[LLD][ELF][AArch64] Add aarch64_elf64_le_vec emulation

[peter.smith]

Summary

Android AOSP has started specifying -m aarch64_elf64_le_vec as supported by gold and BFD. This is a simple change to add the emulation so that LLD doesn't immediately error when used as a linker in an AOSP build.

See https://android.googlesource.com/platform/build/soong/+/master/cc/config/arm64_device.go#31

[grimar]

Looks fine.

[MaskRay]

aarch64_elf64_le_vec is a variable name in BFD (extern const bfd_target aarch64_elf64_le_vec;), instead of a BFD emulation.
@srhines Can this workaround be dropped now?

% aarch64-linux-gnu-ld -m aarch64_elf64_le_vec a.o 
aarch64-linux-gnu-ld: unrecognised emulation mode: aarch64_elf64_le_vec
Supported emulations: aarch64linux aarch64elf aarch64elf32 aarch64elf32b aarch64elfb armelf armelfb aarch64linuxb aarch64linux32 aarch64linux32b armelfb_linux_eabi armelf_linux_eabi

[srhines]

In D46429#2547020, @MaskRay wrote:

aarch64_elf64_le_vec is a variable name in BFD (extern const bfd_target aarch64_elf64_le_vec;), instead of a BFD emulation.
@srhines Can this workaround be dropped now?

% aarch64-linux-gnu-ld -m aarch64_elf64_le_vec a.o 
aarch64-linux-gnu-ld: unrecognised emulation mode: aarch64_elf64_le_vec
Supported emulations: aarch64linux aarch64elf aarch64elf32 aarch64elf32b aarch64elfb armelf armelfb aarch64linuxb aarch64linux32 aarch64linux32b armelfb_linux_eabi armelf_linux_eabi

https://android-review.googlesource.com/c/platform/build/soong/+/1278815 removed the use of this flag a while back, so yes, I am sure that you can drop this patch. Thanks for being proactive about this kind of cleanup.

ELF/Driver.cpp

static std::tuple<ELFKind, uint16_t, uint8_t> parseEmulation(StringRef Emul) {
   StringRef S = Emul;
   if (S.endswith("_fbsd")) {
     S = S.drop_back(5);
     OSABI = ELFOSABI_FREEBSD;
   }
 
   std::pair<ELFKind, uint16_t> Ret =
       StringSwitch<std::pair<ELFKind, uint16_t>>(S)
-          .Cases("aarch64elf", "aarch64linux", {ELF64LEKind, EM_AARCH64})
+          .Cases("aarch64elf", "aarch64linux", "aarch64_elf64_le_vec",
+                 {ELF64LEKind, EM_AARCH64})
           .Cases("armelf", "armelf_linux_eabi", {ELF32LEKind, EM_ARM})
           .Case("elf32_x86_64", {ELF32LEKind, EM_X86_64})
           .Cases("elf32btsmip", "elf32btsmipn32", {ELF32BEKind, EM_MIPS})
           .Cases("elf32ltsmip", "elf32ltsmipn32", {ELF32LEKind, EM_MIPS})
           .Case("elf32ppc", {ELF32BEKind, EM_PPC})
           .Case("elf64btsmip", {ELF64BEKind, EM_MIPS})
           .Case("elf64ltsmip", {ELF64LEKind, EM_MIPS})
           .Case("elf64ppc", {ELF64BEKind, EM_PPC64})

test/ELF/emulation.s

 # MIPS64EL-NEXT:     EF_MIPS_ARCH_64
 # MIPS64EL-NEXT:     EF_MIPS_CPIC
 # MIPS64EL-NEXT:     EF_MIPS_PIC
 # MIPS64EL-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: ld.lld -m aarch64elf %taarch64 -o %t3aarch64
 # RUN: llvm-readobj -file-headers %t3aarch64 | FileCheck --check-prefix=AARCH64 %s
+# RUN: ld.lld -m aarch64_elf64_le_vec %taarch64 -o %t4aarch64
+# RUN: llvm-readobj -file-headers %t4aarch64 | FileCheck --check-prefix=AARCH64 %s
+# RUN: ld.lld %taarch64 -o %t5aarch64
+# RUN: llvm-readobj -file-headers %t5aarch64 | FileCheck --check-prefix=AARCH64 %s
 # AARCH64:      ElfHeader {
 # AARCH64-NEXT:   Ident {
 # AARCH64-NEXT:     Magic: (7F 45 4C 46)
 # AARCH64-NEXT:     Class: 64-bit (0x2)
 # AARCH64-NEXT:     DataEncoding: LittleEndian (0x1)
 # AARCH64-NEXT:     FileVersion: 1
 # AARCH64-NEXT:     OS/ABI: SystemV (0x0)
 # AARCH64-NEXT:     ABIVersion: 0