[Bug tree-optimization/33245] New: Missed opportunities for vectorization due to invariant condition

["spop at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The innermost loop in "j" cannot be vectorized because of the
irregular code in that loop, i.e. the condition "IF ( l.NE.k )".  But
the cond expression is invariant in that loop, so the whole condition
can be hoisted outside that loop, versioning the loop, and potentially
allowing the vectorization of the innermost loop.

      SUBROUTINE DGEFA(A,Lda,N,Ipvt,Info)
      INTEGER Lda , N , Ipvt(*) , Info
      DOUBLE PRECISION A(Lda,*)
      DOUBLE PRECISION t
      INTEGER IDAMAX , j , k , kp1 , l , nm1

      Info = 0
      nm1 = N - 1
      IF ( nm1.GE.1 ) THEN
         DO k = 1 , nm1
            kp1 = k + 1
            l = IDAMAX(N-k+1,A(k,k),1) + k - 1
            Ipvt(k) = l
            IF ( A(l,k).EQ.0.0D0 ) THEN
               Info = k
            ELSE
               IF ( l.NE.k ) THEN
                  t = A(l,k)
                  A(l,k) = A(k,k)
                  A(k,k) = t
               ENDIF
               t = -1.0D0/A(k,k)
               CALL DSCAL(N-k,t,A(k+1,k),1)
               DO j = kp1 , N
                  t = A(l,j)
                  IF ( l.NE.k ) THEN
                     A(l,j) = A(k,j)
                     A(k,j) = t
                  ENDIF
                  CALL DAXPY(N-k,t,A(k+1,k),1,A(k+1,j),1)
               ENDDO
            ENDIF
         ENDDO
      ENDIF
      Ipvt(N) = N
      IF ( A(N,N).EQ.0.0D0 ) Info = N
      CONTINUE
      END

The result of the vectorizer on this testcase is:

/home/seb/ex/linpk.f90:24: note: not vectorized: too many BBs in loop.
/home/seb/ex/linpk.f90:24: note: bad loop form.
/home/seb/ex/linpk.f90:1: note: vectorized 0 loops in function.

Okay, if I'm versioning that loop by hand, I get the same error due to
the PRE as for capacita.f90: the PRE inserts in the loop->latch block
some code: 

      <bb 11>:
        # VUSE <PARM_NOALIAS.16_252> { PARM_NOALIAS.16 }
        pretmp.47_297 = *n_13(D);
        goto <bb 10>;

And with PRE disabled, the fail occurs in the data ref analysis:

./linpk_corrected.f90:26: note: not vectorized: data ref analysis failed t.8_70
= (*a_25(D))[D.1406_69]
./linpk_corrected.f90:26: note: bad data references.


-- 
           Summary: Missed opportunities for vectorization due to invariant
                    condition
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: spop at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33245