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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Mon, 31 Jan 2022 10:33:50 +0000
Message-ID: <CAH6eHdQu1aSamPyqs485TtfywOB0CVhUrrW2YRBCOLbpgNVWbw@mail.gmail.com>
Subject: Re: Enquiry
To: Andreas Schwab <schwab@linux-m68k.org>
Cc: Jonathan Wakely via Gcc <gcc@gcc.gnu.org>, Jakub Jelinek <jakub@redhat.com>,
 Theodore Papadopoulo <Theodore.Papadopoulo@inria.fr>
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On Mon, 31 Jan 2022 at 10:25, Andreas Schwab wrote:
>
> On Jan 30 2022, Jonathan Wakely via Gcc wrote:
>
> > On Sun, 30 Jan 2022, 10:58 Jakub Jelinek, <jakub@redhat.com> wrote:
> >
> >> On Sun, Jan 30, 2022 at 10:50:56AM +0000, Jonathan Wakely wrote:
> >> > We could put a trap instruction at the end of the function though, which
> >> > would make the result a bit less arbitrary.
> >> >
> >> > I've come around to thinking that's preferable for cases like this.
> >>
> >> Depends on which exact cases.
> >> Because for
> >> int foo (int s) { if (s == 123) return 1; }
> >> we want to optimize it into
> >> return 1;
> >> rather than if (s == 123) return 1; else __builtin_trap ();
> >> For debugging we have -fsanitize=undefined
> >
> >
> > What if we inserted the trap for -O0?
>
> Note that in C it is not an error to fall through the end of a non-void
> function if the caller does not use the return value.

Indeed. Theo's program is undefined in C++ and valid in C.

I would love to see the trap inserted for C++ though.