Reapply "Allow "[[FLAGS=<none>]]" value in the ELF Fileheader Flags field (#143845)" (#151094)

This fixes the issues with 0b054e2

This reverts commit b80ce05.

Author: aakanksha555

llvm/include/llvm/ObjectYAML/ELFYAML.h

@@ -117,7 +117,7 @@ struct FileHeader {
   llvm::yaml::Hex8 ABIVersion;
   ELF_ET Type;
   std::optional<ELF_EM> Machine;
-  ELF_EF Flags;
+  std::optional<ELF_EF> Flags;
   llvm::yaml::Hex64 Entry;
   std::optional<StringRef> SectionHeaderStringTable;
 

llvm/lib/ObjectYAML/ELFEmitter.cpp

@@ -481,7 +481,11 @@ void ELFState<ELFT>::writeELFHeader(raw_ostream &OS) {
 
   Header.e_version = EV_CURRENT;
   Header.e_entry = Doc.Header.Entry;
-  Header.e_flags = Doc.Header.Flags;
+  if (Doc.Header.Flags)
+    Header.e_flags = *Doc.Header.Flags;
+  else
+    Header.e_flags = 0;
+
   Header.e_ehsize = sizeof(Elf_Ehdr);
 
   if (Doc.Header.EPhOff)

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -1160,7 +1160,7 @@ void MappingTraits<ELFYAML::FileHeader>::mapping(IO &IO,
   IO.mapOptional("ABIVersion", FileHdr.ABIVersion, Hex8(0));
   IO.mapRequired("Type", FileHdr.Type);
   IO.mapOptional("Machine", FileHdr.Machine);
-  IO.mapOptional("Flags", FileHdr.Flags, ELFYAML::ELF_EF(0));
+  IO.mapOptional("Flags", FileHdr.Flags);
   IO.mapOptional("Entry", FileHdr.Entry, Hex64(0));
   IO.mapOptional("SectionHeaderStringTable", FileHdr.SectionHeaderStringTable);
 

llvm/test/tools/obj2yaml/ELF/eflags.yaml

@@ -0,0 +1,31 @@
+## Check how obj2yaml dumps e_flags field.
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2MSB
+  Type:    ET_EXEC
+  Machine: EM_SPARC32PLUS
+  Flags:   [ [[FLAGS]] ]
+
+# RUN: yaml2obj -DFLAGS="EF_SPARC_32PLUS " %s -o %t2
+# RUN: obj2yaml %t2 | FileCheck %s --check-prefix=FLAG
+
+# FLAG:      --- !ELF
+# FLAG-NEXT: FileHeader:
+# FLAG-NEXT:   Class:   ELFCLASS64
+# FLAG-NEXT:   Data:    ELFDATA2MSB
+# FLAG-NEXT:   Type:    ET_EXEC
+# FLAG-NEXT:   Machine: EM_SPARC32PLUS
+# FLAG-NEXT:   Flags:   [ EF_SPARC_32PLUS ]
+
+# RUN: yaml2obj -DFLAGS="EF_SPARC_HAL_R1 " %s -o %t3
+# RUN: obj2yaml %t3 | FileCheck %s --check-prefix=FLAG2
+
+# FLAG2:      --- !ELF
+# FLAG2-NEXT: FileHeader:
+# FLAG2-NEXT:   Class:   ELFCLASS64
+# FLAG2-NEXT:   Data:    ELFDATA2MSB
+# FLAG2-NEXT:   Type:    ET_EXEC
+# FLAG2-NEXT:   Machine: EM_SPARC32PLUS
+# FLAG2-NEXT:   Flags:   [ EF_SPARC_HAL_R1 ]

llvm/test/tools/yaml2obj/file-header-flags.yaml

@@ -0,0 +1,25 @@
+## Test for FileHeader Flags.
+
+## When FLAGS variable isn't defined, the e_flags value is 0.
+## Otherwise, it's the specified value.
+
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj -h %t | FileCheck %s --check-prefixes=NO-FLAG
+
+# RUN: yaml2obj %s -o %t -DFLAGS=[EF_SPARC_32PLUS]
+# RUN: llvm-readobj -h %t | FileCheck %s --check-prefixes=FLAG
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_SPARC32PLUS
+  Flags:           [[FLAGS=<none>]]
+
+# NO-FLAG:         Flags [ (0x0)
+# NO-FLAG-NEXT:    ]
+
+# FLAG:            Flags [ (0x100)
+# FLAG-NEXT:         EF_SPARC_32PLUS (0x100)
+# FLAG-NEXT:       ]

llvm/tools/obj2yaml/elf2yaml.cpp

@@ -281,7 +281,8 @@ template <class ELFT> Expected<ELFYAML::Object *> ELFDumper<ELFT>::dump() {
   Y->Header.Type = Obj.getHeader().e_type;
   if (Obj.getHeader().e_machine != 0)
     Y->Header.Machine = ELFYAML::ELF_EM(Obj.getHeader().e_machine);
-  Y->Header.Flags = Obj.getHeader().e_flags;
+  if (Obj.getHeader().e_flags != 0)
+    Y->Header.Flags = ELFYAML::ELF_EF(Obj.getHeader().e_flags);
   Y->Header.Entry = Obj.getHeader().e_entry;
 
   // Dump sections