[llvm-dev] [lld] R_MIPS_HI16 / R_MIPS_LO16 calculation

Sun Nov 22 03:05:00 PST 2015

On Sun, Nov 22, 2015 at 1:28 AM, Rui Ueyama <ruiu at google.com> wrote:
> I'm not sure if I understand the semantics of HI16 and LO16 relocations. If
> my understanding is correct, a pair of HI16 and LO16 represents an addend
> AHL. AHL is computed by (AHI<<16) | (ALO&0xFFFF). Can't we apply HI16 and
> LO16 relocations separately and produce the same relocation result? Do we
> have to pair them up before applying relocations?

The correct formula for the combined addend is (AHI << 16) +
(short)ALO. So the combined addend depends on both AHI and ALO
addends, therefore ALO affects result of R_MIPS_HI16 relocation.

Current version of bfd GNU linker looks up R_MIPS_LO16 relocation each
time it needs to calculate R_MIPS_HI16 relocation. It uses
`mips_elf_add_lo16_rel_addend` function for that
(https://goo.gl/P7nb76).

-- 
Simon Atanasyan