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. > > > 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>); > >