Stale relocation after relaxation on LoongArch

[CSharperMantle]

$ mold --version
mold 2.41.0 (45970e661d462fd664e7249a4bfc20ca4d0c6f39; compatible with GNU ld)

For the following assembly:

.section .text
.globl _start
_start:
  .reloc ., R_LARCH_RELAX
  pcalau12i  $t0, %pc_hi20(sym)
  .reloc ., R_LARCH_RELAX
  addi.d     $t0, $t0, %pc_lo12(sym)

  .reloc ., R_LARCH_RELAX
  pcalau12i  $t1, %pc_hi20(sym)
  .reloc ., R_LARCH_RELAX
  addi.d     $t1, $t1, %pc_lo12(sym)

  # exit(0)
  li.d       $a7, 93
  li.d       $a0, 0
  syscall    0

.section .rodata
sym:
  .dword 0

... linking the resulting object file with mold produces the following result:

$ clang --target=loongarch64-unknown-linux-gnu -c repro.s -o repro.o
$ clang -nostdlib -static -fuse-ld=mold -Wl,--emit-relocs -o repro repro.o
$ objdump --disassemble --reloc repro

repro:     file format elf64-loongarch


Disassembly of section .text:

0000000000210278 <_start>:
  210278:       19f7ffcc        pcaddi          $t0, -16386
                        210278: R_LARCH_PCALA_HI20      sym
                        210278: R_LARCH_RELAX   *ABS*
  21027c:       19f7ffad        pcaddi          $t1, -16387
                        21027c: R_LARCH_PCALA_LO12      sym
                        21027c: R_LARCH_RELAX   *ABS*
  210280:       0381740b        li.w            $a7, 0x5d
                        210280: R_LARCH_PCALA_HI20      sym
                        210280: R_LARCH_RELAX   *ABS*
  210284:       03800004        li.w            $a0, 0x0
                        210284: R_LARCH_PCALA_LO12      sym
                        210284: R_LARCH_RELAX   *ABS*
  210288:       002b0000        syscall         0x0

The relocations R_LARCH_PCALA_HI20 and R_LARCH_PCALA_LO12 are copied from the object file, but after linker relaxation, they no longer apply and are attached to the wrong instructions.

LLVM once had a similar issue for RISC-V before at ¹.

The script above and a Makefile can be found at https://github.com/CSharperMantle/mold-issue-1584.

Footnotes

1. https://reviews.llvm.org/D159082 ↩