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

[Simon Atanasyan via llvm-dev]

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?

== 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?

[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