4 May 2020 2:40
#25327
When allocating a large array, if the array size is an integer parameter, I get an ICE, but an integer variable works ok.
PROGRAM monster_1
!
integer*8, parameter :: four = 12 ! 4
integer*8, parameter :: thousand = 1000
integer*8, parameter :: million = thousand*thousand
integer*8, parameter :: billion = thousand*million
INTEGER*8, PARAMETER :: fourgb = four*billion
!
INTEGER*1, allocatable :: i4gb(:)
INTEGER*8 :: i8, step, fgb
integer*4 :: astat, pass
INTEGER*1 :: b
!
call wait_for_response ( 'about to start - not allocated')
!
! fgb = fourgb
! allocate ( i4gb(fgb), stat=astat ) ! this works
allocate ( i4gb(fourgb), stat=astat ) ! this produces ICE with Ver 8.62.0
write (*,*) 'allocate ( i4gb(fourgb), stat=',astat
call wait_for_response ( 'array allocated')
!
do pass = 1,2
if ( pass == 1 ) step = 500*million+3
if ( pass == 2 ) step = 500*thousand+3
write (*,fmt='(a,i0,a,i0)') 'Pass =',pass,' : Step = ',step
!
b = 1
DO i8 = 1,fourgb,step
IF (b == 127) THEN
b = 0
ELSE
b = b+1
END IF
i4gb(i8) = b
END DO
call wait_for_response ( 'initialised loop')
!
DO i8 = 1,fourgb,step
WRITE(*,*)i8,i4gb(i8)
END DO
call wait_for_response ( 'finished check')
!
end do
!
END PROGRAM monster_1
subroutine wait_for_response (prompt)
character prompt*(*), a
write (*,*) prompt,' : Waiting ?'
read (*,fmt='(a)') a
end subroutine wait_for_response
Apart from the ICE, this example is interesting as it demonstrates that sparse use of a large array does not demand physical (possibly virtual) memory being allocated. The above array has a memory 'Comit' of 11.7 GB, but for pass=1 only uses 7Mb of working memory, for pass=2 only uses 126Mb of working memory and no working/physical memory is required at ALLOCATE.
This physical memory would be allocated as 4Kb pages so careful use of a large virtual array can work with much less physical memory. However, get it wrong or use array syntax, then everything will stop !