Re: [rfa/dwarf] Support for attributes pointing to a different CU

[Daniel Jacobowitz <drow at false dot org>]

On Thu, Sep 23, 2004 at 05:17:54PM -0500, Jim Blandy wrote:
> 
> Daniel Jacobowitz <drow@false.org> writes:
> > This is the last logical piece I could break out of the intercu support. 
> > We're almost there.  Only things left after this are dependence tracking
> > and the queueing of multiple full CUs - 400 lines or so.
> > 
> > The core change of this patch is in follow_die_ref.  Instead of taking an
> > offset and returning a DIE, it now takes an attribute and the CU in which
> > the attribute was found, and returns a DIE and the CU in which the DIE
> > was found.
> > 
> > The bulky text change of this patch is in dwarf2_attr.  It divides into two
> > interfaces: anywhere that we are prepared to find a reference to another
> > DIE, we need to call dwarf2_attr_with_cu instead.  Then, any further
> > operations on the returned reference need to be sure to use the returned CU.
> > The reason for this is a little subtle - we're returning a reference.  If
> > the reference is, for example, DW_FORM_ref4, then it's an offset within some
> > particular CU.  If the attribute containing this reference was found by
> > chasing DW_AT_specification, and DW_AT_specification was DW_FORM_ref_addr,
> > then the return value of dwarf2_attr_with_cu is a relative offset to a
> > different CU than the caller passed in.  So if we are going to follow the
> > reference, we need to know the compilation unit in which it resides.
> 
> I may be missing something, but it seems to me some of this effort is
> necessary because we hold some die references as CU-relative, and
> others as .debug_info relative, depending on the form the attribute
> happened to use.  Wouldn't it be a simplification to have
> read_attribute_value always store all DW_FORM_ref* attributes as
> DW_ADDR offsets from the beginning of .debug_info?  Then dwarf2_attr's
> interface could remain unchanged, and we could drop all the spec_cu
> variables carrying the CU to the next call to
> dwarf2_get_ref_die_offset.  Then, dwarf2_get_ref_die_offset could be
> deleted altogether, and its calls replaced with uses of DW_ADDR.
> 
> (This change wasn't possible in the past because we didn't pass a CU
> to dwarf2_attr.)

This is why I like second-person review :-)  That's a brilliant idea. 
I will try it.

-- 
Daniel Jacobowitz