How to express the fact that arguments don't escape?

[Richard Guenther]

2009/8/15 Thomas Koenig <tkoenig@gcc.gnu.org>:
> On Sat, 2009-08-15 at 01:59 +0200, Tobias Schlüter wrote:
>> Hi,
>>
>> I reviewed some old discussions on argument passing, but I found nothing
>> which addressed a seemingly simple optimization for Fortran: what I
>> would like to see is a way to tell the compiler that a function argument
>> (which will be a pointer) will not have escaped after the function has
>> returned.  I.e.
>>    call f(i)
>>    i = 1
>>    call g(j)  ! won't know about i
>>    if (i != 1) then
>>       can't happen
>>    end if
>>
>> Is there a way to do this?  Unless I'm mistaken, this assumption holds
>> for all non-POINTER arguments.

non-POINTER and non-TARGET arguments.  TARGET arguments can
escape to global pointers or pointer return values.

> To stay within the current, C-centric framework of gcc, we'll need to do
> even more copying.

Yes, of course.  With LTO IPA-SRA might do something about it.  And yes,
being able to specify whether an argument escapes, whether it is read from
or written to would be useful - but tracking the information properly will cause
an overhead and thus it has to be carefully designed.

Richard.

> Compare
>
> $ cat g1.f90
> subroutine foo
>  call f(i)
>  i = 1
>  call g(j)
>  if (i /= 1) then
>    call abort
>  end if
> end subroutine foo
> $ gfortran -O2 -S -fdump-tree-optimized g1.f90
> $ cat g1.f90.133t.optimized
>
> ;; Function foo (foo_)
>
> foo ()
> {
>  integer(kind=4) j;
>  integer(kind=4) i;
>  integer(kind=4) i.0;
>
> <bb 2>:
>  f (&i);
>  i = 1;
>  g (&j);
>  i.0_1 = i;
>  if (i.0_1 != 1)
>    goto <bb 3>;
>  else
>    goto <bb 4>;
>
> <bb 3>:
>  _gfortran_abort ();
>
> <bb 4>:
>  return;
>
> }
>
> with
>
> $ cat g2.f90
> subroutine foo
>  call f(i_aux)
>  i = i_aux
>  i = 1
>  call g(j)
>  if (i /= 1) then
>    call abort
>  end if
> end subroutine foo
> $ gfortran -O2 -S -fdump-tree-optimized g2.f90
> $ cat g2.f90.133t.optimized
>
> ;; Function foo (foo_)
>
> foo ()
> {
>  integer(kind=4) j;
>  integer(kind=4) i_aux;
>
> <bb 2>:
>  f (&i_aux);
>  g (&j);
>  return;
>
> }
>
>
>