Re: Fix Fortran rounding issues, PR fortran/96983.

[Michael Meissner <meissner@linux.ibm.com>]

On Wed, Apr 21, 2021 at 10:10:07AM +0200, Tobias Burnus wrote:
> On 20.04.21 08:58, Richard Biener via Fortran wrote:
> >On Mon, Apr 19, 2021 at 9:40 PM Michael Meissner via Fortran
> ><fortran@gcc.gnu.org>  wrote:
> Is there any reason to not only send the email to fortran@ _and_
> gcc-patches@ but sending it to 13 Fortran maintainers explicitly? (Now
> removed)

Sorry about that.  With PowerPC backend changes, I generally do explicitly add
the maintainers so things don't get lost.


> >>Fix Fortran rounding issues, PR fortran/96983.
> >>
> >>Can I check this change into the GCC trunk?
> >The patch looks fine technically and is definitely an improvement since the
> >intermediate conversion looks odd.  But it might be that the standard
> >requires such dance though the preceeding cases handled don't seem to
> >care.  I'm thinking of a FP format where round(1.6) == 3 because of lack
> >of precision but using an intermediate FP format with higher precision
> >would "correctly" compute 2.
> 
> The patched build_round_expr is only called by ANINT / NINT;
> NINT is real → integer; ANINT is real → real
> [And the modified code is only called for NINT, reason: see comment far below.]
> 
> NINT (A[, KIND]) is described (F2018) as "Nearest integer":
> * Result Characteristics. Integer. If KIND is present, the kind type parameter
>   is that specified by the value of KIND;
>   otherwise, the kind type parameter is that of default integer type.
> * The result is the integer nearest A, or if there are two
>   integers equally near A, the result is whichever such integer has the greater
>   magnitude.
> * Example. NINT (2.783) has the value 3.
> 
> ANINT (A[, KIND]) as "Nearest whole number":
> * The result is of type real. If KIND is present, the kind type parameter is that
>   specified by the value of KIND; otherwise, the kind type parameter is that of A.
> * The result is the integer nearest A, or if there are two integers equally near A,
>   the result is whichever such integer has the greater magnitude.
> * Examples. ANINT (2.783) has the value 3.0. ANINT (−2.783) has the value −3.0.
> 
> >Of course the current code doesn't handle this correctly for the
> >case if llroundf either.
> >>I've not contributed to the Fortran front end before.  If the maintainers like
> >>the patch, can somebody point out if I need to do additional things to commit
> >>the patch?
> Nothing special: a testcase already exists, committing is done as usual
> and a PR to update you have as well.

Given GCC 11 has branched, is it ok to backport the patch to the GCC 11 branch
as well?  I assume it is, since it fixes a regression in the compiler.

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.ibm.com, phone: +1 (978) 899-4797