> -----Original Message----- > From: dwarf-discuss-boun...@lists.dwarfstd.org [mailto:dwarf-discuss- > boun...@lists.dwarfstd.org] On Behalf Of Pierre-Marie de Rodat > Sent: Wednesday, May 14, 2014 5:39 AM > To: Agovic, Sanimir > Cc: DWARF Discuss > Subject: Re: [Dwarf-Discuss] About self-referencial sized types > > On 04/28/2014 05:29 PM, Agovic, Sanimir wrote: > >> In GCC, however, computing the upper bound of "A" is more subtle: > >> it is internally represented as: max(0, <record>.N) so that when > >> "N" is negative, 0 is returned. > > > > I`m not being familiar with internal gcc representations but what is > > preventing you of referencing the member N? > > Nothing: actually, the internal GCC representation for Ada arrays bounds > (GENERIC trees) is already there and works well for quite a while. My > problem here is that I do not know what DWARF operations to output in > the DW_AT_{lower,upper}_bound attributes in order to retrieve array > "neighbors" members so that we can compute the array bounds using them. > > >> So what address would this operation [DW_OP_push_object_address] > >> push on top of the stack? The address of the "A" member, or the > >> address of the embedding record? > > > > It pushes the address of the currently evaluated object, in your > > case it is the address of member "A". You may have a look at 'D.2 > > Aggregate Examples' and Figure 51 in the latest dwarf standard. > > DW_OP_push_object_address is usually used to address meta information > > of a type e.g. bound information of an array. This information is > > usually part of the array descriptor hence the address of the object > > is needed and not the embedding type. > > > Can you illustrate the record descriptor representation? e.g. a > > simple representation in form of a C struct. My knowledge about how > > vla work is pretty limited to C99 and Fortran. It might help to > > understand the problem a bit better. > > Sure, I should have done it earlier. The Record_Type I was refering to > in my example would be translated to something like: > > struct record_type { > int n; > int a[1 .. max(n, 0)]; > };
I had to do something like this for a COBOL compiler once, except it was simply [1 .. N] and so I had the upper bound be a reference to the member DIE for N. If you're computing an expression on N then yes it's more complicated. In this case it seems like you could DW_OP_push_object_address to get the address of "A", from there back up to the address of "N", fetch it, and compute the max. > > >> On the other hand, getting the address of the "A" member would not > >> be sufficient: in more complex cases, the offset of the "A" member > >> can depend on discriminants! Does that mean the offset between "A" and "N" is not constant? You'd have to produce a sub-expression to compute that offset... This sounds complicated but not infeasible. --paulr _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
