[flang] DTIO: Illegal instruction when reading stream access file from a specified position

[DanielCChen]

Consider the following code:

module m1
   type, abstract :: base
      character(3) :: c = ''
      contains
         procedure, pass :: setC
   end type

   type, extends(base) :: child
      character(3) :: cc = ''
   end type

contains
   subroutine setC (a, char)
      class(base), intent(inout) :: a
      character(3), intent(in) :: char
      a%c = char
   end subroutine
end module


program array001a
   use m1
   interface read(unformatted)
      subroutine readUnformatted (dtv, unit, iostat, iomsg)
         import base
         class(base), intent(inout) :: dtv
         integer,  intent(in) :: unit
         integer,  intent(out) :: iostat
         character(*),  intent(inout) :: iomsg
      end subroutine
   end interface

   class(base) , allocatable :: b1(:)
   integer :: stat
   character(200) :: msg

   allocate ( b1(4), source = (/ child('xxx','XXX'), child('xxx','XXX'), child('xxx','XXX'), child('xxx','XXX') /) )

   open (unit = 1, file ='array001a.data', form='unformatted', access='stream')

   write (1, iostat=stat, iomsg=msg, pos=31 )             'ghiGHIdefDEFjklJKLabcABC'
   write (1, iostat=stat, iomsg=msg, pos=19 )             'ABCabcDEFdef'
   write (1, iostat=stat, iomsg=msg, pos=13 )             'stuSTU'
   write (1, iostat=stat, iomsg=msg, pos=1  )             'MNOmnoSTUstu'

   read (1, iostat=stat, iomsg=msg, pos=31 )              b1((/3,2,4,1/))
   close ( 1, status ='delete' )

end program

subroutine readUnformatted (dtv, unit, iostat, iomsg)
use m1
   class(base), intent(inout) :: dtv
   integer, intent(in) :: unit
   integer, intent(out) :: iostat
   character(*), intent(inout) :: iomsg

   character(3) :: temp

   read (unit, iostat=iostat ) temp

   call dtv%setC(temp)

   select type (dtv)
      type is (child)
         read (unit, iostat=iostat ) dtv%cc
   end select
end subroutine

Flang failed at the read statement with

> a.out
Illegal instruction(coredump)

[llvmbot]

@llvm/issue-subscribers-flang-runtime

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` module m1 type, abstract :: base character(3) :: c = '' contains procedure, pass :: setC end type

type, extends(base) :: child
character(3) :: cc = ''
end type

contains
subroutine setC (a, char)
class(base), intent(inout) :: a
character(3), intent(in) :: char
a%c = char
end subroutine
end module

program array001a
use m1
interface read(unformatted)
subroutine readUnformatted (dtv, unit, iostat, iomsg)
import base
class(base), intent(inout) :: dtv
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg
end subroutine
end interface

class(base) , allocatable :: b1(:)
integer :: stat
character(200) :: msg

allocate ( b1(4), source = (/ child('xxx','XXX'), child('xxx','XXX'), child('xxx','XXX'), child('xxx','XXX') /) )

open (unit = 1, file ='array001a.data', form='unformatted', access='stream')

write (1, iostat=stat, iomsg=msg, pos=31 ) 'ghiGHIdefDEFjklJKLabcABC'
write (1, iostat=stat, iomsg=msg, pos=19 ) 'ABCabcDEFdef'
write (1, iostat=stat, iomsg=msg, pos=13 ) 'stuSTU'
write (1, iostat=stat, iomsg=msg, pos=1 ) 'MNOmnoSTUstu'

read (1, iostat=stat, iomsg=msg, pos=31 ) b1((/3,2,4,1/))
close ( 1, status ='delete' )

end program

subroutine readUnformatted (dtv, unit, iostat, iomsg)
use m1
class(base), intent(inout) :: dtv
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg

character(3) :: temp

read (unit, iostat=iostat ) temp

call dtv%setC(temp)

select type (dtv)
type is (child)
read (unit, iostat=iostat ) dtv%cc
end select
end subroutine

Flang failed at the `read` statement with

> a.out
Illegal instruction(coredump)

</details>

[eugeneepshteyn]

The crash could be due to vector subscripts. If I replace b1((/3,2,4,1/)) with b(2:3), no crash.

[klausler]

It's a combination of vector subscripts and the ABSTRACT derived type; both have to be present. I suspect that the derived type information in the descriptors passed to the runtime for the READ statement doesn't have a valid code address in its vtable; confirming now.

EDIT: Confirmed, the descriptors passed to the READ statement have the declared type base, not the dynamic type 'child'. Further, the bug is peculiar to defined READs; I can't make it fail in other contexts with type-bound procedures involving vector-valued subscripts into a polymorphic array.

[klausler]

Example that works, unlike the READ example above:

module m
   type, abstract :: base
     integer n
    contains
     procedure, pass :: put
   end type
   type, extends(base) :: child
   end type
 contains
  elemental subroutine put(x, n)
    class(base), intent(in out) :: x
    integer, intent(in) :: n
    x%n = n
  end
end
program test
  use m
  class(base), allocatable :: arr(:)
  arr = [child(1),child(2)]
  print *, arr%n
  call arr(:)%put([3,4])
  print *, arr%n
  call arr([1,2])%put([5,6])
  print *, arr%n
end