[RFC] RELLEB: A compact relocation format for ELF

LLVM Project
,
1

The relocation formats REL and RELA for ELF are inefficient. In a release build of Clang for x86-64, .rela.* sections consume a significant portion (approximately 20.9%) of the file size. I have developed an alternative relocation format (tentatively named RELLEB), which yields a significant reduction: x86-64: 17.2%, aarch64: 16.5%, riscv64: 32.4%!

Elf32_Rel and Elf32_Rela sacrifice flexibility to maintain a smaller size, limiting relocation types to a maximum of 255. RELLEB allows 2**32 relocation types, aligning with Elf64_Rel/Elf64_Rela.

I have analyzed many architectures including Arm (AArch32/AArch64), Power, RISC-V, MIPS, RISC-V, z/Architecture, and x86, written a detailed analysis of the size problem and my solution at A compact relocation format for ELF | MaskRay ,
created a generic ABI proposal https://groups.google.com/g/generic-abi/c/yb0rjw56ORw,
and developed a prototype at GitHub - MaskRay/llvm-project at demo-relleb

  • clang
    • -mrelleb: use SHT_RELLEB instead of SHT_REL/SHT_RELA
    • -Xclang --compress-relocations={none,zlib,zstd}: compress
      SHT_REL/SHT_RELA/SHT_RELLEB
  • ld.lld
    • handle SHT_RELLEB input sections
    • -z relleb: use .relleb.dyn instead of .rel.dyn/.rela.dyn for dynamic
      relocations. Can be used together with --pack-dyn-relocs=relr
    • -r: copy SHT_RELLEB and rewrite
  • llvm-readelf
    • -S: recognize SHT_RELLEB
    • -d: recognize DT_RELLEB
    • -r: dump SHT_RELLEB sections
  • yaml2obj/obj2yaml
  • MCTargetOptions
    • llvm-mc -relleb
    • llc -relleb
    • ld.lld -mllvm -relleb for LTO

Unimplemented yet

  • ld.lld
    • –emit-relocs
  • llvm-objdump
    • -d -r

Example:

myclang++ -fuse-ld-lld =mrelleb a.cc b.cc

I am looking forward to your thoughts:)

Here are a few use cases that will benefit from a compact relocation type:

4 Likes