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From: "ysrumyan at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/56595] New: Tree-ssa-pre can create loop carried dependencies which prevent loop vectorization.
Date: Mon, 11 Mar 2013 13:35:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56595

             Bug #: 56595
           Summary: Tree-ssa-pre can create loop carried dependencies
                    which prevent loop vectorization.
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ysrumyan@gmail.com


In some cases pre can create loop carried dependencies spanning multiple
iterations aka scalar replacement. This deficiency can be illustrated with
attached test-case. After pre for stmt

            DO I = 0,I2
               T1 = 0.5D0 * (U1(I,J,K)    + U1(I+1,J,K))

pre creates loop carried dependence:

  <bb 172>:
...
  pretmp_690 = MEM[(real(kind=8)[0:] *)pretmp_675][pretmp_689];
...
  <bb 107>:
  # i_1 = PHI <0(172), i_437(175)>
  # prephitmp_691 = PHI <pretmp_690(172), _440(175)>


Note that in this particular test-case we have arrays with unknown stride1. If
we have arrays with stride1 == 1 such transformation does not happen as for the
following simple test-case which is successfully vectorized:

    subroutine bar(a,b,c,d,n, m)
    integer n, m
    real*8 a(n,*), b(n,*), c(n,*), d(n,*)
    do j=1,m
    do i=1,m
    x1 = 0.5 * (a(i,j) + a(i+1,j))
    x2 = 0.5 * (b(i,j) + b(i+1,j))
    x3 = 0.5 * (c(i,j) + c(i+1,j))
    d(i,j) = (x1 + x2 + x3) / 3.0
    enddo
    enddo
    end