[Bug tree-optimization/56688] New: Fortran save statement prevents loop vectorization.

[Bug tree-optimization/56688] New: Fortran save statement prevents loop vectorization.

[ysrumyan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

             Bug #: 56688
           Summary: Fortran save statement prevents loop vectorization.
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ysrumyan@gmail.com


Analyzing gcc vectorization on 200.sixtrack from spec2000 suite we found out
that only 6 loops are vectorized in the hottest routine (97% run time). The
reason is that save statement is used. This issue can be illustrated by the
following simple example:

    subroutine bar
    implicit real*8 (a-h,o-z)
    parameter (n=700)
    common/my_data/ x1(n), y1(n), z1(n), t1(n)
        save
    do i=1,n
    x = x1(i) - y1(i)
    z1(i) = t1(i) * x
    enddo
    end

and vectorizer issues the following message:

t1.f:6: note: ==> examining statement: _6 = my_data.x1[_5];

t1.f:6: note: num. args = 4 (not unary/binary/ternary op).
t1.f:6: note: vect_is_simple_use: operand my_data.x1[_5]
t1.f:6: note: not ssa-name.
t1.f:6: note: use not simple.
t1.f:6: note: vect_model_load_cost: aligned.
t1.f:6: note: vect_model_load_cost: inside_cost = 1, prologue_cost = 0 .
t1.f:6: note: vect_is_simple_use: operand my_data.x1
t1.f:6: note: not ssa-name.
t1.f:6: note: use not simple.
t1.f:6: note: not vectorized: live stmt not supported: _6 = my_data.x1[_5];

Note also if we comment down svae stmt loop will be vectorized.

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-03-22
             Blocks|                            |53947
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-22 13:29:57 UTC ---
The issue is that x is kept live by applying store-motion:

  <bb 3>:
  # prephitmp_26 = PHI <1(2), i.3_14(4)>
  # ivtmp_30 = PHI <700(2), ivtmp_15(4)>
  _5 = (integer(kind=8)) prephitmp_26;
  _6 = _5 + -1;
  _7 = my_data.x1[_6];
  _8 = my_data.y1[_6];
  x.1_9 = _7 - _8;
  _11 = my_data.t1[_6];
  _12 = x.1_9 * _11;
  my_data.z1[_6] = _12;
  i.3_14 = prephitmp_26 + 1;
  ivtmp_15 = ivtmp_30 - 1;
  if (ivtmp_15 == 0)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  goto <bb 3>;

  <bb 5>:
  # x_lsm.7_25 = PHI <x.1_9(3)>
  x = x_lsm.7_25;
  i = 701;
  return;

because it appears that 'save' makes all variables global ones.  This kind
of "reduction" is not handled by the vectorizer.  If would be handled
by a pass that re-materializes x_lsm.7_25 from memory and operations
after the loop.  Or by handling the "final" value properly by means
of vector extraction or in the epilogue loop, simply using it, or
forcing at least one iteration of the epilogue loop by adjusting the
number of iterations of the vectorized loop.

I like the last option most ;)

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-22 13:31:23 UTC ---
C testcase:

int x[1024], y[1024];
int z;
void foo (void)
{
  unsigned i;
  for (i = 0; i < 1024; ++i)
    {
      z = x[i] - y[i];
      x[i] = z;
    }
}

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[Joost.VandeVondele at mat dot ethz.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Joost.VandeVondele at mat
                   |                            |dot ethz.ch

--- Comment #3 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> 2013-03-22 14:16:47 UTC ---
(In reply to comment #1)
> because it appears that 'save' makes all variables global ones.  

But this is maybe a frontend issue ? The visibility of x is local to the this
subroutine, but its lifetime extends over the entire run (so different to your
variable z in the C testcase).

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-22 14:24:14 UTC ---
(In reply to comment #3)
> (In reply to comment #1)
> > because it appears that 'save' makes all variables global ones.  
> 
> But this is maybe a frontend issue ? The visibility of x is local to the this
> subroutine, but its lifetime extends over the entire run (so different to your
> variable z in the C testcase).

No, same for

int x[1024], y[1024];
void foo (void)
{
  static int z;
  unsigned i;
  for (i = 0; i < 1024; ++i)
    {
      z = x[i] - y[i];
      x[i] = z;
    }
}

(missed optimization is that the variable and the store to it is not
removed completely).

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-22 14:25:15 UTC ---
Testcase for that:

void foo(int i)
{
  static int x;
  x = i;
}

[Bug tree-optimization/56688] Fortran save statement prevents loop vectorization.

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

alalaw01 at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alalaw01 at gcc dot gnu.org

--- Comment #6 from alalaw01 at gcc dot gnu.org ---
(In reply to Richard Biener from comment #4)

The C testcase vectorizes on gcc 6 development (at -O3 on aarch64 or
x86_64)....

[Bug tree-optimization/56688] static/saved variables prevent loop vectorization.

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56688

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|anlauf at gmx dot de               |anlauf at gcc dot gnu.org

--- Comment #9 from anlauf at gcc dot gnu.org ---
The loop can be vectorized if the final value of the 'saved' scalar can be
determined.

Some vectorizing compilers (Cray, NEC) offer directives (e.g. lstval/nolstval)
to control the evaluation of that scalar, and sometimes suggest to add that
directive in front of the loop.

I think the requirements for handling this are very similar to those for the
OpenMP lastprivate declaration.