[RFC] non-unit stride loads for size power of 2.

["Kumar, Venkataramanan" <Venkataramanan dot Kumar at amd dot com>]

Hi 

The code below it looks like we always call  âvect_permute_load_chainâ to load non-unit strides of size powers of 2.

(---snip---)
/* If reassociation width for vector type is 2 or greater target machine can
     execute 2 or more vector instructions in parallel.  Otherwise try to
     get chain for loads group using vect_shift_permute_load_chain.  */
  mode = TYPE_MODE (STMT_VINFO_VECTYPE (vinfo_for_stmt (stmt)));
  
  if (targetm.sched.reassociation_width (VEC_PERM_EXPR, mode) > 1
      || exact_log2 (size) != -1
      || !vect_shift_permute_load_chain (dr_chain, size, stmt,
                                         gsi, &result_chain))
    vect_permute_load_chain (dr_chain, size, stmt, gsi, &result_chain);

static bool
vect_shift_permute_load_chain (vec<tree> dr_chain,
                               unsigned int length,
                               gimple *stmt,
                               gimple_stmt_iterator *gsi,
                               vec<tree> *result_chain)
{
â...
â...
  if (exact_log2 (length) != -1 && LOOP_VINFO_VECT_FACTOR (loop_vinfo) > 4) â This is not used.
    {
      unsigned int j, log_length = exact_log2 (length);
      for (i = 0; i < nelt / 2; ++i)
        sel[i] = i * 2;
      for (i = 0; i < nelt / 2; ++i)
        sel[nelt / 2 + i] = i * 2 + 1; 
(---snip------)


Is there any reason to do so? 

I have not done any benchmarking,  but tried simple test cases for -mavx targets with sizes 2, 4 and VF > 4 (short/char types).
Looks like using vect_shift_permute_load_chain seems better. 

Should we change it to something like this ?

diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
index d0e20da..b0f0a02 100644
--- a/gcc/tree-vect-data-refs.c
+++ b/gcc/tree-vect-data-refs.c
@@ -5733,9 +5733,9 @@ vect_transform_grouped_load (gimple *stmt, vec<tree> dr_chain, int size,
      get chain for loads group using vect_shift_permute_load_chain.  */
   mode = TYPE_MODE (STMT_VINFO_VECTYPE (vinfo_for_stmt (stmt)));
   if (targetm.sched.reassociation_width (VEC_PERM_EXPR, mode) > 1
-      || exact_log2 (size) != -1
-      || !vect_shift_permute_load_chain (dr_chain, size, stmt,
-                                        gsi, &result_chain))
+      || (!vect_shift_permute_load_chain (dr_chain, size, stmt,
+                                        gsi, &result_chain)
+         && exact_log2 (size) != -1))
     vect_permute_load_chain (dr_chain, size, stmt, gsi, &result_chain);
   vect_record_grouped_load_vectors (stmt, result_chain);
   result_chain.release ();
 
regards,
Venkat.