On Wed, 27 Jun 2018, Jason Merrill wrote:
> On Tue, Jun 26, 2018 at 8:43 AM, Richard Biener <rguent...@suse.de> wrote:
> >
> > A patch from Honza not LTO streaming DECL_ORIGINAL_TYPE ends up
> > ICEing during LTO bootstrap because we end up not re-using the
> > DIE we create late during LTRANS for a subprogram because its
> > parent is a namespace rather than a CU DIE (or a module).
> >
> > I'm wondering of the general reason why we enforce (inconsistently)
> > "We always want the DIE for this function that has the *_pc attributes to
> > be under comp_unit_die so the debugger can find it."
> > We indeed generate a specification DIE rooted at the CU in addition to the
> > declaration DIE inside the namespace for sth as simple as
> >
> > namespace Foo { void foo () {} }
>
> The reason was to make it easier for debuggers to collect function
> definitions by scanning the top-level DIEs. I don't know how
> useful/important that is to actual debuggers nowadays, since there are
> various accelerated lookup tables as well.
Ok.
> > anyhow - the comment also says "We also need to do this [re-use the DIE]
> > for abstract instances of inlines, since the spec requires the out-of-line
> > copy to have the same parent.". Not sure what condition this part of
> > the comment applies to.
>
> I think it's saying that we shouldn't re-use an in-class declaration
> die for the abstract instance of an inline, just like we shouldn't
> re-use it for a non-inline definition.
>
> Incidentally, the same parent was only required by DWARF 2; DWARF 3+
> say it's typical but not required.
Ah, I see.
> > So my fix is to move the || get_AT (old_die, DW_AT_abstract_origin)
> > check I added for early LTO debug to a global override - forcing
> > DIE re-use for any DIE with an abstract origin set. That is, all
> > concrete instances are fine where they are. That also avoids
> > double-indirection DW_AT_specification -> DW_AT_abstract_origin -> DIE.
> >
> > But as said, I wonder about the overall condition, esp. the
> > DW_TAG_module special-casing (but not at the same time
> > special-casing DW_TAG_namespace or DW_TAG_partial_unit).
>
> Hmm, the DW_TAG_module check seems to have come in with the
> early-debug merge. I don't know what the rationale was for that; it
> certainly does weaken the case for treating CU scope specially.
The comment says it was done to fix some ICE.
> As for DW_TAG_partial_unit, probably the is_cu_die check should change
> to is_unit_die.
I'll bootstrap and test that change separately.
> > LTO bootstrap is in progress on x86_64-unknown-linux-gnu.
> >
> > OK if that succeeds?
>
> OK.
It did, so applied.
Thanks,
Richard.
