Re: [debug-early] emit locals early patchset

[Martin Jambor <mjambor at suse dot cz>]

On Tue, Oct 28, 2014 at 03:57:43PM +0100, Richard Biener wrote:
> On Tue, Oct 28, 2014 at 1:00 AM, Aldy Hernandez <aldyh@redhat.com> wrote:
> > Gentlemen!
> >
> > My apologies for the big patch.  In getting locals emitted early (parameters
> > and locally scoped variables), I ran into many things which were in need of
> > surgery, many of which couldn't happen without the other.  Consequently, I
> > ended up fixing everything such that we are now back to no guality.exp
> > failures for any language.
> >
> > [Curiously, I hadn't noticed locals were not being dumped early because they
> > were being picked up by the late dwarf pass.  Fixing this oversight is what
> > caused this entire patch.]
> >
> > There are a lot of changes here, and I would greatly appreciate feedback, so
> > let me at least explain what's going on at a high level...
> >
> > 1. Changes to gen_subprogram_die() to handle early generation of locals, and
> > amending location information on the second pass.  Everything else in this
> > patch, basically stems from this change.
> >
> > 2. Changes to gen_variable_die() to handle multiple passes (early/late dwarf
> > generation).
> >
> > A lot of this is complicated by the fact that old_die's are cached and keyed
> > by `tree', but an abstract instance and an inline instance share trees,
> > while dwarf2out_abstract_function() sets DECL_ABSTRACT_P behind the scenes.
> >
> > The current support (and my changes) maintain this shared and delicate
> > design.  I wonder whether we could simplify a lot of this code by unsharing
> > these trees, but this may be beyond the scope of this work. Richi perhaps
> > you can comment?
> 
> I think that the abstract and inline instances are cases that are _only_
> generated "early" - that is, they don't contain any locations or whatever
> and thus do not need to ameded in the late dwarf pass.  So I'd simply
> generate dwarf for them and not remember their DIEs.

Please have a look at PR 63722, which is a complaint I got from people
exploring possiblities of using dwarf to analyze what gcc did to Linux
kernel and they discovered that DIEs of  abstract origins do not have
their DW_AT_inline set correctly for functions that were IPA-SRAed but
not inlined.  On the original testcase before reduction I've seen the
same problem also when using -fno-ipa-sra -fno-ipa-cp so it is not
just my passes but also probably ipa-split ;-)

In any case, if we want abstract origins to have this field set
correctly in abstract origins, we need to have the possibility to
modify it after IPA.

Martin