RE: [PATCH] Fix bootstrap failure (with g++ 4.8.5) in tree-if-conv.cc.

[Tamar Christina <Tamar.Christina@arm.com>]

> On Mon, Jul 17, 2023 at 12:21 AM Tamar Christina via Gcc-patches <gcc-
> patches@gcc.gnu.org> wrote:
> >
> > > -----Original Message-----
> > > From: Richard Biener <richard.guenther@gmail.com>
> > > Sent: Monday, July 17, 2023 7:19 AM
> > > To: Roger Sayle <roger@nextmovesoftware.com>
> > > Cc: gcc-patches@gcc.gnu.org; Tamar Christina
> > > <Tamar.Christina@arm.com>
> > > Subject: Re: [PATCH] Fix bootstrap failure (with g++ 4.8.5) in tree-if-
> conv.cc.
> > >
> > > On Fri, Jul 14, 2023 at 8:56 PM Roger Sayle
> > > <roger@nextmovesoftware.com>
> > > wrote:
> > > >
> > > >
> > > >
> > > > This patch fixes the bootstrap failure I'm seeing using gcc 4.8.5
> > > > as
> > > >
> > > > the host compiler.  Ok for mainline?  [I might be missing
> > > > something]
> > >
> > > OK.   Btw, while I didn't spot this during review I would appreciate
> > > if the code could use vec.[q]sort, this should work with a lambda as
> > > well I think.
> >
> > That was my first use, but that hits
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99469
> 
> That is not hitting PR 99469 but rather it means your comparison is not
> correct for an (unstable) sort.
> That is qsort comparator should have this relationship `f(a,b) == !f(b, a)` and
> `f(a,a)` should also return false.
	
I'm using the standard std::pair comparator which indicates that f(a,a) is true,
https://en.cppreference.com/w/cpp/utility/pair/operator_cmp 

> If you are running into this for qsort here, you will most likely run into issues
> with std::sort later on too.

Don't see why or how. It needs to have a consistent relationship which std::pair
maintains.  So why would using the standard tuple comparator with a standard
std::sort cause problem?

Thanks,
Tamar

> 
> Thanks,
> Andrew
> 
> >
> > Regards,
> > Tamar