[Bug tree-optimization/58508] New: Redundant vector load of "actual" loop invariant in loop body.

[congh at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58508

            Bug ID: 58508
           Summary: Redundant vector load of "actual" loop invariant in
                    loop body.
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: congh at google dot com

When GCC vectorizes the loop below, it will firstly do loop versioning with
aliasing check on a and b. Since a and b have different strides (1 and 0), the
check guarantees that there is no aliasing between a and b across all
iterations. Then with this precondition *b becomes a loop invariant so that it
can be loaded outside the loop during vectorization (Note that this
precondition always holds when the loop is being vectorized). This can save us
a load and a shuffle instruction in each iteration.


void foo (int* a, int* b, int n)
{
  for (int i = 0; i < n; ++i)
    a[i] += *b;
}


I have a patch handling this case as an optimization. After loop versioning, I
detect all zero-strided data references and hoist the loads of them to the loop
header. The patch is shown below.


thanks,
Cong



Index: gcc/tree-vect-loop-manip.c
===================================================================
--- gcc/tree-vect-loop-manip.c    (revision 202662)
+++ gcc/tree-vect-loop-manip.c    (working copy)
@@ -2477,6 +2477,37 @@ vect_loop_versioning (loop_vec_info loop
       adjust_phi_and_debug_stmts (orig_phi, e, PHI_RESULT (new_phi));
     }

+  /* Extract load and store statements on pointers with zero-stride 
+     accesses.  */
+  if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
+    {
+
+      /* In the loop body, we iterate each statement to check if it is a load 
+     or store. Then we check the DR_STEP of the data reference.  If 
+     DR_STEP is zero, then we will hoist the load statement to the loop 
+     preheader, and move the store statement to the loop exit.  */
+
+      for (gimple_stmt_iterator si = gsi_start_bb (loop->header); 
+        !gsi_end_p (si); )
+    {
+      gimple stmt = gsi_stmt (si);
+      stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
+      struct data_reference *dr = STMT_VINFO_DATA_REF (stmt_info);
+
+      if (dr && integer_zerop (DR_STEP (dr)))
+        {
+          if (DR_IS_READ (dr))
+        {
+          basic_block preheader = loop_preheader_edge (loop)->src;
+          gimple_stmt_iterator si_dst = gsi_last_bb (preheader);
+          gsi_move_after (&si, &si_dst);
+        }
+        }
+      else
+        gsi_next (&si);
+    }
+    } 
+
   /* End loop-exit-fixes after versioning.  */

   if (cond_expr_stmt_list)