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

[Eric Christopher]

On Fri, May 23, 2014 at 6:36 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
> 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.

Sounds fine. I hadn't thought about it more than "do we want an alias
or a new symbol type" but since the discussion was ongoing here I
thought I'd bring it up.

-eric

>
>> 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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