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

[Simon Atanasyan via llvm-dev]

On Mon, Nov 23, 2015 at 6:39 AM, Rui Ueyama <ruiu at google.com> wrote:
> On Sun, Nov 22, 2015 at 3:05 AM, Simon Atanasyan <simon at atanasyan.com>
> wrote:
>>
>> On Sun, Nov 22, 2015 at 1:28 AM, Rui Ueyama <ruiu at google.com> wrote:
>> > I'm not sure if I understand the semantics of HI16 and LO16 relocations.
>> > If
>> > my understanding is correct, a pair of HI16 and LO16 represents an
>> > addend
>> > AHL. AHL is computed by (AHI<<16) | (ALO&0xFFFF). Can't we apply HI16
>> > and
>> > LO16 relocations separately and produce the same relocation result? Do
>> > we
>> > have to pair them up before applying relocations?
>>
>> The correct formula for the combined addend is (AHI << 16) +
>> (short)ALO. So the combined addend depends on both AHI and ALO
>> addends, therefore ALO affects result of R_MIPS_HI16 relocation.
>
>
> Does that mean it is impossible to process HI16 and LO16 separately?
>
> If you apply only HI16 relocation, the relocation target will have a value
> AHI<<16. Next time when you apply LO16 relocation to the same address,
> you'll be able to see the previous result of HI16 relocation. So you don't
> have to combine them before applying relocations, no?

HI16 and LO16 applied to the different place in the code. Take a look
at the typical example below. So you have to apply the relocations
separately but calculate them together.

00000000 <main>:
   0:   3c1c0000        lui     gp,0x0
                        0: R_MIPS_HI16  _gp_disp
   4:   279c0000        addiu   gp,gp,0
                        4: R_MIPS_LO16  _gp_disp

HI16 / LO16 relocations use a combined addend AHL. The R_MIPS_LO16
uses low 16 bits of the (S + AHL) expression so HI16 addend does not
affect its result. But LO16 addend might affect the result of the HI16
relocation.

-- 
Simon Atanasyan