[ELF] Allow target to configure ELF header flags in the output file

[atanasyan]

Summary

The patch configure ELF header flags for MIPS target. For now the flags are hard coded. In fact they depends on ELF flags of input object files and selected emulation.

[ruiu]

ELF/Target.cpp

1257–1261 ↗

(On Diff #43018)

Instead of assigning a value to ELFFlags member, can you move this code to getELFFlags and return a new value without assignment?

[atanasyan]

ELF/Target.cpp

1257–1261 ↗

(On Diff #43018)

Do you suggest to make the getELFFlags virtual or put if (Config->EMachine == EM_MIPS) into it?

[ruiu]

ELF/Target.cpp

1257–1261 ↗

(On Diff #43018)

I have no strong preference, but maybe virtual?

[ruiu]

ELF/Target.cpp

1257–1261 ↗

(On Diff #43018)

On second thought, this doesn't seem to have to belong this file. This can be written in Writer.cpp as a non-member function. Maybe that's better?

[atanasyan]

Simplify the code. Use a static function to get ELF flags. For now it is a good enough solution.

[ruiu]

LGTM with a nit.

ELF/Writer.cpp

1009–1017

Can you return early?

if (Config->EMachine != EM_MIPS)
  return 0;
uint32_t V = EF_MIPS_ABI_O32 | ...;
if (Config->Shared)
  return V | EF_MIPS_PIC;
return V;

ELF/Writer.cpp

template <class ELFT> int Writer<ELFT>::getPhdrsNum() const {
   }
   if (Tls)
     ++I;
   if (HasRelro)
     ++I;
   return I;
 }
 
+static uint32_t getELFFlags() {
+  uint32_t ELFFlags = 0;
+  if (Config->EMachine == EM_MIPS) {
+    // FIXME: In fact ELF flags depends on ELF flags of input object files
+    // and selected emulation. For now just use hadr coded values.
+    ELFFlags = EF_MIPS_ABI_O32 | EF_MIPS_CPIC | EF_MIPS_ARCH_32R2;
+    if (Config->Shared)
+      ELFFlags |= EF_MIPS_PIC;
+  }
+  return ELFFlags;
+}
+
 template <class ELFT> void Writer<ELFT>::writeHeader() {
   uint8_t *Buf = Buffer->getBufferStart();
   memcpy(Buf, "\177ELF", 4);
 
   // Write the ELF header.
   auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
   EHdr->e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
   EHdr->e_ident[EI_DATA] = ELFT::TargetEndianness == llvm::support::little
                                ? ELFDATA2LSB
                                : ELFDATA2MSB;
   EHdr->e_ident[EI_VERSION] = EV_CURRENT;
 
   auto &FirstObj = cast<ELFFileBase<ELFT>>(*Config->FirstElf);
   EHdr->e_ident[EI_OSABI] = FirstObj.getOSABI();
 
   EHdr->e_type = Config->Shared ? ET_DYN : ET_EXEC;
   EHdr->e_machine = FirstObj.getEMachine();
   EHdr->e_version = EV_CURRENT;
   EHdr->e_entry = getEntryAddr();
   EHdr->e_phoff = sizeof(Elf_Ehdr);
   EHdr->e_shoff = SectionHeaderOff;
+  EHdr->e_flags = getELFFlags();
   EHdr->e_ehsize = sizeof(Elf_Ehdr);
   EHdr->e_phentsize = sizeof(Elf_Phdr);
   EHdr->e_phnum = Phdrs.size();
   EHdr->e_shentsize = sizeof(Elf_Shdr);
   EHdr->e_shnum = getNumSections();
   EHdr->e_shstrndx = Out<ELFT>::ShStrTab->SectionIndex;
 
   // Write the program header table.

test/ELF/basic-mips.s

 # CHECK-NEXT:     Unused: (00 00 00 00 00 00 00)
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Type: Executable (0x2)
 # CHECK-NEXT:   Machine: EM_MIPS (0x8)
 # CHECK-NEXT:   Version: 1
 # CHECK-NEXT:   Entry: 0x20000
 # CHECK-NEXT:   ProgramHeaderOffset: 0x34
 # CHECK-NEXT:   SectionHeaderOffset: 0x20084
-# CHECK-NEXT:   Flags [ (0x0)
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     EF_MIPS_ABI_O32
+# CHECK-NEXT:     EF_MIPS_ARCH_32R2
+# CHECK-NEXT:     EF_MIPS_CPIC
 # CHECK-NEXT:   ]
 # CHECK-NEXT:   HeaderSize: 52
 # CHECK-NEXT:   ProgramHeaderEntrySize: 32
 # CHECK-NEXT:   ProgramHeaderCount: 5
 # CHECK-NEXT:   SectionHeaderEntrySize: 40
 # CHECK-NEXT:   SectionHeaderCount: 9
 # CHECK-NEXT:   StringTableSectionIndex: 7
 # CHECK-NEXT: }

test/ELF/emulation.s

 # MIPS-NEXT:     Unused: (00 00 00 00 00 00 00)
 # MIPS-NEXT:   }
 # MIPS-NEXT:   Type: Executable (0x2)
 # MIPS-NEXT:   Machine: EM_MIPS (0x8)
 # MIPS-NEXT:   Version: 1
 # MIPS-NEXT:   Entry:
 # MIPS-NEXT:   ProgramHeaderOffset: 0x34
 # MIPS-NEXT:   SectionHeaderOffset:
-# MIPS-NEXT:   Flags [ (0x0)
+# MIPS-NEXT:   Flags [
+# MIPS-NEXT:     EF_MIPS_ABI_O32
+# MIPS-NEXT:     EF_MIPS_ARCH_32R2
+# MIPS-NEXT:     EF_MIPS_CPIC
 # MIPS-NEXT:   ]
 
 # RUN: llvm-mc -filetype=obj -triple=mipsel-unknown-linux %s -o %tmipsel
 # RUN: ld.lld -m elf32ltsmip -e _start %tmipsel -o %t2mipsel
 # RUN: llvm-readobj -file-headers %t2mipsel | FileCheck --check-prefix=MIPSEL %s
 # RUN: ld.lld -melf32ltsmip -e _start %tmipsel -o %t2mipsel
 # RUN: llvm-readobj -file-headers %t2mipsel | FileCheck --check-prefix=MIPSEL %s
 # RUN: ld.lld %tmipsel -e _start -o %t3mipsel

 # MIPSEL-NEXT:     Unused: (00 00 00 00 00 00 00)
 # MIPSEL-NEXT:   }
 # MIPSEL-NEXT:   Type: Executable (0x2)
 # MIPSEL-NEXT:   Machine: EM_MIPS (0x8)
 # MIPSEL-NEXT:   Version: 1
 # MIPSEL-NEXT:   Entry:
 # MIPSEL-NEXT:   ProgramHeaderOffset: 0x34
 # MIPSEL-NEXT:   SectionHeaderOffset:
-# MIPSEL-NEXT:   Flags [ (0x0)
+# MIPSEL-NEXT:   Flags [
+# MIPSEL-NEXT:     EF_MIPS_ABI_O32
+# MIPSEL-NEXT:     EF_MIPS_ARCH_32R2
+# MIPSEL-NEXT:     EF_MIPS_CPIC
 # MIPSEL-NEXT:   ]
 
 # RUN: llvm-mc -filetype=obj -triple=aarch64-unknown-linux %s -o %taarch64
 # RUN: ld.lld -m aarch64linux %taarch64 -o %t2aarch64
 # RUN: llvm-readobj -file-headers %t2aarch64 | FileCheck --check-prefix=AARCH64 %s
 # RUN: ld.lld %taarch64 -o %t3aarch64
 # RUN: llvm-readobj -file-headers %t3aarch64 | FileCheck --check-prefix=AARCH64 %s
 # AARCH64:      ElfHeader {

test/ELF/mips-elf-flags.s

+# Check generation of MIPS specific ELF header flags.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -shared -o %t.so
+# RUN: llvm-readobj -h %t.so | FileCheck -check-prefix=SO %s
+# RUN: ld.lld %t.o -o %t.exe
+# RUN: llvm-readobj -h %t.exe | FileCheck -check-prefix=EXE %s
+
+# REQUIRES: mips
+
+  .text
+  .globl  __start
+__start:
+  nop
+
+# SO:      Flags [
+# SO-NEXT:   EF_MIPS_ABI_O32
+# SO-NEXT:   EF_MIPS_ARCH_32R2
+# SO-NEXT:   EF_MIPS_CPIC
+# SO-NEXT:   EF_MIPS_PIC
+# SO-NEXT: ]
+
+# EXE:      Flags [
+# EXE-NEXT:   EF_MIPS_ABI_O32
+# EXE-NEXT:   EF_MIPS_ARCH_32R2
+# EXE-NEXT:   EF_MIPS_CPIC
+# EXE-NEXT: ]