[Bug middle-end/31738] New: Fortran dot product vectorization is restricted

[Bug middle-end/31738] New: Fortran dot product vectorization is restricted

[jb at gcc dot gnu dot org]

It seems that dot products are vectorized only for very restricted cases. For
the following example

subroutine testvectdp (a, b, c, n)
  integer, intent(in) :: n
  real, intent(in) :: a(n), b(n)
  real, intent(out) :: c
  c = dot_product (a, b)
end subroutine testvectdp

subroutine testvectdp2 (a, b, c, n)
  integer, intent(in) :: n
  real, intent(in) :: a(n), b(n)
  real, intent(out) :: c
  integer :: i
  c = 0.0
  do i = 1, n
     c = c + a(i) * b(i)
  end do
end subroutine testvectdp2

module testvec
contains
  subroutine testvecm (a, b, c)
    real, intent(in) :: a(:), b(:)
    real, intent(out) :: c
    c = dot_product (a, b)
  end subroutine testvecm

  subroutine testvecm2 (a, b, c)
    real, intent(in) :: a(:), b(:)
    real, intent(out) :: c
    integer :: i
    c = 0.0
    do i = 1, size (a)
       c = c + a(i) * b(i)
    end do
  end subroutine testvecm2
end module testvec

program testvec_p
  use testvec
  implicit none
  real :: a(9), b(9), c
  external testvectdp, testvectdp2

  call random_number(a)
  call random_number(b)

  call testvectdp(a,b,c,9)
  print *, c
  call testvectdp2(a,b,c,9)
  print *, c
  call testvecm(a,b,c)
  print *, c
  call testvecm2(a,b,c)
  print *, c
end program testvec_p

Only the first one, testvectdp vectorizes.


-- 
           Summary: Fortran dot product vectorization is restricted
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jb at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2007-04-28 22:04 -------
None is vectorized for me.  I guess this is why gas_dyn regressed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-04-28 22:04:58
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[dorit at il dot ibm dot com]

------- Comment #2 from dorit at il dot ibm dot com  2007-05-08 21:00 -------
Here is what happens in the three loops that don't get vectorized:

(1) the loop in testvectdp2: 
This is the loop we analyze:

  # prephitmp.192_37 = PHI <storetmp.191_30(3), D.1443_42(5)>
  # i_1 = PHI <1(3), i_44(5)>
<L15>:;
  D.1437_32 = prephitmp.192_37;
  D.1438_33 = (int8) i_1;
  D.1439_34 = D.1438_33 + -1;
  D.1440_36 = (*a_35(D))[D.1439_34];
  D.1441_40 = (*b_39(D))[D.1439_34];
  D.1442_41 = D.1441_40 * D.1440_36;
  D.1443_42 = prephitmp.192_37 + D.1442_41;
  storetmp.191_38 = D.1443_42;
  c__lsm.199_17 = D.1443_42;
  i_44 = i_1 + 1;
  if (i_1 == D.1429_5)
    goto <bb 6> (<L21>);
  else
    goto <bb 5> (<L20>);

We recognize the reduction, but we think that it is used in the loop:
  pr31738.f90:14: note: reduction used in loop.

and indeed, prephitmp.192_37 is used in:
  D.1443_42 = prephitmp.192_37 + D.1442_41;
which is ok, because this is the reduction stmt,
but also used here:
  D.1437_32 = prephitmp.192_37;
which is indeed something that we normally don't allow.
so the vectorizer is ok, except that in this case D.1437_32 doesn't seem to be
used anywhere in the function, so this stmt looks dead to me, but for some
reason it is not cleaned away before the vectorizer...  Still need to
investigate why. 


(2) the loop in testvecm:
This looks like the problem reported in PR31756:

failed to compute offset or step for (*a.0_11)[D.1559_52]
create_data_ref: failed to create a dr for (*a.0_11)[D.1559_52]
pr31738.f90:24: note: not vectorized: unhandled data-ref

(3) the loop in testvecm2
Same story (the PR31756 problem):

failed to compute offset or step for (*a.0_10)[D.1509_52]
create_data_ref: failed to create a dr for (*a.0_10)[D.1509_52]
pr31738.f90:32: note: not vectorized: unhandled data-ref


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[dorit at il dot ibm dot com]

------- Comment #3 from dorit at il dot ibm dot com  2007-05-16 20:45 -------
(In reply to comment #2)
> Here is what happens in the three loops that don't get vectorized:
> (1) the loop in testvectdp2: 
...
> so the vectorizer is ok, except that in this case D.1437_32 doesn't seem to > be used anywhere in the function, so this stmt looks dead to me, but for 
> some reason it is not cleaned away before the vectorizer...  Still need to
> investigate why. 

So looks like the stmt 
   D.1437_32 = prephitmp.192_37
became dead by pass pr31738a.f90.089t.copyprop3.

So the question is what's the most appropriate fix:
(1) fix copyprop3 to also clean away any dead code it creates?
(2) add a dce pass after copyprop3?
(3) work around it in the vectorizer. I think it should be easy - just move the
check of the uses of the reduction in the loop until after the vectorizer
analysis pass that marks relevant stmts.

If (3) sounds like the way to go - I can prepare a patch for that.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2007-05-29 04:43 -------
(In reply to comment #3)
> (2) add a dce pass after copyprop3?

This is already done now after:
2007-05-24  Zdenek Dvorak  <dvorakz@suse.cz>

        * passes.c (init_optimization_passes):  Add dceloop after
        copy propagation in loop optimizer.  Add predictive commoning
        to loop optimizer passes.
But still none of the functions are vectorized (at least for me).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[jb at gcc dot gnu dot org]

------- Comment #5 from jb at gcc dot gnu dot org  2008-02-16 22:40 -------
Still occurs with 

 4.3.0 20080216 (experimental) [trunk revision 132367]

i.e. only the first procedure (testvectdp) vectorizes, and that only with
-ffast-math. 


-- 

jb at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2007-04-28 22:04:58         |2008-02-16 22:40:16
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738

[Bug middle-end/31738] Fortran dot product vectorization is restricted

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2008-04-28 09:39 -------
For testvectdp2 we now miss to apply store-motion so the reduction is no longer
recognized.  This is the bad interaction between PRE and lim for which we have
PR36009.  If you add -fno-tree-pre vectorization fails because of

t.f90:7: note: reduction: not commutative/associative: D.1011_34

even with -ffast-math.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |36009


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31738