HELP: MIPS PC Relative Addressing

[Maciej W. Rozycki]

On Wed, 24 Feb 2021, Jiaxun Yang wrote:

> For RISC-V, %pcrel_lo shall point to the label of corresponding %pcrel_hi,
> like
> 
> .LA0:
>     auipc    a0, %pcrel_hi(sym)
>     addi      a0, a0, %pcrel_lo(.LA0)

 I commented on it once, in the course of the FDPIC design project, and I 
find it broken by design.  Sadly it has made it into the RISC-V psABI and 
it is hard to revert at this time, too many places have started relying on 
it.

> However, for MIPS %pcrel_lo simply calculate LO16 of the symbol to current
> PC, thus PC relative addressing will look like:
> 
> .LA0:
>     auipc  a0, %pcrel_hi(sym)
> .LA1:
>     addi    a0, %pcrel_lo(sym + (.LA1 - .LA0))
> 
> I found it's very difficult for GCC to generate this kind of pcrel_lo
> expression,
> RTX label_ref can't be lower into such LOW_SUM expression.

 You may want to use composed relocations to refer to .LA1 (R_MIPS_32) and 
.LA0 (R_MIPS_SUB).  There may or may not be linker updates needed; unlike 
the RISC-V one the MIPS BFD backend already supports composed relocations 
with the usual ELF gABI semantics.  It would be good to switch to RELA at 
this point universally too; none of new stuff will work with old linkers 
anyway.

 HTH,

  Maciej