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

[Jason Merrill <jason@redhat.com>]

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?

> 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>);