[yaml2obj] - Don't crash when `FileHeader` declares an empty `Flags` key in specific situations.

[grimar]

Summary

We currently call the llvm_unreachable for the following YAML:

--- !ELF
FileHeader:
  Class:   ELFCLASS32
  Data:    ELFDATA2LSB
  Type:    ET_REL
  Machine: EM_NONE
  Flags:   [ ]

it happens because the Flags key is present, though EM_NONE is a
machine type that has no known EF_* values and we call llvm_unreachable by mistake.

[jhenderson]

LGTM. Is there testing for the other flags discussed in that function? If not, we might want to add it.

[grimar]

In D86138#2223614, @jhenderson wrote:

LGTM. Is there testing for the other flags discussed in that function? If not, we might want to add it.

There is nothing yet, except the mips-eflags.yaml test for obj2yaml that does it for mips. Though I'd expect to see
a test for yaml2obj that uses something different, like llvm-readobj. So, yes, we might want to add more testing for all kind of EF_*.

llvm/lib/ObjectYAML/ELFYAML.cpp

case ELF::EM_AMDGPU:
     BCaseMask(EF_AMDGPU_MACH_AMDGCN_GFX1010, EF_AMDGPU_MACH);
     BCaseMask(EF_AMDGPU_MACH_AMDGCN_GFX1011, EF_AMDGPU_MACH);
     BCaseMask(EF_AMDGPU_MACH_AMDGCN_GFX1012, EF_AMDGPU_MACH);
     BCaseMask(EF_AMDGPU_MACH_AMDGCN_GFX1030, EF_AMDGPU_MACH);
     BCaseMask(EF_AMDGPU_MACH_AMDGCN_GFX1031, EF_AMDGPU_MACH);
     BCase(EF_AMDGPU_XNACK);
     BCase(EF_AMDGPU_SRAM_ECC);
     break;
-  case ELF::EM_X86_64:
-    break;
   default:
-    llvm_unreachable("Unsupported architecture");
+    break;
   }
 #undef BCase
 #undef BCaseMask
 }
 
 void ScalarEnumerationTraits<ELFYAML::ELF_SHT>::enumeration(
     IO &IO, ELFYAML::ELF_SHT &Value) {
   const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());

llvm/test/tools/yaml2obj/ELF/eflags.yaml

+## Check how the 'Flags' key can be used to encode e_flags field values.
+
+## Check we are able to produce no flags for EM_NONE. EM_NONE is an arbitrary
+## e_machine type that has no EF_* values defined for it.
+# RUN: yaml2obj %s -o %t-no-flags
+# RUN: llvm-readelf --file-headers %t-no-flags | FileCheck %s --check-prefix=NOFLAGS
+
+# NOFLAGS: Flags: 0x0{{$}}
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_NONE
+  Flags:   [ ]