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

[Jason Merrill <jason@redhat.com>]

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?

Jason