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Date: Wed, 25 Feb 2009 12:16:00 -0000
Subject: [Bug tree-optimization/39300]  New: vectorizer confused by predictive commoning
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From: "matz at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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The loop in this test is not vectorized if either PRE or predictive
commoning is active:
% cat vecttest2.f
      subroutine Bench_StaggeredLeapfrog2( cctk_dim,XADM_curv_stag0,
     &ADM_kzz_stag,lgxx,nx)
      implicit none
      INTEGER cctk_dim
      INTEGER XADM_curv_stag0
      REAL*8 ADM_kzz_stag(XADM_curv_stag0)

      integer :: i
      integer :: nx
      REAL*8,DIMENSION(cctk_dim):: lgxx
      do i=2,nx-1
        ADM_kzz_stag(i) = ADM_kzz_stag(i)+lgxx(i)+lgxx(i-1)+lgxx(i+1)
      end do
      end subroutine Bench_StaggeredLeapfrog2
% gfortran -c -O3 -g -ffast-math -ftree-vectorizer-verbose=2 vecttest2.f
vecttest2.f:11: note: not vectorized: unsupported use in stmt.
vecttest2.f:12: note: not vectorized: unsupported use in stmt.
% add -fno-tree-pre -fno-predictive-commoning to above command:
vecttest2.f:11: note: LOOP VECTORIZED.
% add only -fno-tree-pre (so predictive commoning is active):
vecttest2.f:11: note: LOOP VECTORIZED.
vecttest2.f:12: note: not vectorized: unsupported use in stmt.

The one vectorized loop in case it mentions two of them is the tail loop
for the one produced by predictive commoning.  That one doesn't contain
any loop carried values.  Somehow the vectorizer doesn't like the 
PHI nodes in the loop created by predictive commoning.

This testcase comes from 436.cactusADM, where it's very important to
vectorize a certain inner loop, and this (PRE and predcom) is one reason this
doesn't happen already.


-- 
           Summary: vectorizer confused by predictive commoning
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: matz at gcc dot gnu dot org
  GCC host triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39300