[LLVMdev] [lld] ELF weak aliases

[Michael Spencer]

On Wed, Jan 9, 2013 at 6:07 AM,  <shankare at codeaurora.org> wrote:
>> On Tue, Jan 8, 2013 at 8:56 PM,  <shankare at codeaurora.org> wrote:
>>> Hi Michael,
>>>
>>> Does ELF support aliasing ?
>>>
>>> How is the relationship captured in ELF symbol table, that one symbol is
>>> a
>>> alias of another symbol ?
>>
>> It is not explicitly captured. It's an implicit relationship due to
>> the symbols having the same address.
>
> Got it.
>
>>
>>>
>>>> Note that __stdout_used is the last symbol in the .rodata section.
>>>> This means that the reader assigns the data (16 bytes of 0) to
>>>> __stdout_used. Because dummy_file and the other __stdx_used symbols
>>>> come before it, they end up in the right place in the final file.
>>>
>>> Did you change the Reader too ?
>>
>> No. I just made another symbol to steal the actual content.
> We could change the Reader so that if the symbol is the last symbol in the
> section and the symbol is weak, treat the size of the symbol differently.

This doesn't only occur when the symbol is the last in the section. It
occurs any time a weak symbol shares content with another.

>>
>>>
>>> The Reader doesnot allocate any space for __stdout_used. The size of the
>>> current symbol = (value of next symbol - current symbol). In this case
>>> its
>>> zero.
>>
>> __stdout_used is the last symbol at that address, so it gets the data.
>> The hack was to make __stdout_used not get the data.
>
> Got it. Thanks for explainining things.
>
>>
>>>
>>>>
>>>> This works great until another object file provides a definition of
>>>> __stdout_used. The weak definition of it gets totally removed, meaning
>>>> so does the content for the other __stdx_used symbols.
>>>
>>> When the other object provides a definition for __stdout_used, the atom
>>> gets the property of the other object which defines the atom isnt it,
>>> and
>>> so as the ordinal too riht ?
>>>
>>> Couldnt follow how did the others move ?
>>
>> I'm not quite sure what you mean here.
>
> Sorry for not making it clear. I was not sure how did the content of the
> other symbols change when another object file provided a definition of
> __stdout_used ?

Because __stdout_used in __stdio_exit.o was the only symbol with the 8
null bytes, when another object file defined a non weak version of
__stdout_used, the __stdio_exit.o::__stdout_used was removed along
with its 8 null bytes. The non-weak definition ends up in a different
location.

>
> Thanks
>
> Shankar Easwaran
>

- Michael Spencer