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[PATCH v4] MIPS: Support PCREL GOT access

YunQiang Su syq@gcc.gnu.org
Thu Apr 18 10:56:27 GMT 2024 
Fangrui Song <i@maskray.me> 于2024年4月3日周三 12:11写道：
>
> On Sat, Mar 16, 2024 at 1:51 AM YunQiang Su <syq@gcc.gnu.org> wrote:
> >
> > Current if we need to access a GOT entry, we use the
> > got_base + offset. Thus we have GOT and XGOT.
> > From MIPSr6, we have PCREL instructions like ALUIPC,
> > so we have no need to use got_base now.
> > For pre-R6, we can use BAL to get the the value of PC.
>
> Currently loading a GOT entry requires loading the GOT base address
> first, then adding an offset.
> Therefore, (elaborate the reason) we need both GOT and XGOT.
> MIPSr6 introduced PC-relative instructions like ..., which allow us to
> use a pair of .. and .. to materialize the GOT entry address.
>

Thanks.

Currently loading a GOT entry requires loading the GOT base address
first, then adding an offset. Therefore, (elaborate the reason)
we need both GOT and XGOT. MIPSr6 introduced PC-relative instructions
like ALUIPC/AUIPC, which allow us to use a pair of GOTPC_HI and
GOTPC_LO to materialize the GOT entry address.


> > In this patch, we add 8 new relocs:
> >         R_MIPS_GOTPC_HI16, R_MIPS_GOTPC_LO16,
> >         R_MIPS_GOTPC_CALL_HI16, R_MIPS_GOTPC_CALL_LO16,
> >         R_MIPS_GOTPC_AHI16, R_MIPS_GOTPC_ALO16,
> >         R_MIPS_GOTPC_CALL_AHI16, R_MIPS_GOTPC_CALL_ALO16.
> > These asm notes can be used for them:
> >         %gotpc_hi(sym), %gotpc_lo(sym),
> >         %gotpc_call_hi(sym), %gotpc_call_lo(sym),
> >         %gotpc_ahi(sym), %gotpc_alo(sym),
> >         %gotpc_call_ahi(sym), %gotpc_call_alo(sym).
>
> GCC's mips port names assembler relocation operators
> (gas/config/tc-mips.c names them `mips_percent_op`).
> Other ports seem to prefer "modifiers" (assembler modifiers,
> expression modifiers).
>

These percent_op can be used for them


> >         * bfd-in2.h: Add new MIPS GOTPC BFD_RELOC defination.
>
> typo
>
Thanks.

Ping  again.
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