Index: COFF/Driver.h
===================================================================
--- COFF/Driver.h
+++ COFF/Driver.h
@@ -103,6 +103,7 @@
   // entry point name.
   StringRef findDefaultEntry();
   WindowsSubsystem inferSubsystem();
+  MachineTypes inferMachineType();
 
   // Driver is the owner of all opened files.
   // InputFiles have MemoryBufferRefs to them.
Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -30,9 +30,7 @@
 #include <memory>
 
 using namespace llvm;
-using llvm::COFF::IMAGE_SUBSYSTEM_UNKNOWN;
-using llvm::COFF::IMAGE_SUBSYSTEM_WINDOWS_CUI;
-using llvm::COFF::IMAGE_SUBSYSTEM_WINDOWS_GUI;
+using namespace llvm::COFF;
 using llvm::sys::Process;
 using llvm::sys::fs::OpenFlags;
 using llvm::sys::fs::file_magic;
@@ -236,6 +234,16 @@
   return IMAGE_SUBSYSTEM_UNKNOWN;
 }
 
+MachineTypes LinkerDriver::inferMachineType() {
+  for (ObjectFile *F : Symtab.ObjectFiles) {
+    // Try to infer machine type from the magic byte of the object file.
+    auto MT = static_cast<MachineTypes>(F->getCOFFObj()->getMachine());
+    if (MT != IMAGE_FILE_MACHINE_UNKNOWN)
+      return MT;
+  }
+  return IMAGE_FILE_MACHINE_UNKNOWN;
+}
+
 bool LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
   // Needed for LTO.
   llvm::InitializeAllTargetInfos();
@@ -625,6 +633,15 @@
     }
   }
 
+  // Windows specific -- if no /machine is givne, infer that.
+  if (Config->MachineType == IMAGE_FILE_MACHINE_UNKNOWN) {
+    Config->MachineType = inferMachineType();
+    if (Config->MachineType == IMAGE_FILE_MACHINE_UNKNOWN) {
+      llvm::errs() << "/machine must be specified\n";
+      return false;
+    }
+  }
+
   // Windows specific -- when we are creating a .dll file, we also
   // need to create a .lib file.
   if (!Config->Exports.empty())
Index: COFF/Writer.cpp
===================================================================
--- COFF/Writer.cpp
+++ COFF/Writer.cpp
@@ -298,17 +298,6 @@
              RoundUpToAlignment(FileOff - SizeOfHeaders, FileAlignment);
 }
 
-static MachineTypes
-inferMachineType(const std::vector<ObjectFile *> &Files) {
-  for (ObjectFile *F : Files) {
-    // Try to infer machine type from the magic byte of the object file.
-    auto MT = static_cast<MachineTypes>(F->getCOFFObj()->getMachine());
-    if (MT != IMAGE_FILE_MACHINE_UNKNOWN)
-      return MT;
-  }
-  return IMAGE_FILE_MACHINE_UNKNOWN;
-}
-
 void Writer::writeHeader() {
   // Write DOS stub
   uint8_t *Buf = Buffer->getBufferStart();
@@ -323,15 +312,10 @@
   memcpy(Buf, PEMagic, sizeof(PEMagic));
   Buf += sizeof(PEMagic);
 
-  // Determine machine type, infer if needed. TODO: diagnose conflicts.
-  MachineTypes MachineType = Config->MachineType;
-  if (MachineType == IMAGE_FILE_MACHINE_UNKNOWN)
-    MachineType = inferMachineType(Symtab->ObjectFiles);
-
   // Write COFF header
   auto *COFF = reinterpret_cast<coff_file_header *>(Buf);
   Buf += sizeof(*COFF);
-  COFF->Machine = MachineType;
+  COFF->Machine = Config->MachineType;
   COFF->NumberOfSections = OutputSections.size();
   COFF->Characteristics = IMAGE_FILE_EXECUTABLE_IMAGE;
   COFF->Characteristics |= IMAGE_FILE_LARGE_ADDRESS_AWARE;