CREL relocation format for ELF (was: RELLEB)

[Fangrui Song <maskray@gcc.gnu.org>]

On Thu, Mar 14, 2024 at 5:16 PM Fangrui Song <maskray@gcc.gnu.org> wrote:
>
> 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 propose RELLEB, a new format offering significant file size
> reductions: 17.2% (x86-64), 16.5% (aarch64), and even 32.4% (riscv64)!
>
> Your thoughts on RELLEB are welcome!
>
> Detailed analysis:
> https://maskray.me/blog/2024-03-09-a-compact-relocation-format-for-elf
> generic ABI (ELF specification):
> https://groups.google.com/g/generic-abi/c/yb0rjw56ORw
> binutils feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=31475
> LLVM: https://discourse.llvm.org/t/rfc-relleb-a-compact-relocation-format-for-elf/77600
>
> Implementation primarily involves binutils changes. Any volunteers?
> For GCC, a driver option like -mrelleb in my Clang prototype would be
> needed. The option instructs the assembler to use RELLEB.

The format was tentatively named RELLEB. As I refine the original pure
LEB-based format, “RELLEB” might not be the most fitting name.

I have switched to SHT_CREL/DT_CREL/.crel and updated
https://maskray.me/blog/2024-03-09-a-compact-relocation-format-for-elf
and
https://groups.google.com/g/generic-abi/c/yb0rjw56ORw/m/eiBcYxSfAQAJ

The new format is simpler and better than RELLEB even in the absence
of the shifted offset technique.

Dynamic relocations using CREL are even smaller than Android's packed
relocations.

// encodeULEB128(uint64_t, raw_ostream &os);
// encodeSLEB128(int64_t, raw_ostream &os);

Elf_Addr offsetMask = 8, offset = 0, addend = 0;
uint32_t symidx = 0, type = 0;
for (const Reloc &rel : relocs)
  offsetMask |= crels[i].r_offset;
int shift = std::countr_zero(offsetMask)
encodeULEB128(relocs.size() * 4 + shift, os);
for (const Reloc &rel : relocs) {
  Elf_Addr deltaOffset = (rel.r_offset - offset) >> shift;
  uint8_t b = deltaOffset * 8 + (symidx != rel.r_symidx) +
              (type != rel.r_type ? 2 : 0) + (addend != rel.r_addend ? 4 : 0);
  if (deltaOffset < 0x10) {
    os << char(b);
  } else {
    os << char(b | 0x80);
    encodeULEB128(deltaOffset >> 4, os);
  }
  if (b & 1) {
    encodeSLEB128(static_cast<int32_t>(rel.r_symidx - symidx), os);
    symidx = rel.r_symidx;
  }
  if (b & 2) {
    encodeSLEB128(static_cast<int32_t>(rel.r_type - type), os);
    type = rel.r_type;
  }
  if (b & 4) {
    encodeSLEB128(std::make_signed_t<uint>(rel.r_addend - addend), os);
    addend = rel.r_addend;
  }
}

---

While alternatives like PrefixVarInt (or a suffix-based variant) might
excel when encoding larger integers, LEB128 offers advantages when
most integers fit within one or two bytes, as it avoids the need for
shift operations in the common one-byte representation.

While we could utilize zigzag encoding (i>>31) ^ (i<<1) to convert
SLEB128-encoded type/addend to use ULEB128 instead, the generate code
is inferior to or on par with SLEB128 for one-byte encodings.