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

[Rui Ueyama via llvm-dev]

On Fri, Nov 20, 2015 at 10:13 PM, Simon Atanasyan <simon at atanasyan.com>
wrote:

> Hi,
>
> I am working on support R_MIPS_HI16 / R_MIPS_LO16 in the new LLD and
> have a couple of questions.
>
> == Q1
> In case of MIPS O32 ABI we have to find a matching R_MIPS_LO16
> relocation to calculate R_MIPS_HI16 one because R_MIPS_HI16 uses
> combined addend (AHI << 16) + (short)ALO where AHI is original
> R_MIPS_HI16 addend and ALO is addend of the matching R_MIPS_LO16
> relocation [1]. There are two methods to do matching and R_MIPS_HI16
> calculation.
>
> Method A:
> 1. Postpone R_MIPS_HI16 relocation calculation and record its arguments.
> 2. When R_MIPS_LO16 is found, iterate over recorded R_MIPS_HI16
> relocations, calculate combined addend and apply relocations.
> 3. At the end check orphaned (without R_MIPS_LO16 pair) R_MIPS_HI16
> relocations, show warnings and apply them with zero addend.
>
> Method B:
> 1. Each time we have found R_MIPS_HI16 relocation, iterate remaining
> relocations list to find matching R_MIPS_LO16.
> 2. Calculate combined adddend and apply relocation or show warning if
> the R_MIPS_LO16 is not found.
>
> Method A requires some sort of container to keep postponed HI16
> relocations. If we add the container to the `MipsTargetInfo` class we
> will be able to hide all this unusual scheme inside MIPS specific code
> and will not need to perform LO16 lookups. But the `MipsTargetInfo`
> becomes stateful.
>
> Method B keeps the `MipsTargetInfo` stateless but requires forward
> LO16 lookup for each HI16 relocation and requires to provide an
> interface for such lookup to the `MipsTargetInfo`.
>
> Sure we can implement each of these methods somewhere in the
> `InputSectionBase` class under `if (MIPS)` statements.
>
> Any opinions about the best method / approach?
>

If I understand that spec correctly, an R_MIPS_HI16 should immediately be
followed by an R_MIPS_LO16. Can't you use that property? It doesn't seem to
me that you really have to search and pair up HI16 and LO16 relocations.

== Q2
>
> R_MIPS_HI16 and R_MIPS_LO16 relocations perform a special calculation
> if a target symbol's name is `_gp_disp` [2]. AFAIK now in the target
> `relocateOne` method there is no chance to get the traget symbol name.
> Is it okay to pass the target symbol index and provide
> `MipsTargetInfo` access to the symbol table of the processing input
> file?
>

One way is to add a SymbolBody* field to Out<ELFT> struct, and let it have
a pointer to _gp_disp symbol. And do pointer comparison to check if a
relocation target is _gp_disp or not.

[1] ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf page 4-18
> [2] ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf page 4-19
>
> --
> Simon Atanasyan
>
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