[PATCH] Vtable pointer verification, C++ front end changes (patch 1 of 3)

Jason Merrill jason@redhat.com
Fri Feb 1 16:49:00 GMT 2013

On 01/31/2013 07:24 PM, Caroline Tice wrote:
> On Wed, Jan 30, 2013 at 9:26 AM, Jason Merrill <jason@redhat.com> wrote:
>>> @@ -17954,6 +17954,10 @@ mark_class_instantiated (tree t, int ext
>>> +      if (flag_vtable_verify)
>>> +        vtv_save_class_info (t);
>> Why do you need this here as well as in finish_struct_1?
> If we don't have this in both places, then we miss getting vtable
> pointers for instantiated templates.

Why?  instantiated templates also go through finish_struct_1.  And we 
only hit this function for explicit instantiations, not implicit.

>>> +  base_id = DECL_ASSEMBLER_NAME (TREE_CHAIN (base_class));
>> I think you want TYPE_LINKAGE_IDENTIFIER here.
> I don't know the difference between DECL_ASSEMBLER_NAME and
> TYPE_LINKAGE_IDENTIFIER.  We are just trying to get the mangled name
> for the class.

Ah, I guess you don't want TYPE_LINKAGE_IDENTIFIER, as that's the simple 
name rather than the mangled one.  But for the external name you always 
want to look at TYPE_NAME, not TREE_CHAIN (which corresponds to 
TYPE_STUB_DECL); in the case of an anonymous class that gets a name for 
linkage purposes from a typedef, the latter will have the original 
placeholder name, while the former will have the name used in mangling.

>> I don't understand what the qualifier business is trying to accomplish,
>> especially since you never use type_decl_type.  You do this in several
>> places, but it should never be necessary; classes don't have any qualifiers.
> We used to not have the "qualifier business", assuming that classes
> did not have any type qualifiers.  This turned out not to be a true
> assumption.  Occasionally we were getting a case where a class had a
> "const" qualifier attached to it *sometimes*.

Why?  You are getting a qualified variant of the class somehow.  Where 
is it coming from?

>> Here you're doing two hash table lookups when one would be enough.
> As written the insert function doesn't return anything to let you know
> whether the item was already there or not, which we need to know (we
> use the results here to avoid generating redundant calls to
> __VLTRegisterPair.  I suppose we could modify the insert function to
> return a boolean indicating if the item was already in the hashtable,
> and then we could get by with just one call here...

Yep, that's what I was thinking.

>> For that matter, you don't need the array, either; you can just use TYPE_UID
>> for a bitmap key and use htab_traverse to iterate over all elements.
> I don't understand how this would work.  I think we need the vec, at
> least, to have direct access based on TYPE_UID (which is also the vec
> index).

TYPE_UID is already a property of the type, different from the class_uid 
in your patch.

But yes, I guess you do need some way to get from your index back to the 
type, so never mind.

>>> +guess_num_vtable_pointers (struct vtv_graph_node *class_node)
>> I would think it would be better to pass the unrounded count to the library,
>> and let the library decide how to adjust that number for allocation.
> If there is any computation we can do at compile-time rather than
> run-time, we would rather do it at compile time.

I guess that makes sense.

>>> +  var_name = ACONCAT (("_ZN4_VTVI", IDENTIFIER_POINTER (base_id),
>>> +                       "E12__vtable_mapE", NULL));
> $ c++filt _ZN4_VTVISt13bad_exceptionE12__vtable_mapE
> _VTV<std::bad_exception>::__vtable_map

Interesting.  Does this _VTV template appear anywhere else?

Even if we stay with this approach to producing the name, I'd like it to 
happen in a (new) function in mangle.c.

>>> +reset_type_qualifiers (unsigned int new_quals, tree type_node)
>> This function is not safe and should be removed; as mentioned above, it
>> shouldn't be needed anyway.
> As I explained above, we originally didn't have it and then found we
> really needed it.   If you know of a safer or better way to accomplish
> the same thing we would be happy to hear about it.

TYPE_MAIN_VARIANT will give you an unqualified variant of any qualified 


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