Can I ask you to see whether -O3 -fno-tree-vectorize avoids the issue and whether -O3 -fno-vect-cost-model makes the issue appear in more versions?  It could be that this is a resolved issue that awaits backporting of a fix (or where backporting is too difficult).

I can confirm the -O3 -march=core-avx2 failure with GCC 10 and success with GCC 11.

Ok so we have: 

gfortran | [...] -fno-tree-vectorize | [...] -fno-vect-cost-model
    10.3 |                        OK |                     ERROR
     9.3 |                        OK |                     ERROR 
     8.3 |                        OK |                     ERROR

-fno-vect-cost model also does not change behavior on gcc 11/7/6/5 (all OK)

Fixed on master with r11-3718-g91ae6930ed4a87d7.

(In reply to Martin Liška from comment #4)
> Fixed on master with r11-3718-g91ae6930ed4a87d7.

If anything that just made the issue latent I suspect.

Would be interesting to find what patch broke this and what patch fixed the -mtune=generic case.

We don't know the failure mode and the identified fix isn't likely the fix.  Leaving open as non-regression.

(In reply to Andrew Pinski from comment #6)
> Would be interesting to find what patch broke this and what patch fixed the
> -mtune=generic case.

It is not easy bisecting with old compilers - compilation issues keep
coming up on more modern systems, and sometimes newer compilers do
not compile older compilers...

I can bisect it if someone gives me a case which aborts with the wrong value.

Hi Sam, 

Thanks for looking into it - here is a simplified version of the test program: you can also test it live at the Compiler Explorer, at this link: https://godbolt.org/z/r63G348hM

Thanks,
Federico

module subs
    use iso_fortran_env, only: ip => int8
    implicit none

    contains

    ! from two indices in [1:6] and [1:32], create a unique int8 code
    elemental integer(ip) function ENCODE(i,j)
        integer(ip), intent(in) :: i   ! loops in [1:6] range
        integer(ip), intent(in) :: j   ! loops in [1:32] range
        integer(ip) :: k
        ENCODE = -122_ip + i 
        do k=1_ip,j+1_ip
          ENCODE = ENCODE+6_ip
        end do  
    end function ENCODE

    elemental subroutine DECODE(code,i,j)
        integer(ip), intent(in)  :: code
        integer(ip), intent(out) :: i,j

        select case (code)
           case (-121_ip:82_ip)

              i = code
              j = -1_ip

              ! Child ID loop
              do while (i>-116_ip)
                 j = j+1_ip
                 i = i-6_ip
              end do

              i = i+122_ip

           case default
              i = -huge(i)
              j = -huge(j)
        end select
    end subroutine DECODE
end module subs

program test_int1
  use iso_fortran_env, only: ip => int8
  use subs
  implicit none

  integer(ip) :: i,j,code,id,jd
  logical :: success
  integer :: errors

  errors = 0
  do j=1_ip,32_ip
     do i=1_ip,6_ip
        code = ENCODE(i,j)
        call DECODE(code,id,jd)
        if (i/=id .or. j/=jd) then 
           errors = errors+1
           print *, 'i=',i,' j=',j,' code=',code, &
                    merge('error','     ',i/=id.or.j/=jd) 
        endif 
     end do
  end do

  success = errors==0

  print *, merge('SUCCESS!','ERROR   ',errors==0)
  if (errors>0) print "(*(i0,a))", errors,'/',6_ip*32_ip,' decoding errors '

end program test_int1