[llvm] r207719 - Revert "Emit DW_AT_object_pointer once, on the declaration, for each function."

[Eric Christopher]

On Thu, May 1, 2014 at 10:27 AM, David Blaikie <dblaikie at gmail.com> wrote:
> On Thu, May 1, 2014 at 8:50 AM, Robinson, Paul
> <Paul_Robinson at playstation.sony.com> wrote:
>>> -----Original Message-----
>>> From: David Blaikie [mailto:dblaikie at gmail.com]
>>> On Wed, Apr 30, 2014 at 7:40 PM, Robinson, Paul
>>> <Paul_Robinson at playstation.sony.com> wrote:
>>> > Yeah, I was wondering.  Attributes like object_pointer seem more
>>> > appropriate to a definition than a declaration, as the declaration
>>> > typically wouldn't provide the actual location.
>>>
>>> I'm not sure the actual location is a prerequisite for making use of
>>> object_pointer - I figured callers might like to know about it (but I
>>> suppose you can't /call/ a function from just the DWARF declaration?
>>> Never really thought about whether debuggers would manage that - they
>>> could /maybe/ build the mangled name, etc). Useful for rendering
>>> functions in pretty printers (well, the first parameter is flagged as
>>> artifiical in the declaration, perhaps that's sufficient for printing
>>> purposes - another case where it'd be nice if we didn't have to
>>> duplicate that information in the definition, though).
>>>
>>> But, sure, it'd be great if there was no reason to put object_pointer
>>> on declarations - not insignificant space savings when emitting types
>>> all over the place.
>>>
>>> - David
>>>
>>> > --paulr
>>
>> If you have a definition, then the location is definitely relevant;
>> a location expression might want to push the object address, and for
>> that you need to know where the object is!
>
> I'm not suggesting it's not helpful at the definition, just that it
> shouldn't need to be duplicated there.
>
>> But if you're inheriting
>> the object_pointer from the declaration, which only points to the
>> declaration's formal_parameter, which has no location... not very
>> helpful.
>
> I don't know why an implementation wouldn't derive it from the
> declaration - they must have the same parameters in the same order,
> right?
>

Exactly.

>> That's why I'd expect the definition to have one, and if
>> the definition has one, then having one on the declaration is not
>> necessary.
>>
>> If you have a declaration with no definition... I suppose it would
>> be "for completeness" to have object_pointer point to the 'this'
>> parameter (in C++) although some debuggers I've worked with are
>> happy to infer that from the first formal_parameter being named
>> 'this' and marked artificial.
>
> Right - and I assume they'd be able to infer that in the definition
> too. Evidently GCC doesn't infer it in the definition, nor derive it
> from the declaration.

They should be able to, but probably hasn't come up and so it hasn't
been thought about.

-eric

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