On Wed, 15 Dec 2021, Jason Merrill wrote:
> On 12/15/21 12:49, Patrick Palka wrote:
> > This patch makes us remember the function selected by overload
> > resolution during ahead of time processing of a non-dependent call
> > expression, so that we avoid repeating most of the work of overload
> > resolution at instantiation time. This mirrors what we already do for
> > non-dependent operator expressions via build_min_non_dep_op_overload.
> >
> > Some caveats:
> >
> > * When processing ahead of time a non-dependent call to a member
> > function template inside a class template (as in
> > g++.dg/template/deduce4.C), we end up generating an "inverted" partial
> > instantiation such as S<T>::foo<int, int>(), the kinds of which we're
> > apparently not prepared to fully instantiate (e.g. tsubst_baselink
> > mishandles it). So this patch disables this optimization for such
> > functions and adds a FIXME.
>
> I wonder if it would be worthwhile to build a TEMPLATE_ID_EXPR to remember the
> deduced template args, even if we are failing to remember the actual function?
Hmm, that transformation could have observable effects, since overload
resolution for f<int>(0) might end up instantiating more things than for
f(0) due to the explicit-args substitution step:
template<class T> struct A { using type = typename T::type; };
template<class T> void f(T);
template<class T, class U = typename T::type> typename A<T>::type f(T);
Here overload resolution for f(0) succeeds and selects the first
overload but for f<int>(0) induces a hard error. Also I worry that such
a transformation might affect declaration matching in weird ways due
to conflating f(0) with f<int>(0).
We could at least though prune the overload set to the corresponding
selected function template rather than the "inside-out" specialization;
I'll try to implement that.
>
> > * WHen trying to make the instantiation machinery handle these partial
> > instantiations, I made a couple of changes in register_specialization
> > and tsubst_function_decl that get us closer to handling such partial
> > instantiations and that seem like improvements on their own, so this
> > patch includes these changes.
>
> The tsubst_function_decl change makes me nervous; surely there was some reason
> that function wasn't that way in the first place. Let's hold these changes
> for stage 1 if they aren't actually fixing anything.
Will do.
>
> > * This change triggered a latent FUNCTION_DECL pretty printing issue
> > in cpp0x/error2.C -- since we now resolve the call to foo<0> ahead
> > of time, the error now looks like:
> >
> > error: expansion pattern ‘foo()()=0’ contains no parameter pack
> >
> > where the FUNCTION_DECL foo is clearly misprinted. But this
> > pretty-printing issue could be reproduced without this patch if
> > we replace foo with an ordinary function. Since this testcase was
> > added to verify pretty printing of TEMPLATE_ID_EXPR, I work around
> > this test failure by making the call to foo type-dependent and thus
> > immune to this ahead of time pruning.
> >
> > * We now reject parts of cpp0x/fntmp-equiv1.C because we notice that
> > the call d(f, b) in
> >
> > template <unsigned long f, unsigned b, typename> e<d(f, b)> d();
> >
> > isn't constexpr because the (resolved) d isn't. I tried fixing this
> > by making d constexpr, but then the call to d from main becomes
> > ambiguous. So I settled with removing this part of the testcase.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk? Also tested on cmcstl2 and range-v3.
> >
> > gcc/cp/ChangeLog:
> >
> > * call.c (build_new_method_call): For a non-dependent call
> > expression inside a template, returning a templated tree
> > whose overload set contains just the selected function.
> > * pt.c (register_specialization): Check only the innermost
> > template args for dependence in the early exit test.
> > (tsubst_function_decl): Simplify obtaining the template arguments
> > for a partial instantiation.
> > * semantics.c (finish_call_expr): As with build_new_method_call.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp0x/error2.C: Make the call to foo type-dependent in
> > order to avoid latent pretty-printing issue for FUNCTION_DECL
> > inside MODOP_EXPR.
> > * g++.dg/cpp0x/fntmp-equiv1.C: Remove ill-formed parts of
> > testcase that we now diagnose.
> > * g++.dg/template/non-dependent16.C: New test.
> > * g++.dg/template/non-dependent16a.C: New test.
> > ---
> > gcc/cp/call.c | 17 +++++++++
> > gcc/cp/pt.c | 18 ++-------
> > gcc/cp/semantics.c | 15 ++++++++
> > gcc/testsuite/g++.dg/cpp0x/error2.C | 4 +-
> > gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C | 4 --
> > .../g++.dg/template/non-dependent16.C | 37 +++++++++++++++++++
> > .../g++.dg/template/non-dependent16a.C | 36 ++++++++++++++++++
> > 7 files changed, 111 insertions(+), 20 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16.C
> > create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16a.C
> >
> > diff --git a/gcc/cp/call.c b/gcc/cp/call.c
> > index 53a391cbc6b..92d96c19f5c 100644
> > --- a/gcc/cp/call.c
> > +++ b/gcc/cp/call.c
> > @@ -11165,6 +11165,23 @@ build_new_method_call (tree instance, tree fns,
> > vec<tree, va_gc> **args,
> > }
> > if (INDIRECT_REF_P (call))
> > call = TREE_OPERAND (call, 0);
> > +
> > + /* Prune all but the selected function from the original overload
> > + set so that we can avoid some duplicate work at instantiation time.
> > */
> > + if (really_overloaded_fn (fns))
> > + {
> > + if (DECL_TEMPLATE_INFO (fn)
> > + && DECL_MEMBER_TEMPLATE_P (DECL_TI_TEMPLATE (fn))
> > + && dependent_type_p (DECL_CONTEXT (fn)))
> > + /* FIXME: We're not prepared to fully instantiate "inverted"
> > + partial instantiations such as A<T>::f<int>(). */;
> > + else
> > + {
> > + orig_fns = copy_node (orig_fns);
> > + BASELINK_FUNCTIONS (orig_fns) = fn;
> > + }
> > + }
> > +
> > call = (build_min_non_dep_call_vec
> > (call,
> > build_min (COMPONENT_REF, TREE_TYPE (CALL_EXPR_FN (call)),
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index 2340139b238..b114114e617 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -1566,18 +1566,10 @@ register_specialization (tree spec, tree tmpl, tree
> > args, bool is_friend,
> > && TREE_CODE (spec) == NONTYPE_ARGUMENT_PACK));
> > if (TREE_CODE (spec) == FUNCTION_DECL
> > - && uses_template_parms (DECL_TI_ARGS (spec)))
> > + && uses_template_parms (INNERMOST_TEMPLATE_ARGS (DECL_TI_ARGS
> > (spec))))
> > /* This is the FUNCTION_DECL for a partial instantiation. Don't
> > - register it; we want the corresponding TEMPLATE_DECL instead.
> > - We use `uses_template_parms (DECL_TI_ARGS (spec))' rather than
> > - the more obvious `uses_template_parms (spec)' to avoid problems
> > - with default function arguments. In particular, given
> > - something like this:
> > -
> > - template <class T> void f(T t1, T t = T())
> > -
> > - the default argument expression is not substituted for in an
> > - instantiation unless and until it is actually needed. */
> > + register it; we want to register the corresponding TEMPLATE_DECL
> > + instead. */
> > return spec;
> > if (optimize_specialization_lookup_p (tmpl))
> > @@ -13960,9 +13952,7 @@ tsubst_function_decl (tree t, tree args,
> > tsubst_flags_t complain,
> > /* Calculate the complete set of arguments used to
> > specialize R. */
> > - argvec = tsubst_template_args (DECL_TI_ARGS
> > - (DECL_TEMPLATE_RESULT
> > - (DECL_TI_TEMPLATE (t))),
> > + argvec = tsubst_template_args (DECL_TI_ARGS (t),
> > args, complain, in_decl);
> > if (argvec == error_mark_node)
> > return error_mark_node;
> > diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
> > index 7078af03d3c..57f689042b9 100644
> > --- a/gcc/cp/semantics.c
> > +++ b/gcc/cp/semantics.c
> > @@ -2893,6 +2893,21 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args,
> > bool disallow_virtual,
> > {
> > if (INDIRECT_REF_P (result))
> > result = TREE_OPERAND (result, 0);
> > +
> > + /* Prune all but the selected function from the original overload
> > + set so that we can avoid some duplicate work at instantiation time.
> > */
> > + if (TREE_CODE (result) == CALL_EXPR
> > + && really_overloaded_fn (orig_fn))
> > + {
> > + orig_fn = CALL_EXPR_FN (result);
> > + if (TREE_CODE (orig_fn) == COMPONENT_REF)
> > + {
> > + /* The result of build_new_method_call. */
> > + orig_fn = TREE_OPERAND (orig_fn, 1);
> > + gcc_assert (BASELINK_P (orig_fn));
> > + }
> > + }
> > +
> > result = build_call_vec (TREE_TYPE (result), orig_fn, orig_args);
> > SET_EXPR_LOCATION (result, input_location);
> > KOENIG_LOOKUP_P (result) = koenig_p;
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/error2.C
> > b/gcc/testsuite/g++.dg/cpp0x/error2.C
> > index e6af294c180..eb966362ccb 100644
> > --- a/gcc/testsuite/g++.dg/cpp0x/error2.C
> > +++ b/gcc/testsuite/g++.dg/cpp0x/error2.C
> > @@ -3,7 +3,7 @@
> > template<int> int foo();
> > -template<typename F> void bar(F f)
> > +template<typename F, int N> void bar(F f)
> > {
> > - f((foo<0>()=0)...); // { dg-error "pattern '\\(foo\\<0\\>\\)\\(\\)=0'" }
> > + f((foo<N>()=0)...); // { dg-error "pattern '\\(foo\\<N\\>\\)\\(\\)=0'" }
> > }
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
> > b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
> > index 833ae6fc85c..60ebad8d1d3 100644
> > --- a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
> > +++ b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
> > @@ -1,10 +1,7 @@
> > // PR c++/86946, DR 1321
> > // { dg-do compile { target c++11 } }
> > -int d(int, int);
> > template <long> class e {};
> > -template <unsigned long f, unsigned b, typename> e<sizeof(d(f, b))> d();
> > -template <unsigned long f, unsigned b, typename> e<d(f, b)> d();
> > template <class T, class U> constexpr T d2(T, U) { return 42; }
> > template <unsigned long f, unsigned b, typename> e<d2(f, b)> d2();
> > @@ -17,7 +14,6 @@ template <unsigned long f, unsigned b, typename>
> > e<sizeof(d3(f, b))> d3();
> > int main()
> > {
> > - d<1,2,int>();
> > d2<1,2,int>();
> > d3<1,2,int>();
> > }
> > diff --git a/gcc/testsuite/g++.dg/template/non-dependent16.C
> > b/gcc/testsuite/g++.dg/template/non-dependent16.C
> > new file mode 100644
> > index 00000000000..ee8ef902529
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/non-dependent16.C
> > @@ -0,0 +1,37 @@
> > +// This test verifies that after resolving a non-dependent call expression
> > +// ahead of time, we prune all but the selected candidate from the overload
> > +// set. Without this optimization, overload resolution for the final call
> > to
> > +// f<void>() would be exponential in the size of the overload set.
> > +
> > +// { dg-do compile { target c++11 } }
> > +
> > +template<class T> void f();
> > +template<class T> auto f() -> decltype(f<void>(), 1, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 2, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 3, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 4, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 5, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 6, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 7, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 8, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 9, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 10, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 11, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 12, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 13, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 14, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 15, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 16, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 17, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 18, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 19, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 20, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 21, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 22, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 23, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 24, *T());
> > +template<class T> auto f() -> decltype(f<void>(), 25, *T());
> > +
> > +int main() {
> > + f<void>();
> > +}
> > diff --git a/gcc/testsuite/g++.dg/template/non-dependent16a.C
> > b/gcc/testsuite/g++.dg/template/non-dependent16a.C
> > new file mode 100644
> > index 00000000000..0e04d646c0b
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/non-dependent16a.C
> > @@ -0,0 +1,36 @@
> > +// Like non-dependent16.C, but using member functions.
> > +
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct A {
> > + template<class T> static void f();
> > + template<class T> static auto f() -> decltype(f<void>(), 1, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 2, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 3, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 4, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 5, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 6, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 7, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 8, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 9, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 10, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 11, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 12, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 13, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 14, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 15, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 16, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 17, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 18, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 19, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 20, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 21, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 22, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 23, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 24, *T());
> > + template<class T> static auto f() -> decltype(f<void>(), 25, *T());
> > +};
> > +
> > +int main() {
> > + A::f<void>();
> > +}
>
>
