lto gimple types and debug info

[Kenneth Zadeck]

Daniel Berlin wrote:
> On Sun, Jul 27, 2008 at 1:18 PM, Mark Mitchell <mark@codesourcery.com> wrote:
>   
>> David Edelsohn wrote:
>>
>>     
>>>        I do not expect LTO (or WHOPR) to work on AIX -- at least not
>>> without a lot of work on wrappers around the AIX linker.  However, I do
>>> not understand why enhancing GCC to support LTO -- when GCC is run without
>>> enabling LTO -- requires locking GCC completely into DWARF debugging.
>>>       
>> I agree that, at least in principle, it should be possible to emit the debug
>> info (whether the format is DWARF, Stabs, etc.) once.
>>     
>
> No, you can't.
> You would at least have to emit the variables separate from the types
> (IE emit debug info twice).
>
>   
>>  So, I don't see a
>> reason that this makes us a DWARF-only compiler either.
>>
>> Others have raised the issue of types which are fundamentally transformed by
>> the compiler (such as by removing fields).  I think that such opportunities
>> are going to be relatively rare; the global "struct Window" object in a GUI
>> library full of functions taking "struct Window *" parameters probably isn't
>> optimizable in this way.  But there will be situations where this is
>> possible and profitable of course.
>>
>> In that case, I'm not sure that *type* ought to be modified at all, from the
>> debug perspective.  To the extent there's still an object of type "struct X"
>> around, it's type is still what it was.
>>     
>
> Uh, except that if you only write things out once, and have already
> written out the variables, the variable no longer has the correct type
> if you've rewritten the type, and if we've already emitted debug info,
> it won't display properly anymore (since the locations of data members
> the type specifies will now be incorrect).
>
> So are you suggesting we emit debug info at multiple times
>   
it is my guess that we are still going to have to generate the debugging 
info for the variables late, if for no other reasons, that things like 
stack offsets are not set until then.   If that is true, we could 
possibly just generate a new type and abandon the first one.  

kenny