Re: [PATCH] ipa-icf: skip variables with body_removed

[Jan Hubicka <hubicka@kam.mff.cuni.cz>]

> @Honza: PING
> 
> On 5/20/22 09:46, Martin Liška wrote:
> > On 5/19/22 17:02, Jan Hubicka wrote:
> >>> Similarly to cgraph_nodes, it may happen that body_removed is set
> >>> during merging of symbols.
> >>>
> >>> 	PR ipa/105600
> >>>
> >>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> >>>
> >>> Ready to be installed?
> >>> Thanks,
> >>> Martin
> >>>
> >>> gcc/ChangeLog:
> >>>
> >>> 	* ipa-icf.cc (sem_item_optimizer::filter_removed_items):
> >>> 	Skip variables with body_removed.
> >>> ---
> >>>  gcc/ipa-icf.cc | 7 ++++---
> >>>  1 file changed, 4 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/gcc/ipa-icf.cc b/gcc/ipa-icf.cc
> >>> index 765ae746745..6528a7a10b2 100644
> >>> --- a/gcc/ipa-icf.cc
> >>> +++ b/gcc/ipa-icf.cc
> >>> @@ -2411,10 +2411,11 @@ sem_item_optimizer::filter_removed_items (void)
> >>>  	    {
> >>>  	      /* Filter out non-readonly variables.  */
> >>>  	      tree decl = item->decl;
> >>> -	      if (TREE_READONLY (decl))
> >>> -		filtered.safe_push (item);
> >>> -	      else
> >>> +	      varpool_node *vnode = static_cast <sem_variable *>(item)->get_node ();
> >>> +	      if (!TREE_READONLY (decl) || vnode->body_removed)
> >>>  		remove_item (item);
> >>> +	      else
> >>> +		filtered.safe_push (item);
> >>
> >> So the situation here is that we merge symbols but keep syntactic alias
> >> because the declarations are not compatible (have different attributes
> >> perhaps because of fortify source)?
> > 
> > The test-case is more about a constexpr symbol or so, I'm not familiar enough
> > with these modern C++.
> > 
> > cgraph_node looks like:
> > 
> > (gdb) p item->node->debug()
> > _ZN8nlohmann6detail12static_constINS0_10to_json_fnEE5valueE/10 (value) @0x7ffff7fb3200
> >   Type: variable
> >   Body removed by symtab_remove_unreachable_nodes
> >   Visibility: externally_visible semantic_interposition preempted_reg external public weak comdat comdat_group:_ZN8nlohmann6detail12static_constINS0_10to_json_fnEE5valueE one_only
> >   References: 
> >   Referring: _Z7to_jsonRN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmiSaNS_14adl_serializerES2_IhSaIhEEEERK8Settings/1 (addr) _Z7to_jsonRN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmiSaNS_14adl_serializerES2_IhSaIhEEEERK8Settings/1 (addr) 
> >   Read from file: a.o
> >   Availability: not_available
> >   Varpool flags: initialized read-only

OK, I think I worked out what happens here.  The thing is that the
symbol has "external" flag set.  In my mental picture we should be
keeping their definitions around since they are used for constant
folding, but it seems it is not what happens. We special case inline
functions but we do not special case external symbols.

This is a missed optimization oppurtunity but not completely trivial to
fix, since external symbol may reffer to internal one (such as comdat)
but even if it does so we want to optimize out that comdat.  So we need
some bookkeeping for referneces from external symbols.  I will implement
that but I guess we still want your patch since this is optional
optimization.

So patch is OK.
Honza
> > 
> > 
> >>
> >> Will ICF still see through the aliases and do merging?
> > 
> > No.
> > 
> >> I think you can
> >> craft a testcase by triggering the attribute mismatch and see what
> >> happens.  At the time we implemented ICF these aliases did not exists,
> >> so maybe some TLC is needed here.
> > 
> > Please come up with such test case :)
> > 
> > Thanks,
> > Martin
> > 
> >>
> >> Honza
> > 
>