Re: [pushed] c++: fix tourney logic

[Patrick Palka <ppalka@redhat.com>]

On Fri, 27 Oct 2023, Patrick Palka wrote:

> On Fri, 27 Oct 2023, Patrick Palka wrote:
> 
> > On Fri, 20 Oct 2023, Jason Merrill wrote:
> > 
> > > Tested x86_64-pc-linux-gnu, applying to trunk.  Patrick, sorry I didn't apply
> > > this sooner.
> > > 
> > > -- 8< --
> > > 
> > > In r13-3766 I changed the logic at the end of tourney to avoid redundant
> > > comparisons, but the change also meant skipping any less-good matches
> > > between the champ_compared_to_predecessor candidate and champ itself.
> > > 
> > > This should not be a correctness issue, since we believe that joust is a
> > > partial order.  But it can lead to missed warnings, as in this testcase.
> > 
> > I suppose this rules out optimizing tourney via transitivity when in
> > a non-SFINAE context since it'd cause missed warnings such as these.
> > But maybe we'd still want to optimize the second pass via transitivity
> > in a SFINAE context?
> 
> Eh, maybe it's not worth it either way..  According to some quick
> experiments, making the second pass in tourney assume transitivity by
> going up to the most recent tie even in non-SFINAE contexts reduces the
> total number of non-trivial calls to tourney by about 5%.  Doing the

total number of non-trivial calls to joust, rather

> same in only SFINAE contexts reduces the number of calls by less than 1%.
> 
> > 
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* call.cc (tourney): Only skip champ_compared_to_predecessor.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/warn/Wsign-promo1.C: New test.
> > > ---
> > >  gcc/cp/call.cc                           |  5 +++--
> > >  gcc/testsuite/g++.dg/warn/Wsign-promo1.C | 15 +++++++++++++++
> > >  2 files changed, 18 insertions(+), 2 deletions(-)
> > >  create mode 100644 gcc/testsuite/g++.dg/warn/Wsign-promo1.C
> > > 
> > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > index 657eca93d23..a49fde949d5 100644
> > > --- a/gcc/cp/call.cc
> > > +++ b/gcc/cp/call.cc
> > > @@ -13227,10 +13227,11 @@ tourney (struct z_candidate *candidates, tsubst_flags_t complain)
> > >       been compared to.  */
> > >  
> > >    for (challenger = candidates;
> > > -       challenger != champ
> > > -	 && challenger != champ_compared_to_predecessor;
> > > +       challenger != champ;
> > >         challenger = challenger->next)
> > >      {
> > > +      if (challenger == champ_compared_to_predecessor)
> > > +	continue;
> > >        fate = joust (champ, challenger, 0, complain);
> > >        if (fate != 1)
> > >  	return NULL;
> > > diff --git a/gcc/testsuite/g++.dg/warn/Wsign-promo1.C b/gcc/testsuite/g++.dg/warn/Wsign-promo1.C
> > > new file mode 100644
> > > index 00000000000..51b76eee735
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/warn/Wsign-promo1.C
> > > @@ -0,0 +1,15 @@
> > > +// Check that we get joust warnings from comparing the final champ to a
> > > +// candidate between it and the previous champ.
> > > +
> > > +// { dg-additional-options -Wsign-promo }
> > > +
> > > +struct A { A(int); };
> > > +
> > > +enum E { e };
> > > +
> > > +int f(int, A);
> > > +int f(unsigned, A);
> > > +int f(int, int);
> > > +
> > > +int i = f(e, 42);		// { dg-warning "passing 'E'" }
> > > +// { dg-warning "in call to 'int f" "" { target *-*-* } .-1 }
> > > 
> > > base-commit: 084addf8a700fab9222d4127ab8524920d0ca481
> > > -- 
> > > 2.39.3
> > > 
> > > 
> > 
>