Re: [PATCH] c++: ahead-of-time overload set pruning for non-dep calls

[Patrick Palka <ppalka@redhat.com>]

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>();
> > +}
> 
>