Re: [PATCH] c++/modules: local class merging [PR99426]

[Patrick Palka <ppalka@redhat.com>]

On Wed, 10 Apr 2024, Jason Merrill wrote:

> On 4/10/24 14:48, Patrick Palka wrote:
> > On Tue, 9 Apr 2024, Jason Merrill wrote:
> > 
> > > On 3/5/24 10:31, Patrick Palka wrote:
> > > > On Tue, 27 Feb 2024, Patrick Palka wrote:
> > > > 
> > > > Subject: [PATCH] c++/modules: local type merging [PR99426]
> > > > 
> > > > One known missing piece in the modules implementation is merging of a
> > > > streamed-in local type (class or enum) with the corresponding in-TU
> > > > version of the local type.  This missing piece turns out to cause a
> > > > hard-to-reduce use-after-free GC issue due to the entity_ary not being
> > > > marked as a GC root (deliberately), and manifests as a serialization
> > > > error on stream-in as in PR99426 (see comment #6 for a reduction).  It's
> > > > also reproducible on trunk when running the xtreme-header tests without
> > > > -fno-module-lazy.
> > > > 
> > > > This patch makes us merge such local types according to their position
> > > > within the containing function's definition, analogous to how we merge
> > > > FIELD_DECLs of a class according to their index in the TYPE_FIELDS
> > > > list.
> > > > 
> > > > 	PR c++/99426
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* module.cc (merge_kind::MK_local_type): New enumerator.
> > > > 	(merge_kind_name): Update.
> > > > 	(trees_out::chained_decls): Move BLOCK-specific handling
> > > > 	of DECL_LOCAL_DECL_P decls to ...
> > > > 	(trees_out::core_vals) <case BLOCK>: ... here.  Stream
> > > > 	BLOCK_VARS manually.
> > > > 	(trees_in::core_vals) <case BLOCK>: Stream BLOCK_VARS
> > > > 	manually.  Handle deduplicated local types..
> > > > 	(trees_out::key_local_type): Define.
> > > > 	(trees_in::key_local_type): Define.
> > > > 	(trees_out::get_merge_kind) <case FUNCTION_DECL>: Return
> > > > 	MK_local_type for a local type.
> > > > 	(trees_out::key_mergeable) <case FUNCTION_DECL>: Use
> > > > 	key_local_type.
> > > > 	(trees_in::key_mergeable) <case FUNCTION_DECL>: Likewise.
> > > > 	(trees_in::is_matching_decl): Be flexible with type mismatches
> > > > 	for local entities.
> > > > 
> > > > diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> > > > index 80b63a70a62..d9e34e9a4b9 100644
> > > > --- a/gcc/cp/module.cc
> > > > +++ b/gcc/cp/module.cc
> > > > @@ -6714,7 +6720,37 @@ trees_in::core_vals (tree t)
> > > >        case BLOCK:
> > > >          t->block.locus = state->read_location (*this);
> > > >          t->block.end_locus = state->read_location (*this);
> > > > -      t->block.vars = chained_decls ();
> > > > +
> > > > +      for (tree *chain = &t->block.vars;;)
> > > > +	if (tree decl = tree_node ())
> > > > +	  {
> > > > +	    /* For a deduplicated local type or enumerator, chain the
> > > > +	       duplicate decl instead of the canonical in-TU decl.  Seeing
> > > > +	       a duplicate here means the containing function whose body
> > > > +	       we're streaming in is a duplicate too, so we'll end up
> > > > +	       discarding this BLOCK (and the rest of the duplicate function
> > > > +	       body) anyway.  */
> > > > +	    if (is_duplicate (decl))
> > > > +	      decl = maybe_duplicate (decl);
> > > > +	    else if (DECL_IMPLICIT_TYPEDEF_P (decl)
> > > > +		     && TYPE_TEMPLATE_INFO (TREE_TYPE (decl)))
> > > > +	      {
> > > > +		tree tmpl = TYPE_TI_TEMPLATE (TREE_TYPE (decl));
> > > > +		if (DECL_TEMPLATE_RESULT (tmpl) == decl && is_duplicate
> > > > (tmpl))
> > > > +		  decl = DECL_TEMPLATE_RESULT (maybe_duplicate (tmpl));
> > > > +	      }
> > > 
> > > This seems like a lot of generally-applicable code for finding the
> > > duplicate,
> > > which other calls to maybe_duplicate/odr_duplicate don't use.  If the
> > > template
> > > is a duplicate, why isn't its result?  If there's a good reason for that,
> > > should this template handling go into maybe_duplicate?
> > 
> > Ah yeah, that makes sense.
> > 
> > Some context: IIUC modules treats the TEMPLATE_DECL instead of the
> > DECL_TEMPLATE_RESULT as the canonical decl, which in turn means we'll
> > register_duplicate only the TEMPLATE_DECL.  But BLOCK_VARS never contains
> > a TEMPLATE_DECL, always the DECL_TEMPLATE_RESULT (i.e. a TYPE_DECL),
> > hence the extra handling.
> > 
> > Given that it's relatively more difficult to get at the TEMPLATE_DECL
> > from the DECL_TEMPLATE_RESULT rather than vice versa, maybe we should
> > just register both as duplicates from register_duplicate?  That way
> > callers can just simply pass the DECL_TEMPLATE_RESULT to maybe_duplicate
> > and it'll do the right thing.
> 
> Sounds good.
> 
> > > > @@ -10337,6 +10373,83 @@ trees_in::fn_parms_fini (int tag, tree fn, tree
> > > > existing, bool is_defn)
> > > >        }
> > > >    }
> > > >    +/* Encode into KEY the position of the local type (class or enum)
> > > > +   declaration DECL within FN.  The position is encoded as the
> > > > +   index of the innermost BLOCK (numbered in BFS order) along with
> > > > +   the index within its BLOCK_VARS list.  */
> > > 
> > > Since we already set DECL_DISCRIMINATOR for mangling, could we use it+name
> > > for
> > > the key as well?
> > 
> > We could (and IIUc that'd be more robust to ODR violations), but
> > wouldn't it mean we'd have to do a linear walk over all BLOCK_VARs of
> > all BLOCKS in order to find the one with the matching
> > name+discriminator?  That'd be slower than the current approach which
> > lets us skip to the correct BLOCK and walk only its BLOCK_VARS.
> 
> Ah, good point.  How about block number + name instead of the index?

It seems DECL_DISCRIMINATOR is only set at instantiation time and so for
local types from a function template pattern the field is empty, which
means we can't use it as the key in general :/

> 
> Jason
> 
>