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

[Rui Ueyama via llvm-dev]

On Fri, Nov 20, 2015 at 11:02 PM, Simon Atanasyan <simon at atanasyan.com>
wrote:

> On Sat, Nov 21, 2015 at 9:28 AM, Rui Ueyama <ruiu at google.com> wrote:
> > On Fri, Nov 20, 2015 at 10:13 PM, Simon Atanasyan <simon at atanasyan.com>
> > wrote:
> >>
> >> 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.
>
> It is a question what the ABI authors did mean by the "R_MIPS_HI16
> must have an associated R_MIPS_LO16 entry immediately following it"
> phrase. In fact you can get from a compiler this code:
>
> lui    $t0,%hi(sym1+4)      # R_MIPS_HI16
> lui    $t0,%hi(sym1+8)      # R_MIPS_HI16
> lui    $t0,%hi(sym1+12)     # R_MIPS_HI16
> addi   $t0,$t0,%lo(sym1+16) # R_MIPS_LO16
>

The first two relocations don't conform to the standard because there are
no corresponding LO16 relocations, no?

and even such code:
>
> lui    $t0,%hi(sym1)     # R_MIPS_HI16 on sym1
> lui    $t0,%hi(sym2)     # R_MIPS_HI16 on sym2
> addi   $t0,$t0,%lo(sym1) # R_MIPS_LO16 on sym1
> addi   $t0,$t0,%lo(sym2) # R_MIPS_LO16 on sym2
>

Hmm, isn't this a violation of the ABI? My interpretation of "[e]ach
relocation type of R_MIPS_HI16 must have an associated R_MIPS_LO16 entry
immediately following it in the list of relocations" is not ambiguous to
allow them. Is there any chance to fix the compiler? (I guess there isn't,
though.)

fortunately I have never seen such code:
>
> lui    $t0,%hi(sym1+12)     # R_MIPS_HI16
> ... other type (except HI16 / LO16) of relocation here
> addi   $t0,$t0,%lo(sym1+16) # R_MIPS_LO16
>
> --
> Simon Atanasyan
>
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