Re: [PATCH] c++: consider built-in operator candidates first

[Patrick Palka <ppalka@redhat.com>]

On Mon, 20 Sep 2021, Jason Merrill wrote:

> On 9/20/21 12:46, Patrick Palka wrote:
> > During operator overload resolution, we currently consider non-member
> > candidates before built-in candidates.  This didn't make a difference
> > before r12-3346, but after this change add_candidates will avoid
> > computing excess argument conversions if we've already seen a strictly
> > viable candidate, so it's better to consider built-in candidates first.
> 
> Doesn't r12-3346 stop considering conversions after it sees a bad one, and
> later return to the bad candidate if there is no strictly viable candidate?
> How does this patch change that?

Yes, but add_candidates also looks for a strictly viable candidate among
the already-considered candidates in the 'candidates' list via the line:

  bool seen_strictly_viable = any_strictly_viable (*candidates);

So by considering the built-in candidates first, the subsequent call to
add_candidates that considers the non-member functions in will be aware
of any (built-in) strictly viable candidate.

> 
> Depending on the order of the candidates seems fragile.

Yeah.. :/  I guess in general it'd be better to build up the entire
overload set first and then call add_candidates exactly once (which
would also make the perfect candidate optimization more consistent/effective).
But I'm not sure if we can easily build up such an overload set in this
case since built-in candidates are represented and handled differently
than non-built-in candidates..

FWIW, although the test case added by this patch is contrived, this
opportunity was found in the real world by instrumenting the 'bad_fns'
mechanism added by r12-3346 to look for situations where we still end up
using it (and thus end up redundantly considering some candidates twice),
and this built-in operator situation was the most common in the
codebases that I tested (although still quite rare in the codebases that
I tested).

> 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* call.c (add_operator_candidates): Consider built-in operator
> > 	candidates before considering non-member candidates.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/template/conv17.C: Extend test.
> > ---
> >   gcc/cp/call.c                          | 13 +++++++------
> >   gcc/testsuite/g++.dg/template/conv17.C |  7 +++++++
> >   2 files changed, 14 insertions(+), 6 deletions(-)
> > 
> > diff --git a/gcc/cp/call.c b/gcc/cp/call.c
> > index c5601d96ab8..c0da083758f 100644
> > --- a/gcc/cp/call.c
> > +++ b/gcc/cp/call.c
> > @@ -6321,7 +6321,6 @@ add_operator_candidates (z_candidate **candidates,
> >   			 vec<tree, va_gc> *arglist,
> >   			 int flags, tsubst_flags_t complain)
> >   {
> > -  z_candidate *start_candidates = *candidates;
> >     bool ismodop = code2 != ERROR_MARK;
> >     tree fnname = ovl_op_identifier (ismodop, ismodop ? code2 : code);
> >   @@ -6333,6 +6332,12 @@ add_operator_candidates (z_candidate **candidates,
> >     if (rewritten && code != EQ_EXPR && code != SPACESHIP_EXPR)
> >       flags &= ~LOOKUP_REWRITTEN;
> >   +  /* Add built-in candidates to the candidate set.  The standard says to
> > +     rewrite built-in candidates, too, but there's no point.  */
> > +  if (!rewritten)
> > +    add_builtin_candidates (candidates, code, code2, fnname, arglist,
> > +			    flags, complain);
> > +
> >     bool memonly = false;
> >     switch (code)
> >       {
> > @@ -6352,6 +6357,7 @@ add_operator_candidates (z_candidate **candidates,
> >       /* Add namespace-scope operators to the list of functions to
> >        consider.  */
> > +  z_candidate *start_candidates = *candidates;
> >     if (!memonly)
> >       {
> >         tree fns = lookup_name (fnname, LOOK_where::BLOCK_NAMESPACE);
> > @@ -6423,11 +6429,6 @@ add_operator_candidates (z_candidate **candidates,
> >       if (!rewritten)
> >       {
> > -      /* The standard says to rewrite built-in candidates, too,
> > -	 but there's no point.  */
> > -      add_builtin_candidates (candidates, code, code2, fnname, arglist,
> > -			      flags, complain);
> > -
> >         /* Maybe add C++20 rewritten comparison candidates.  */
> >         tree_code rewrite_code = ERROR_MARK;
> >         if (cxx_dialect >= cxx20
> > diff --git a/gcc/testsuite/g++.dg/template/conv17.C
> > b/gcc/testsuite/g++.dg/template/conv17.C
> > index f0f10f2ef4f..87ecefb8de3 100644
> > --- a/gcc/testsuite/g++.dg/template/conv17.C
> > +++ b/gcc/testsuite/g++.dg/template/conv17.C
> > @@ -61,3 +61,10 @@ concept E = requires { T().h(nullptr); };
> >     static_assert(!E<C>);
> >   #endif
> > +
> > +// Verify that the strictly viable built-in operator+ candidate precludes
> > +// us from computing all argument conversions for the below non-strictly
> > +// viable non-member candidate.
> > +enum N { n };
> > +int operator+(N&, B);
> > +int f = n + 42;
> > 
> 
>