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From: "federico.perini at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/101757] New: Simple integer assigment fails (off-seted
 by -1) when compiling with -O3
Date: Tue, 03 Aug 2021 16:22:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101757

            Bug ID: 101757
           Summary: Simple integer assigment fails (off-seted by -1) when
                    compiling with -O3
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: federico.perini at gmail dot com
  Target Milestone: ---

Created attachment 51251
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51251&action=3Dedit
test program

An extremely simple logical-flag-based assignment, which sets an integer to=
 1
if the logical is .true., and to 0 if .false., fails with gfortran 7.1.0 if
compiled with -O3. Using `WHERE`, `MERGE`, or a simple loop does not change=
 the
result.

Test program:


program test_merge_
   use iso_fortran_env
   implicit none
   integer, parameter :: N=3D10000
   logical :: test(N)
   integer :: i,itest(N)

   do i=3D1,N
      test(i) =3D mod(i,2)=3D=3D0
   end do

   itest =3D merge(1,0,test)
   print *, 'MERGE  itest min (0) =3D',minval(itest),' max (1)=3D',maxval(i=
test)

   where (test)
     itest =3D 1
   elsewhere
     itest =3D 0
   end where

   print *, 'WHERE  itest min (0) =3D',minval(itest),' max (1)=3D',maxval(i=
test)

   do i=3D1,N
     if (test(i)) then
        itest(i) =3D 1
     else
        itest(i) =3D 0
     endif
   end do

   print *, 'IFLOOP itest min (0) =3D',minval(itest),' max (1)=3D',maxval(i=
test)
end program test_merge_

Expected result is OK if "-O2", wrong if "-O3":=20

werc 1073% gfortran -O3 test_merge.f90
werc 1074% ./a.out
 MERGE  itest min (0) =3D          -1  max (1)=3D           0
 WHERE  itest min (0) =3D          -1  max (1)=3D           0
 IFLOOP itest min (0) =3D          -1  max (1)=3D           0
werc 1075% gfortran -O2 test_merge.f90
werc 1076% ./a.out
 MERGE  itest min (0) =3D           0  max (1)=3D           1
 WHERE  itest min (0) =3D           0  max (1)=3D           1
 IFLOOP itest min (0) =3D           0  max (1)=3D           1

Tested OK on gfortran 9.2.0, 10.2.0, 10.3.0.

Thanks,
Federico=