[PATCH] c++: ahead-of-time overload set pruning for non-dep calls
Jason Merrill
jason@redhat.com
Wed Dec 15 21:29:03 GMT 2021
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?
> * 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.
> * 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>();
> +}
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