Index: lib/Target/X86/MCTargetDesc/X86WinCOFFObjectWriter.cpp
===================================================================
--- lib/Target/X86/MCTargetDesc/X86WinCOFFObjectWriter.cpp
+++ lib/Target/X86/MCTargetDesc/X86WinCOFFObjectWriter.cpp
@@ -79,7 +79,8 @@
     case FK_SecRel_4:
       return COFF::IMAGE_REL_AMD64_SECREL;
     default:
-      llvm_unreachable("unsupported relocation type");
+      Ctx.reportError(Fixup.getLoc(), "unsupported relocation type");
+      return COFF::IMAGE_REL_AMD64_ADDR32;
     }
   } else if (getMachine() == COFF::IMAGE_FILE_MACHINE_I386) {
     switch (FixupKind) {
@@ -100,7 +101,8 @@
     case FK_SecRel_4:
       return COFF::IMAGE_REL_I386_SECREL;
     default:
-      llvm_unreachable("unsupported relocation type");
+      Ctx.reportError(Fixup.getLoc(), "unsupported relocation type");
+      return COFF::IMAGE_REL_I386_DIR32;
     }
   } else
     llvm_unreachable("Unsupported COFF machine type.");
Index: test/MC/COFF/unsupported-relocations.s
===================================================================
--- /dev/null
+++ test/MC/COFF/unsupported-relocations.s
@@ -0,0 +1,5 @@
+// RUN: not llvm-mc -filetype=obj -triple i386-pc-win32 %s 2>&1 | FileCheck %s
+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-win32 %s 2>&1 | FileCheck %s
+// CHECK: unsupported relocation type
+        .text
+        mov $_GLOBAL_OFFSET_TABLE_, %eax