Re: [PATCH] Check DECL_CONTEXT of new/delete operators.

[Jan Hubicka <hubicka@ucw.cz>]

> On Mon, Mar 30, 2020 at 10:41 AM Martin Liška <mliska@suse.cz> wrote:
> >
> > Hi.
> >
> > The patch ensures that a deleted new/delete pair has a same context.
> > That will fix the issue presented in the PR.
> >
> > Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> 
> I think this will break the DCE with LTO since we strip quite some
> DECL_CONTEXT in free-lang-data.
> 
> Honza - do you remember why we strip DECL_CONTEXT for methods
> which will most likely keep their containing record types live through
> their this
> arguments?  Quoting:
> 
>   /* We need to keep field decls associated with their trees. Otherwise tree
>      merging may merge some fileds and keep others disjoint wich in turn will
>      not do well with TREE_CHAIN pointers linking them.
> 
>      Also do not drop containing types for virtual methods and tables because
>      these are needed by devirtualization.
>      C++ destructors are special because C++ frontends sometimes produces
>      virtual destructor as an alias of non-virtual destructor.  In
>      devirutalization code we always walk through aliases and we need
>      context to be preserved too.  See PR89335  */
>   if (TREE_CODE (decl) != FIELD_DECL
>       && ((TREE_CODE (decl) != VAR_DECL && TREE_CODE (decl) != FUNCTION_DECL)
>           || (!DECL_VIRTUAL_P (decl)
>               && (TREE_CODE (decl) != FUNCTION_DECL
>                   || !DECL_CXX_DESTRUCTOR_P (decl)))))
>     DECL_CONTEXT (decl) = fld_decl_context (DECL_CONTEXT (decl));
> 
> and fld_decl_context stripping up to the next non-TYPE context (unless VLA).

Well, I basically went through all pointers and tried to get rid of as
many of them as possible.  CONTEXT pointers do increase size of SCCs
that increases chance they will not get merged and also processing time
of merging algorithm.  I guess if we need to stream more contexts we
could do that (and check the effect on merging and average SCC size)

Honza
> 
> Richard.
> 
> > Ready to be installed?
> > Thanks,
> > Martin
> >
> > gcc/ChangeLog:
> >
> > 2020-03-30  Martin Liska  <mliska@suse.cz>
> >
> >         PR c++/94314
> >         * tree-ssa-dce.c (propagate_necessity): Verify that
> >         DECL_CONTEXT of a new/delete operators do match.
> >
> > gcc/testsuite/ChangeLog:
> >
> > 2020-03-30  Martin Liska  <mliska@suse.cz>
> >
> >         PR c++/94314
> >         * g++.dg/pr94314.C: New test.
> > ---
> >   gcc/testsuite/g++.dg/pr94314.C | 84 ++++++++++++++++++++++++++++++++++
> >   gcc/tree-ssa-dce.c             | 31 +++++++++----
> >   2 files changed, 105 insertions(+), 10 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/pr94314.C
> >
> >