[Bug fortran/53309] New: Unnecessary temporary array creation in subroutine call

[Bug fortran/53309] New: Unnecessary temporary array creation in subroutine call

[townsend at astro dot wisc.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53309

             Bug #: 53309
           Summary: Unnecessary temporary array creation in subroutine
                    call
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: townsend@astro.wisc.edu


Created attachment 27367
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27367
Example program

I'm encountering a situation where an unnecessary array is being created when
passing an assumed-shape dummy argument as the argument to a routine without an
explicit interface.

The attached example program illustrates the problem; when compiled with
-Warray-temporaries, I get the warning:

test_array_temp.f90:19.25:

    call sub_set_a(3, 3, a)
                         1
Warning: Creating array temporary at (1)

At compile time, there is no way of determining whether a is contiguous, and so
it would seem an array temporary is necessary. However, is it possible to do a
*runtime* check on whether a is contiguous, and if so avoid the copy? Intel
Fortran seems to manage this.

BTW, this issue arises when calling many of the LAPACK and BLAS routines from
Fortran 9x; it seems array temporaries are being created all over the place,
doubtless with a performance impact.

[Bug fortran/53309] Unnecessary temporary array creation in subroutine call

[tkoenig at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53309

--- Comment #1 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
The runtime test is done by _gfortran_internal_pack.  -Warray-temporaries
creates the warning if this is called.

If you want to know if a temporary has actually been created,
use -fcheck-array-temporaries.

Does this answer your concern?

[Bug fortran/53309] Unnecessary temporary array creation in subroutine call

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53309

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |burnus at gcc dot gnu.org
         Depends on|                            |57959
         Resolution|---                         |WORKSFORME

--- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Let's close this as WONTFIX.

-Warray-temporaries warns if the compiler adds code for temporaries. In case of
actual arguments, there is a run-time check, which only does the
copy-in/copy-out if the variable is noncontiguous.

As Thomas wrote, you can use -fcheck-array-temporaries which prints at runtime
a warning if the copy-in/copy-out actually happened.

If the compiler knows that the actual argument is contiguous, it can avoid the
copy-in altogether. For your example, simply mark the dummy argument "a" of
sub_wrap as CONTIGUOUS (or use an explicit/assumed-size array instead).

[Bug fortran/53309] Unnecessary temporary array creation in subroutine call

[townsend at astro dot wisc.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53309

--- Comment #3 from Rich Townsend <townsend at astro dot wisc.edu> ---
Thanks for the explanation about -Warray-temporaries vs.
-fcheck-array-temporaries -- got it!

Might be worth changing the output text from the former to something like
"Warning: Array temporary might be created at (1)"