[COFF] [X86] Don't use llvm_unreachable for unsupported relocation types

[mstorsjo]

Summary

This can happen if assembling a reference to _GLOBAL_OFFSET_TABLE_.

While it doesn't make sense to try to assemble that for COFF, the fact that we previously used llvm_unreachable meant that the code had undefined behaviour if something tried to assemble that.

The configure script of libgmp would try to assemble such a snippet (which should signal a failure). If llvm is built without assertions, the undefined behaviour meant a (near) infinite loop.

[rnk]

lgtm, thanks for improving the error handling. MC needs a lot of work in that area.

lib/Target/X86/MCTargetDesc/X86WinCOFFObjectWriter.cpp

case X86::reloc_signed_4byte_relax:
       return COFF::IMAGE_REL_AMD64_ADDR32;
     case FK_Data_8:
       return COFF::IMAGE_REL_AMD64_ADDR64;
     case FK_SecRel_2:
       return COFF::IMAGE_REL_AMD64_SECTION;
     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) {
     case FK_PCRel_4:
     case X86::reloc_riprel_4byte:
     case X86::reloc_riprel_4byte_movq_load:
       return COFF::IMAGE_REL_I386_REL32;
     case FK_Data_4:
     case X86::reloc_signed_4byte:
     case X86::reloc_signed_4byte_relax:
       if (Modifier == MCSymbolRefExpr::VK_COFF_IMGREL32)
         return COFF::IMAGE_REL_I386_DIR32NB;
       if (Modifier == MCSymbolRefExpr::VK_SECREL)
         return COFF::IMAGE_REL_AMD64_SECREL;
       return COFF::IMAGE_REL_I386_DIR32;
     case FK_SecRel_2:
       return COFF::IMAGE_REL_I386_SECTION;
     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.");
 }
 
 std::unique_ptr<MCObjectTargetWriter>
 llvm::createX86WinCOFFObjectWriter(bool Is64Bit) {
   return llvm::make_unique<X86WinCOFFObjectWriter>(Is64Bit);
 }

test/MC/COFF/unsupported-relocations.s

+// 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