for (i=0; i<N; i++)
{
D[i] = A[i] + Y;
A[i+1] = B[i] + X;
}
Even though, this loop contains a backward-carried dependence between A[i+1]
and A[i], it is vectorizable - the stmts of the loop should be interchanged to
get:
for (i=0; i<N; i++)
{
A[i+1] = B[i] + X;
D[i] = A[i] + Y;
}
which will be vectorizable once the pacth in PR 32377 (comment #14) is
committed.
The interchange is possible since there is no loop-independent dependence
between the stmts. This requires working with a dependence graph when
vectorizing, or have a separate optimization before the vectorizer to take care
of this.
Ira
--
Summary: Missing optimization to remove backword dependencies
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: irar at il dot ibm dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32806
