Re: [PATCH 2/1] c++: optimize specialization of templated member functions

[Patrick Palka <ppalka@redhat.com>]

On Thu, 9 Jun 2022, Jason Merrill wrote:

> On 6/9/22 09:00, Patrick Palka wrote:
> > This performs one of the optimizations added by the previous
> > patch to lookup_template_class, to instantiate_template as well.
> > (For the libstdc++ ranges tests this optimization appears to be
> > effective around 30% of the time, i.e. 30% of the time context of 'tmpl'
> > is non-dependent while the context of 'gen_tmpl' is dependent.)
> 
> If this is a significant optimization, how about doing it in tsubst_aggr_type
> rather than its callers?

I'm not sure how we'd do this optimization in tsubst_aggr_type?

I haven't observed any significant time/memory improvements based on my
limited benchmarking, but I can imagine for deeply nested templates it
could be significant.  And avoiding redundant work should hopefully help
streamline debugging I suppose.

> 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.cc (instantiate_template): Don't substitute the context
> > 	of the most general template if that of the partially
> > 	instantiated template is non-dependent.
> > ---
> >   gcc/cp/pt.cc | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index e021c254872..208daad298a 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -21661,8 +21661,14 @@ instantiate_template (tree tmpl, tree orig_args,
> > tsubst_flags_t complain)
> >       ++processing_template_decl;
> >     if (DECL_CLASS_SCOPE_P (gen_tmpl))
> >       {
> > -      tree ctx = tsubst_aggr_type (DECL_CONTEXT (gen_tmpl), targ_ptr,
> > -				   complain, gen_tmpl, true);
> > +      tree ctx;
> > +      if (!uses_template_parms (DECL_CONTEXT (tmpl)))
> > +	/* If the context of the partially instantiated template is already
> > +	   non-dependent, then we might as well use it.  */
> > +	ctx = DECL_CONTEXT (tmpl);
> > +      else
> > +	ctx = tsubst_aggr_type (DECL_CONTEXT (gen_tmpl), targ_ptr,
> > +				complain, gen_tmpl, true);
> >         push_nested_class (ctx);
> >       }
> >   
> 
>