[Bug fortran/109186] nearest(huge(x),-1.0_kind(x)) half of correct value

["john.harper at vuw dot ac.nz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109186

--- Comment #2 from john.harper at vuw dot ac.nz ---
The results I expected were that

nearest(huge(sp1),-sp1) would be near huge(sp1)/(1+epsilon(sp1)),
nearest(huge(dp1),-dp1) would be near huge(dp1)/(1+epsilon(dp1)),
nearest(huge(ep1),-ep1) would be near huge(ep1)/(1+epsilon(ep1)),
nearest(huge(qp1),-qp1) would be near huge(qp1)/(1+epsilon(qp1)).

The results from gfortran were different from those by a factor near 2.
On compiling with ifort I got results close to what I had expected, which
is why I thought the bug was not in my program. The g95 compiler (now 
about 10 years old) gave results identical to those of ifort except
for ep = selected_real_kind(18) which ifort treats as if it were 
selected_real_kind(33) because ifort has only three real kinds.

By the way I thought I was using gfortran 12.0. Compiling with -v said
GNU Fortran (Ubuntu 12.1.0-2ubuntu1~22.04) version 12.1.0 
(x86_64-linux-gnu)
I wish to apologise for that error in my bug report. I got the same output
on another machine with GNU Fortran (GCC) version 12.2.0 
(x86_64-pc-linux-gnu)

The factor 2 discrepancies become clearer in this version of my program,
which has 4 more lines printing huge/nearest for the 4 real kinds.

program nearesttest3
   implicit none
   integer,parameter::sp = kind(1.0), dp=kind(1d0), ncases = 2, &
        ep = selected_real_kind(18),qp = selected_real_kind(33)
   character(*),parameter:: fmt(4) = &
        "(A,ES"//["16.08E2","26.17E3","30.20E4","44.34E4"]//")"
   character(24),dimension(2):: splist,dplist,eplist,qplist
   integer n
   real(sp),parameter::sp1 = 1.0_sp
   real(sp):: spx(ncases) = [huge(sp1),nearest(huge(sp1),-sp1)]
   data splist/'huge(sp1)','nearest(huge(sp1),-sp1)'/
   real(dp),parameter::dp1 = 1.0_dp
   real(dp):: dpx(ncases) = [huge(dp1),nearest(huge(dp1),-dp1)]
   data dplist/'huge(dp1)','nearest(huge(dp1),-dp1)'/
   real(ep),parameter::ep1 = 1.0_ep
   real(ep):: epx(ncases) = [huge(ep1),nearest(huge(ep1),-ep1)]
   data eplist/'huge(ep1)','nearest(huge(ep1),-ep1)'/
   real(qp),parameter::qp1 = 1.0_qp
   real(qp):: qpx(ncases) = [huge(qp1),nearest(huge(qp1),-qp1)]
   data qplist/'huge(qp1)','nearest(huge(qp1),-qp1)'/

   print fmt(1),(splist(n),spx(n),n=1,ncases)
   print fmt(1),'huge/nearest = ',huge(sp1)/nearest(huge(sp1),-sp1)
   print fmt(2),(dplist(n),dpx(n),n=1,ncases)
   print fmt(2),'huge/nearest = ',huge(dp1)/nearest(huge(dp1),-dp1)
   print fmt(3),(eplist(n),epx(n),n=1,ncases)
   print fmt(3),'huge/nearest = ',huge(ep1)/nearest(huge(ep1),-ep1)
   print fmt(4),(qplist(n),qpx(n),n=1,ncases)
   print fmt(4),'huge/nearest = ',huge(qp1)/nearest(huge(qp1),-qp1)
end program nearesttest3

Its gfortran output:

huge(sp1)                 3.40282347E+38
nearest(huge(sp1),-sp1)   1.70141173E+38
huge/nearest =   2.00000000E+00
huge(dp1)                 1.79769313486231571E+308
nearest(huge(dp1),-dp1)   8.98846567431157854E+307
huge/nearest =   2.00000000000000000E+000
huge(ep1)                 1.18973149535723176502E+4932
nearest(huge(ep1),-ep1)   5.94865747678615882511E+4931
huge/nearest =   2.00000000000000000000E+0000
huge(qp1)                 1.1897314953572317650857593266280070E+4932
nearest(huge(qp1),-qp1)   5.9486574767861588254287966331400351E+4931
huge/nearest =   2.0000000000000000000000000000000000E+0000

Its g95 output:

uge(sp1)                 3.40282347E+38
nearest(huge(sp1),-sp1)   3.40282326E+38
huge/nearest =   1.00000012E+00
huge(dp1)                 1.79769313486231571E+308
nearest(huge(dp1),-dp1)   1.79769313486231551E+308
huge/nearest =   1.00000000000000022E+000
huge(ep1)                 1.18973149535723176502E+4932
nearest(huge(ep1),-ep1)   1.18973149535723176496E+4932
huge/nearest =   1.00000000000000000011E+0000
huge(qp1)                 1.1897314953572317650857593266280070E+4932
nearest(huge(qp1),-qp1)   1.1897314953572317650857593266280069E+4932
huge/nearest =   1.0000000000000000000000000000000002E+0000

On Sun, 19 Mar 2023, jvdelisle at gcc dot gnu.org wrote:

> Date: Sun, 19 Mar 2023 05:06:47 +0000
> From: jvdelisle at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>
> To: John Harper <john.harper@vuw.ac.nz>
> Subject: [Bug fortran/109186] nearest(huge(x),
>     -1.0_kind(x)) half of correct value
> Resent-Date: Sun, 19 Mar 2023 18:06:59 +1300 (NZDT)
> Resent-From: <john.harper@vuw.ac.nz>
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109186
>
> Jerry DeLisle <jvdelisle at gcc dot gnu.org> changed:
>
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                 CC|                            |jvdelisle at gcc dot gnu.org
>
> --- Comment #1 from Jerry DeLisle <jvdelisle at gcc dot gnu.org> ---
> Can you clarify what you expect the correct results should be that you expect?
>
> -- 
> You are receiving this mail because:
> You reported the bug.
>


-- John Harper, School of Mathematics and Statistics
Victoria Univ. of Wellington, PO Box 600, Wellington 6140, New Zealand.
e-mail john.harper@vuw.ac.nz phone +64(0) 4 463 5276