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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/106790] New: Weird SIGSEV using polymorphic routine
 with "select type" and optimization (-O3)
Date: Wed, 31 Aug 2022 12:41:58 +0000
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X-Bugzilla-Component: fortran
X-Bugzilla-Version: 12.1.0
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X-Bugzilla-Who: federico.perini at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106790

            Bug ID: 106790
           Summary: Weird SIGSEV using polymorphic routine with "select
                    type" and optimization (-O3)
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: federico.perini at gmail dot com
  Target Milestone: ---

I'm getting a weird SIGSEV error when running the following code with full
optimization (-O3). Error happens if:=20

- type guard ("select type") is is "type is", no segfault if "class is"
- the non-abstract derived type contains more than 1 scalar

I suspect something is wrong with the compiler trying to optimize out the c=
ode
in the main program (it's all constants).=20

- Fails only on gfortran 11, 12.=20
- Works on gfortran 7,8,9,10

Test it here: https://godbolt.org/z/qKPMvKnzf

Here is the minimum working example:=20

module ttt
implicit none
    type, abstract :: t0
        contains
            procedure(merge), deferred :: mergeWith
            procedure(pprint), deferred :: print
    end type
    type,extends(t0) :: t
        integer :: from1,to1,face
        contains=20
            procedure :: mergeWith =3D> t_merge
            procedure :: print =3D> link_print
    end type t

abstract interface
    pure subroutine merge(this,that)
        import t0
        class(t0), intent(inout) :: this
        class(t0), intent(in) :: that
    end subroutine merge
    function pprint(this) result(msg)
        import t0
        class(t0), intent(in) :: this
        character(len=3D:), allocatable :: msg
    end function pprint
end interface

contains

    function link_print(this) result(msg)
       class(t), intent(in) :: this
       character(len=3D:), allocatable :: msg
       character(len=3D1024) :: buffer
       integer :: lt

       write(buffer,1) this%to1,this%from1,this%face
       lt =3D len_trim(buffer)
       allocate(character(len=3Dlt) :: msg)
       if (lt>0) msg(1:lt) =3D buffer(1:lt)

       1 format('to1=3D',i0,' from1=3D',i0,' face=3D',i0)
    end function link_print

pure subroutine t_merge(this,that)
class(t), intent(inout) :: this
class(t0), intent(in) :: that

       select type (ttype =3D> that)

       type is (t) ! Does not SIGSEV if using "class is (t)"

           ! SIGSEV at any of the following lines
           ! Does not crash anymore if commenting any of them
           if (this%to1<0 .and. this%from1<0) then
              this%to1   =3D ttype%to1
              this%face =3D ttype%face
           end if

       end select

end subroutine t_merge
end module

program test_t
use ttt
implicit none

type(t) :: t1,t2


t1 =3D t(from1=3D123,to1=3D435,face=3D789)
t2 =3D t(from1=3D-1,to1=3D-1,face=3D-1)
call t1%mergeWith(t2)
print *, 't1=3D',t1%print(),' t2=3D',t2%print()

t1 =3D t(from1=3D123,to1=3D435,face=3D789)
t2 =3D t(from1=3D-1,to1=3D-1,face=3D-1)

! Crash here on -O3, apparently during the unrolling of constants
call t2%mergeWith(t1)
print *, 't1=3D',t1%print(),' t2=3D',t2%print()

end program


Thanks,
Federico=