[Bug tree-optimization/56595] New: Tree-ssa-pre can create loop carried dependencies which prevent loop vectorization.

[ysrumyan at gmail dot com]

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: unassig...@gcc.gnu.org

        ReportedBy: ysrum...@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