Re: [PATCH] c++: unnecessary instantiation of constexpr var [PR99130]

[Jason Merrill <jason@redhat.com>]

On 9/7/22 16:40, Patrick Palka wrote:
> On Wed, 7 Sep 2022, Jason Merrill wrote:
> 
>> On 9/7/22 15:41, Patrick Palka wrote:
>>> Here the use of the constexpr member/variable specialization 'value'
>>> from within an unevaluated context causes us to overeagerly instantiate
>>> it, via maybe_instantiate_decl called from mark_used, despite only its
>>> declaration not its definition being needed.
>>
>> If the issue is with unevaluated context, maybe maybe_instantiate_decl should
>> guard the call to decl_maybe_constant_var_p with !cp_unevaluated_operand?
> 
> Hmm, that seems to work too.  But IIUC this would mean in an evaluated
> (but non-constexpr) context we'd continue to instantiate constexpr
> variables _immediately_ rather than ideally allowing mark_used to
> postpone their instantiation until the end of TU processing (which is
> what happens with the below approach).
> 
> Another benefit of the below approach is that from within a template
> definition we we now avoid instantiation altogether e.g. for
> 
>    template<class T> constexpr int value = /* blah */;
> 
>    template<class T>
>    int f() { return value<int>; }
> 
> we no longer instantiate value<int> which IIUC is consistent with how we
> handle other kinds of specializations used within a template definition.
> So making mark_used no longer instantiate constexpr variables immediately
> (in both evaluated and unevaluated contexts) seems to yield the most
> benefits.

Makes sense.  The patch is OK.

>>> We used to have the same issue for constexpr function specializations
>>> until r6-1309-g81371eff9bc7ef made us delay their instantiation until
>>> necessary during constexpr evaluation.
>>>
>>> So this patch makes us avoid unnecessarily instantiating constexpr
>>> variable template specializations from mark_used as well.  To that end
>>> this patch pulls out the test in maybe_instantiate_decl
>>>
>>>     (decl_maybe_constant_var_p (decl)
>>>      || (TREE_CODE (decl) == FUNCTION_DECL
>>>          && DECL_OMP_DECLARE_REDUCTION_P (decl))
>>>      || undeduced_auto_decl (decl))
>>>
>>> into each of its three callers (including mark_used) and refines the
>>> test appropriately.  The net result is that only mark_used is changed,
>>> because the other two callers, resolve_address_of_overloaded_function
>>> and decl_constant_var_p, already guard the call appropriately.  And
>>> presumably decl_constant_var_p will take care of instantiation when
>>> needed for e.g. constexpr evaluation.
>>>
>>> Bootstrapped and regteste on x86_64-pc-linux-gnu, does this look OK for
>>> trunk?
>>>
>>> 	PR c++/99130
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* decl2.cc (maybe_instantiate_decl): Adjust function comment. >>> 	Check VAR_OR_FUNCTION_DECL_P. Pull out the disjunction into ...
>>> 	(mark_used): ... here, removing the decl_maybe_constant_var_p
>>> 	part of it.
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp1y/var-templ70.C: New test.
>>> ---
>>>    gcc/cp/decl2.cc                          | 33 ++++++++----------------
>>>    gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 19 ++++++++++++++
>>>    2 files changed, 30 insertions(+), 22 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>>
>>> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
>>> index 89ab2545d64..cd188813bee 100644
>>> --- a/gcc/cp/decl2.cc
>>> +++ b/gcc/cp/decl2.cc
>>> @@ -5381,24 +5381,15 @@ possibly_inlined_p (tree decl)
>>>      return true;
>>>    }
>>>    -/* Normally, we can wait until instantiation-time to synthesize DECL.
>>> -   However, if DECL is a static data member initialized with a constant
>>> -   or a constexpr function, we need it right now because a reference to
>>> -   such a data member or a call to such function is not value-dependent.
>>> -   For a function that uses auto in the return type, we need to instantiate
>>> -   it to find out its type.  For OpenMP user defined reductions, we need
>>> -   them instantiated for reduction clauses which inline them by hand
>>> -   directly.  */
>>> +/* If DECL is a function or variable template specialization, instantiate
>>> +   its definition now.  */
>>>      void
>>>    maybe_instantiate_decl (tree decl)
>>>    {
>>> -  if (DECL_LANG_SPECIFIC (decl)
>>> +  if (VAR_OR_FUNCTION_DECL_P (decl)
>>> +      && DECL_LANG_SPECIFIC (decl)
>>>          && DECL_TEMPLATE_INFO (decl)
>>> -      && (decl_maybe_constant_var_p (decl)
>>> -	  || (TREE_CODE (decl) == FUNCTION_DECL
>>> -	      && DECL_OMP_DECLARE_REDUCTION_P (decl))
>>> -	  || undeduced_auto_decl (decl))
>>>          && !DECL_DECLARED_CONCEPT_P (decl)
>>>          && !uses_template_parms (DECL_TI_ARGS (decl)))
>>>        {
>>> @@ -5700,15 +5691,13 @@ mark_used (tree decl, tsubst_flags_t complain)
>>>          return false;
>>>        }
>>>    -  /* Normally, we can wait until instantiation-time to synthesize DECL.
>>> -     However, if DECL is a static data member initialized with a constant
>>> -     or a constexpr function, we need it right now because a reference to
>>> -     such a data member or a call to such function is not value-dependent.
>>> -     For a function that uses auto in the return type, we need to
>>> instantiate
>>> -     it to find out its type.  For OpenMP user defined reductions, we need
>>> -     them instantiated for reduction clauses which inline them by hand
>>> -     directly.  */
>>> -  maybe_instantiate_decl (decl);
>>> +  /* If DECL has a deduced return type, we need to instantiate it now to
>>> +     find out its type.  For OpenMP user defined reductions, we need them
>>> +     instantiated for reduction clauses which inline them by hand directly.
>>> */
>>> +  if (undeduced_auto_decl (decl)
>>> +      || (TREE_CODE (decl) == FUNCTION_DECL
>>> +	  && DECL_OMP_DECLARE_REDUCTION_P (decl)))
>>> +    maybe_instantiate_decl (decl);
>>>        if (processing_template_decl || in_template_function ())
>>>        return true;
>>> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>> b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>> new file mode 100644
>>> index 00000000000..80965657c32
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>> @@ -0,0 +1,19 @@
>>> +// PR c++/99130
>>> +// { dg-do compile { target c++14 } }
>>> +
>>> +template<class T>
>>> +struct A {
>>> +  static constexpr int value = T::value;
>>> +};
>>> +
>>> +struct B {
>>> +  template<class T>
>>> +  static constexpr int value = T::value;
>>> +};
>>> +
>>> +template<class T>
>>> +constexpr int value = T::value;
>>> +
>>> +using ty1 = decltype(A<int>::value);
>>> +using ty2 = decltype(B::value<int>);
>>> +using ty3 = decltype(value<int>);
>>
>>
>