[Bug fortran/82567] New: [6/7/8] gfortran takes a long time to compile a simple implied-do with -Optimization.

[chinoune.mehdi at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82567

            Bug ID: 82567
           Summary: [6/7/8] gfortran takes a long time to compile a simple
                    implied-do with -Optimization.
           Product: gcc
           Version: 6.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chinoune.mehdi at hotmail dot com
  Target Milestone: ---

gfortran takes about 11s to compile this simple subroutine with -O1 :

  SUBROUTINE sub()
  IMPLICIT NONE

  INTEGER, PARAMETER :: n = 10000
  REAL, ALLOCATABLE :: x(:)
  REAL :: xc, h
  INTEGER :: i

  ALLOCATE( x(n) )
  xc = 100.
  h = xc/n
  x = h*[(i,i=1,n)]
  ! Replacing the previous line with one of these lines gives faster
compilation
  ! x = [(i*h,i=1,n)]
  ! x = (100./n) *[(i,i=1,n)] 

END SUBROUTINE sub

time gfortran -O1 -c test.f90:
real    0m11.167s

the time increases with n (-O1) :
n    | 10000 | 20000 | 30000 | 40000
-------------------------------------
time | 11s   | 25s   | 43s   | 1m7s

and decrease with the level of optimization (n=20000) :
-Ox  | -O1 | -O2 | -O3 | -Ofast
-------------------------------
time | 20s | 15s | 15s | 8s

tested with gfortran 6/7/8 Linux-64 and gfortran-7 MinGW-64