[LLVMdev] Changing the design of GlobalAliases to represent what is actually possible in object files.

[Rafael Espíndola]

According to

http://www.airs.com/blog/archives/403

>From the compiler & assembler point of view this is just a special symbol type.

Given that it cannot be used directly I would say it is not a
ConstantExpr, it is a property of a label.

Given that the special IFUNC label has the same value as the resolver,
it sounds like it is not a GlobalObject, just a special type of
GlobalAlias.

In summary, from a very quick look it looks like it is independent
from this discussion: All that is needed is a bit saying it is a
"IFUNC GlobalAlias", not a plain GlobalAlias, regardless of how we end
up representing GlobalAliases.

> On May 23, 2014, at 18:15, Eric Christopher <echristo at gmail.com> wrote:
>
> On Fri, May 23, 2014 at 3:06 PM, Rafael Avila de Espindola
> <rafael.espindola at gmail.com> wrote:
>>
>>
>> Sent from my iPhone
>>
>>> On May 23, 2014, at 17:51, Eric Christopher <echristo at gmail.com> wrote:
>>>
>>> I'm not there yet, but at some point I'm going to need the notion of a
>>> global callable function like symbol that's resolved at runtime. I've
>>> not given it much thought but I may need a new callable entity here
>>> (this is for the gnu ifunc stuff).
>>>
>>> Don't even know if this fits into the discussion, but since we were
>>> talking about weird symbols...
>>
>> It is a symbol or a value that is loaded? If it is an symbol, what does it point to? That is, what is the value that shows up in the .o?
>
> Relocation resolved at compile time to one of N symbols that will be
> the call that's made.
>
> -eric
>
>>
>>
>>> -eric
>>>
>>>> On Fri, May 23, 2014 at 1:26 PM, Reid Kleckner <rnk at google.com> wrote:
>>>> On Thu, May 22, 2014 at 7:14 PM, Rafael Espíndola
>>>> <rafael.espindola at gmail.com> wrote:
>>>>>
>>>>>> For example, it would not be possible to define an absolute symbol to be
>>>>>> the offset between two symbols.  In some restricted circumstances —
>>>>>> if both symbols are global variables in the same section and defined
>>>>>> in the same translation unit — this could be worked around.
>>>>>>
>>>>>> But I’ll gladly admit that I don’t have a use case in mind for that
>>>>>> feature.
>>>>>> Absolute symbols are useful, and storing offsets between symbols into
>>>>>> global memory is useful, but I don’t know why you’d combine them.
>>>>>
>>>>> That is funny. I, on the other hand, think that this is the best
>>>>> argument I have seen for keeping aliases pointing to ConstantExpr so
>>>>> far.
>>>>
>>>>
>>>> IMO if we want to support defining symbols at absolute addresses, we should
>>>> add a separate construct for this.  So far everyone has gotten by with
>>>> linker flags and scripts, though.
>>>>
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