[Bug debug/42959] g++ does not emit DW_AT_default_value

["dodji at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from dodji at gcc dot gnu dot org  2010-03-11 16:17 -------
Created an attachment (id=20085)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20085&action=view)
Draft patch

Please find below a copy/paste of the introductory comment of the attached
patch.

Basically the value of DW_AT_default_value -- as explained by the DWARF4
draft specification -- is either a constant if the default argument of
the function is a constant, or a reference to a DIE if the value of the
default argument is a language construct like a variable.

I believe there are cases however where it's hard (impossible yet?) to
represent the value of DW_AT_default_value. For instance, when the
default argument of a function F is an arbitrary expression that
yields temporaries.  In that case, the default argument expression
must be evaluated in the context of the caller of F. Just taking in
account the default argument expression in the context of the F
declaration is not enough.
I am not sure we can represent that in DWARF today, can we?

So this patch is limited to the simple cases were the default argument
is either a constant or a variable that does not involve any temporary.
When a default argument expression involves some temporaries, no debug
info is generated for it.

For a class, it appears that we were calling finish_struct right before
parsing default arguments of the member functions.
finish_struct does the layout of the class. So we do the layout of the
class before parsing the defaut arguments because they can refer to
parts of the struct that need to be constructed before. The problem is
finish_struct also generates debug info for the class. That means, when
we generate debug info for the struct, we don't have any information
ready about the default arguments.

The patch allows finish_struct to not emit debug info systematically. We
can then keep calling finish_struct "early" and emit debug info only after
we parsed the default arguments.
We do something similar at class template instantiation time. We emit debug
info only after the template arguments are substituted into the default
arguments expressions.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42959