在 2021/2/25 上午1:30, Maciej W. Rozycki 写道:
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.
Thanks for your hint;-)
I'm unsure about how should we express composed relocations in assembly :-/
MIPS N32/N64 ABI is already using RELA, do you mean switch to RELA for o32
as well?
Thanks.
- Jiaxun
HTH,
Maciej
