[LLVMdev] [llvm] r205200 - [mips] Implement missing relocations in the integrated assembler.

[Simon Atanasyan]

CC to the group.

On Mon, Mar 31, 2014 at 9:53 PM, Simon Atanasyan <simon at atanasyan.com> wrote:
> On Mon, Mar 31, 2014 at 9:27 PM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>>> AFAIK the patch for the gold linker is under review now. Here is the
>>> git repository:
>>> https://dmz-portal.mips.com/git/?p=gold.git;a=blob_plain;f=src/gold/mips.cc;hb=HEAD
>>>
>>> Function rello16().
>>>
>>> The gold linker uses slightly different approach. It postpones
>>> handling of "HI" relocations and save them into containers. When it
>>> handles "LO" relocation, it iterates over postponed "HI" relocations,
>>> find "paired" ones and process them.
>>
>>
>> It looks like they produce different results for
>>
>> R_MIPS_HI16   (1)
>> R_MIPS_HI16   (2)
>> R_MIPS_LO16  (3)
>> R_MIPS_LO16  (4)
>>
>> The gas implementation will match (1,3) and (2,3) while the gold
>> implementation will match (1,3) and (2,4), no?
>
> Note 1: R_MIPS_LO16 can be alone and does not have a paired R_MIPS_HI16
> Note 2: Several R_MIPS_HI16s can be attached to the same R_MIPS_LO16
>
> R_MIPS_HI16  (1)
> R_MIPS_HI16  (2)
> R_MIPS_LO16  (3)
> R_MIPS_LO16  (4)
>
> If all relocations above are against the same symbol then:
> bfd linker:
>   a) process 1 and find 3
>   b) process 2 and find 3 again
>   c) 4 is alone
>
> gold linker:
>   a) save 1
>   b) save 2
>   c) process 3 and handle 1 and 2
>   d) 4 is alone
>
>> Just as a curiosity (from the MC perspective), why is the linker
>> required to match these relocations?
>
> To handle R_MIPS_HI16 and R_MIPS_LO16 relocations we need a "combined"
> addend "AHL". This addend is calculated using addends from both
> R_MIPS_HI16 ("AHI") and R_MIPS_LO16 ("ALO"). That is why we need to
> pair these relocations.
>
> AHL = (AHI << 16) + (short)ALO
>
> R_MIPS_LO16: AHL + symbol
> R_MIPS_HI16: ((AHL + symbol) – (short)(AHL + symbol)) >> 16

-- 
Simon Atanasyan