[Bug tree-optimization/49955] New: Fails to do partial basic-block SLP

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49955

           Summary: Fails to do partial basic-block SLP
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: rgue...@gcc.gnu.org
                CC: i...@gcc.gnu.org


410.bwaves in shell_lam.f has a lot of arrays with inner dimension 5 operated
on in loops that are either unrolled by early unrolling or manually unrolled
in source.  All but one loop in shell_lam.f are not vectorized.

One reason is that basic-block vectorization gives up if it sees
interleaving size that is not a multiple of a supported vectorization
factor.  Testcase:

double a[1024], b[1024];

void foo (int k)
{
  int j;
  a[k*5 + 0] = a[k*5 + 0] + b[k*5 + 0];
  a[k*5 + 1] = a[k*5 + 1] + b[k*5 + 1];
  a[k*5 + 2] = a[k*5 + 2] + b[k*5 + 2];
  a[k*5 + 3] = a[k*5 + 3] + b[k*5 + 3];
  a[k*5 + 4] = a[k*5 + 4] + b[k*5 + 4];
}

taken from the last loop in shell_lam.f which has its innermost loop unrolled
(and loop SLP refuses to vectorize as well, see separate bug).

For the above we get:

t.c:6: note: === vect_analyze_data_ref_accesses ===
t.c:6: note: Detected interleaving of size 5
t.c:6: note: Detected interleaving of size 5
t.c:6: note: Detected interleaving of size 5
t.c:6: note: Vectorizing an unaligned access.
t.c:6: note: Vectorizing an unaligned access.
t.c:6: note: Vectorizing an unaligned access.
t.c:6: note: === vect_analyze_slp ===
t.c:6: note: get vectype with 2 units of type double
t.c:6: note: vectype: vector(2) double
t.c:6: note: Build SLP failed: unrolling required in basic block SLP
t.c:6: note: Failed to SLP the basic block.
t.c:6: note: not vectorized: failed to find SLP opportunities in basic block.

but of course we could simply vectorize with an interleaving size of 4
leaving the excess operations unvectorized (with optimization opportunity
if we can pick a properly sized and aligned set of accesses).