[PATCH] Initial support for Attribute subjects

[Richard Smith]

+      else if ((*I)->isSubClassOf("SubsetSubject")) {
+        std::string FN, TN;
+        bool IsDecl = writeAppertainsToSubjectSubsetDefinition(OS, *I, FN, TN);
+        std::string C = FN + "(cast<" + TN + ">(P))";

This cast<> doesn't look right: we can get here if the potential
subject is not a TN, can't we?

How are you intending that process of creating and adding attributes
will work once this lands? Right now, the usual flow of handleFooAttr
is:

For each entity where we want to apply the attribute:
 * Check attribute appertains to entity
 * Perform other semantic checks on the attribute and on whether it
can be attached to the entity
 * Create FooAttr instance and add it

With appertainsTo being a method on FooAttr, that won't work. Perhaps
we can flip this around:

 * Perform semantic checks on the attribute by itself
 * Create FooAttr from the AttributeList
 * For each entity where we want to apply the attribute:
    * Check whether it appertains to the entity
    * Perform semantic checks on the (attribute, entity) pair
    * Add it

There are a couple of cases where we need to modify the attribute once
we see what it's being applied to; those don't look too hard to
support but are something to keep in mind.

On Mon, Jun 24, 2013 at 10:34 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> Based on feedback from Richard, here is another pass at modifying
> tablegen for Subjects support.  The delta here is that templates have
> been removed and replaced by a method pair (one for Decl and one for
> Stmt).  The usage is still the same as before: you can call
> appertainsTo and pass in something you want to know whether the
> attribute appertains to it or not, and it will return a Boolean.
>
> ~Aaron
>
> On Thu, Jun 20, 2013 at 5:34 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>> This patch adds support for the Subjects part of the Attr tablegen
>> file.  Specifically, it modifies tablegen to pay attention to
>> Subjects, and emit a templated member function for attribute
>> subclasses called appertainTo.  The purpose is to be able to determine
>> whether a given attribute appertains to a particular subject.
>>
>> template <typename Subject>
>> bool appertainTo(const Subject *S) const;
>>
>> If the attribute has no subjects, then the default behavior of
>> appertainTo is to return true.  This makes it backwards compatible
>> with the fact that we've always ignored subjects previously (and many
>> attributes do not have a specific subject specified).
>>
>> If the attribute does have subjects, then the default appertainTo will
>> return false, but specializations will be generated that return true
>> for each subject case.  If the subject happens to be a SubsetSubject,
>> the code from CheckCode is emitted (hence the reason for having the
>> subject passed into the function) and the proper base is taken from
>> the subject.
>>
>> ~Aaron