Re: Fortran loops

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Jun 28, 2016 at 2:10 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> IIRC this has already been discussed in the past (gfortran.dg/do_1.f90).
>> The Fortran standard requires that the variable i is equal to array(2)+1 on the loop exit, which causes an overflow if array(2) is equal to huge(i).
>> So the code is invalid when b(2) = huge(x), but I donât see why the overflow of the exit value should lead to an infinite loop.
>
> The reason why I ask (again as I just noticed - thanks for reminding :) is that
> the C equivalent:
>
> for(i=low;i<=high;i++)
>   ... body ...
>
> is a lot easier to compile, but it will loop infinitly when the upper bound
> happens to be INT_MAX.  To avoid that Fortran FE expand the loop as:
>
> if (low<high)
>   {
>     i=low;
>     do {
>       ... body ...
>       if (i==high)
>         break;
>       i++
>     }
>     while (1)
>   }
>
> This results in at least one extra compare and usually also extra register needed.
> If we could avoid this codegen based on fact that such loops are undefined, we would
> get improvements in thight and deeply nested loops. I.e. in exchange2 benchmark.
>
> Martin implemented the C-style codegen and it does improve code quality.
> If the fact that we produce the complex loop code above is just QOI, perhaps
> we can introduce a flag controlling this and turn into C-style loop expansion
> by default when optimizing?
> The do-3.f90 testcase tests some of those so it will need to be split into
> valid loops and invalid loops.

I guess it might be GCC on its own removing the exit test based on
undefined-overflow
and computing an upper bound on the iteration.

So if the testcases are really invalid I suppose that at least with
-Ofast the Fortran
FE could avoid the extra check.  But IIRC the issue appeared on valid
loops as well.
(that said - I've been here before)

Richard.

> Honza
>
>> Dominique
>> > On Jun 27, 2016, at 6:56 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >
>> > Hello,
>> > I have a Fortran question.  The following program loops infinitly when compiled
>> > with ifort -O0, while it does the expected number of iterations with gfortran.
>> > The issue is that we do produce quite ineffective code to deal with
>> > side cases like this.
>> >
>> > is the program bellow valid and expected to terminate?  If so I guess we want
>> > to have it as a testcase.
>> >
>> > Thanks,
>> > Honza
>> >
>> > program test_program
>> > integer(4) :: b(2), a(22), x
>> > integer(8) :: suma
>> >
>> > b(1) = huge(x)-10
>> > b(2) = huge(x)
>> >
>> > call test2(b, suma)
>> > print *,suma
>> >
>> > end program test_program
>> > function test2(array, s)
>> >  integer(4) :: i, block(9), array(2)
>> >  integer (8) :: s
>> >  s = 1
>> >
>> >  do i = array(1), array(2)
>> >      s = s + 1
>> >  end do
>> >
>> > end function test2
>> >