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
