On Wed, Mar 20, 2019 at 6:36 PM Andrew Haley <a...@redhat.com> wrote:
>
> On 3/20/19 2:08 PM, Moritz Strübe wrote:
> >
> > Ok, I played around a bit. Interestingly, if I set
> > -fsanitize=udefined and -fsanitize-undefined-trap-on-error the
> > compiler detects that it will always trap, and optimizes the code
> > accordingly (the code after the trap is removed).* Which kind of
> > brings me to David's argument: Shouldn't the compiler warn if there
> > is undefined behavior it certainly knows of?
>
> Maybe an example would help.
>
> Consider this code:
>
> for (int i = start; i < limit; i++) {
> foo(i * 5);
> }
>
> Should GCC be entitled to turn it into
>
> int limit_tmp = i * 5;
> for (int i = start * 5; i < limit_tmp; i += 5) {
> foo(i);
> }
>
> If you answered "Yes, GCC should be allowed to do this", would you
> want a warning? And how many such warnings might there be in a typical
> program?
I assume i is signed int. Even then GCC may not do this unless it knows
the loop is entered (start < limit).
Richard.
>
> --
> Andrew Haley
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com>
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
