 
   Args.AddAllArgs(CmdArgs, options::OPT_e);
   // FIXME: add -N, -n flags
Index: clang/test/Driver/mingw-implicit-extension-cross.c
===================================================================
--- /dev/null
+++ clang/test/Driver/mingw-implicit-extension-cross.c
@@ -0,0 +1,9 @@
+// Test how an implicit .exe extension is added. If not running the compiler
+// on windows, no implicit extension is added. (Therefore, this test is skipped
+// when running on windows.)
+
+// UNSUPPORTED: system-windows
+
+// RUN: %clang -target i686-windows-gnu -### --sysroot=%S/Inputs/mingw_clang_tree/mingw32 %s -o outputname 2>&1 | FileCheck %s
+
+// CHECK: "-o" "outputname"
Index: clang/test/Driver/mingw-implicit-extension-windows.c
===================================================================
--- /dev/null
+++ clang/test/Driver/mingw-implicit-extension-windows.c
@@ -0,0 +1,14 @@
+// Test how an implicit .exe extension is added. If running the compiler
+// on windows, an implicit extension is added if none is provided in the
+// given name. (Therefore, this test is skipped when not running on windows.)
+
+// REQUIRES: system-windows
+
+// RUN: %clang -target i686-windows-gnu -### --sysroot=%S/Inputs/mingw_clang_tree/mingw32 %s -o outputname 2>&1 | FileCheck %s --check-prefix=CHECK-OUTPUTNAME-EXE
+
+// RUN: %clang -target i686-windows-gnu -### --sysroot=%S/Inputs/mingw_clang_tree/mingw32 %s -o outputname.exe 2>&1 | FileCheck %s --check-prefix=CHECK-OUTPUTNAME-EXE
+
+// RUN: %clang -target i686-windows-gnu -### --sysroot=%S/Inputs/mingw_clang_tree/mingw32 %s -o outputname.q 2>&1 | FileCheck %s --check-prefix=CHECK-OUTPUTNAME-Q
+
+// CHECK-OUTPUTNAME-EXE: "-o" "outputname.exe"
+// CHECK-OUTPUTNAME-Q: "-o" "outputname.q"