Untested code

[Simon Atanasyan via llvm-commits]

On Wed, Apr 13, 2016 at 11:02 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
> On 13 April 2016 at 10:58, Simon Atanasyan <simon at atanasyan.com> wrote:
>> On Wed, Apr 13, 2016 at 12:02 AM, Rafael Espíndola
>> <rafael.espindola at gmail.com> wrote:
>>> I think now there is only one if left untested (see the attached patch) :-)
>>
>> Attached patch adds test case and tries to fix the problem.
>>
>> _gp_disp designates offset between start of function and 'gp' pointer into GOT.
>> Usually you can see the following code in the beginning of a function:
>>
>> lui    $t0, %hi(_gp_disp)
>> addi   $t0, $t0, %lo(_gp_disp)
>>
>> When you calculate result of %hi(_gp_disp), you use %hi(_gp - P)
>> expression. For the first instruction "_gp - P" gives the distance
>> between the function and _gp. But for the second instruction you have
>> to add four byte offset %hi(_gp - P + 4). Code in the trunk
>> incorrectly calculates %lo(_gp_disp) as an absolute expression
>> %lo(_gp).
>>
>> I am not sure that the fix is ideal. So I would appreciate any
>> suggestions / objections.
>
> Is it guaranteed that _gp_disp can only ever be used as the first two
> instructions in a function? Is that restriction documented somewhere?
> If so, please add a link to the documentation.

Yes. This pair of instructions is a standard function preamble. I will
try to find a link to spec and add it to the code.

> Instead of changing InputSection.cpp, can you put the +4  in the addend?

Where do you suggest to fix addend? It is impossible to do in the
getImplicitAddend routine because we do not pass SymbolBody to this
function. Without getImplicitAddend signature changing I can adjust
addend in the place where we call getImplicitAddend (Writer.cpp).

-- 
Simon Atanasyan