[PATCH] Do not vectorize loops that do not roll

[Richard Guenther]

I noticed we vectorize

int objects[2];

void foo (int n)
{
  int i;
  for (i = 0; i < n; ++i)
    objects[i] = i;
}

which is of course pointless (with a vectorization factor > 2).

The following patch avoids doing this.

Bootstrap & regtest pending on x86_64-unknown-linux-gnu.

Richard.

2012-04-19  Richard Guenther  <rguent...@suse.de>

        * tree-vect-loop.c (vect_analyze_loop_operations): Do not
        vectorize loops that can never run more often than the
        vectorization factor.

Index: gcc/tree-vect-loop.c
===================================================================
*** gcc/tree-vect-loop.c        (revision 186589)
--- gcc/tree-vect-loop.c        (working copy)
*************** vect_analyze_loop_operations (loop_vec_i
*** 1235,1240 ****
--- 1235,1241 ----
    int min_scalar_loop_bound;
    unsigned int th;
    bool only_slp_in_loop = true, ok;
+   HOST_WIDE_INT max_niter;
  
    if (vect_print_dump_info (REPORT_DETAILS))
      fprintf (vect_dump, "=== vect_analyze_loop_operations ===");
*************** vect_analyze_loop_operations (loop_vec_i
*** 1407,1414 ****
          "vectorization_factor = %d, niters = " HOST_WIDE_INT_PRINT_DEC,
          vectorization_factor, LOOP_VINFO_INT_NITERS (loop_vinfo));
  
!   if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
!       && (LOOP_VINFO_INT_NITERS (loop_vinfo) < vectorization_factor))
      {
        if (vect_print_dump_info (REPORT_UNVECTORIZED_LOCATIONS))
          fprintf (vect_dump, "not vectorized: iteration count too small.");
--- 1408,1417 ----
          "vectorization_factor = %d, niters = " HOST_WIDE_INT_PRINT_DEC,
          vectorization_factor, LOOP_VINFO_INT_NITERS (loop_vinfo));
  
!   if ((LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
!        && (LOOP_VINFO_INT_NITERS (loop_vinfo) < vectorization_factor))
!       || ((max_niter = max_loop_iterations_int (loop)) != -1
!         && max_niter < vectorization_factor))
      {
        if (vect_print_dump_info (REPORT_UNVECTORIZED_LOCATIONS))
          fprintf (vect_dump, "not vectorized: iteration count too small.");