[patch, fortran] Fix PR 82567

[Thomas Koenig]

Hi Jerry and Steve,

>> Well I know 42 is the answer to the ultimate question of the universe so this
>> must be OK.  I just don't know what the question is.
>>
>> OK and thanks,
>>
>> Jerry
>>
>> +#define CONSTR_LEN_MAX 42
> Actually, I was wondering about the choice myself.  With
> most common hardware having fairly robust L1 and L2 cache
> sizes, a double precision array constructor with 42
> elements only occupies 336 bytes.  Seems small.

Well, the answer is that I didn't know how to chose a reasonable
constant.  I now actually ran some benchmarks using rdtsc, and
these seem to indicate that the optimum value for CONST_LEN_MAX
is actually quite short, 3 or 4, otherwise I just got a slowdown
or a break even.

So, I committed (r253872) with a length of 4 as a limit.  If anybody
comes up with a better number, we can always change this.

So, thanks for the review and the comments.

Regards

	Thomas

If somebody wants to check, here is the test case:

main.f90:

module tick
   interface
      function rdtsc()
        integer(kind=8) :: rdtsc
      end function rdtsc
   end interface
end module tick

program main
   use tick
   use tst
   implicit none
   integer(8) :: t1, t2
   t1 = rdtsc()
   call sub1(2.0)
   t2 = rdtsc()
   !  print *,"sub1 : ", t2-t1

   t1 = rdtsc()
   do i=1,10000
      call sub1(2.0)
   end do
   t2 = rdtsc()
   print *,"sub1 : ", t2-t1

   t1 = rdtsc()
   do i=1,10000
      call sub2(2.0)
   end do
   t2 = rdtsc()
   print *,"sub2 : ", t2-t1

end program main

tst.f90:
module tst
   integer, parameter :: n=4
   real, dimension(n) :: x
   real, dimension(n), parameter :: s = [(i,i=1,n)]
contains
   subroutine sub1(a)
     real, intent(in) :: a
     x(1) = a * 1.0
     x(2) = a * 2.0
     x(3) = a * 3.0
     x(4) = a * 3.0

   end subroutine sub1
   subroutine sub2(a)
     x(:) = a * s(:)
   end subroutine sub2
end module tst

rdtsc.s:
         .file   "rdtsc.s"
         .text
         .globl  rdtsc_
         .type   rdtsc_, @function
rdtsc_:
.LFB0:
         .cfi_startproc
         rdtsc
         shl     $32, %rdx
         or      %rdx, %rax
         ret
         .cfi_endproc
.LFE0:
         .size   rdtsc_, .-rdtsc_
         .section        .note.GNU-stack,"",@progbits