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

[Jason Merrill <jason@redhat.com>]

On 6/9/22 15:37, Patrick Palka wrote:
> 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?

Oops, I was overlooking the gen_tmpl vs. tmpl difference.

> 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.

OK.

>>
>>> 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);
>>>        }
>>>    
>>
>>
>