Re: [PATCH] c++: cache partial template specialization selection

[Jason Merrill <jason@redhat.com>]

On 6/28/23 12:51, Patrick Palka wrote:
> There's currently no cheap way to obtain the partial template
> specialization (and arguments relative to it) that was selected for a
> class or variable template specialization.  Our only option is to
> compute the result from scratch via most_specialized_partial_spec.
> 
> For class templates this isn't really an issue because we usually need
> this information just once, upon instantiation.  But for variable
> templates we need it upon specialization and later upon instantiation.
> It'd be good for this information to be readily available in general
> however.
> 
> To that end, this patch adds a TI_PARTIAL_INFO field to TEMPLATE_INFO
> that holds another TEMPLATE_INFO consisting of the partial template and
> arguments relative to it, which most_specialized_partial_spec then
> uses to transparently cache its (now TEMPLATE_INFO) result.
> > Similarly, there's no easy way to go from the DECL_TEMPLATE_RESULT of a
> partial TEMPLATE_DECL back to the TEMPLATE_DECL.  (Our best option is to
> walk the DECL_TEMPLATE_SPECIALIZATIONS list of the primary TEMPLATE_DECL.)
> So this patch also uses this new field to link these entities in this
> other direction.

You had talked about this possibly replacing the deferred_access_checks; 
could they share the same slot by anonymous union?  Your second use 
would conflict, but perhaps the checks could move to the TEMPLATE_INFO 
of the TEMPLATE_DECL now that we have a way to get at it?

In any case, this patch is OK.

Jason