[PATCH] Avoid useless work in loop vectorization

[Richard Biener]

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk.

Richard.

2015-11-13  Richard Biener  <rguent...@suse.de>

        * tree-vect-loop.c (vect_analyze_loop_2): Add fatal parameter.
        Signal fatal failure if early checks fail.
        (vect_analyze_loop): If vect_analyze_loop_2 fails fatally
        do not bother testing further vector sizes.

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c        (revision 230260)
+++ gcc/tree-vect-loop.c        (working copy)
@@ -1709,13 +1709,16 @@ vect_analyze_loop_operations (loop_vec_i
    for it.  The different analyses will record information in the
    loop_vec_info struct.  */
 static bool
-vect_analyze_loop_2 (loop_vec_info loop_vinfo)
+vect_analyze_loop_2 (loop_vec_info loop_vinfo, bool &fatal)
 {
   bool ok;
   int max_vf = MAX_VECTORIZATION_FACTOR;
   int min_vf = 2;
   unsigned int n_stmts = 0;
 
+  /* The first group of checks is independent of the vector size.  */
+  fatal = true;
+
   /* Find all data references in the loop (which correspond to vdefs/vuses)
      and analyze their evolution in the loop.  */
 
@@ -1795,7 +1798,6 @@ vect_analyze_loop_2 (loop_vec_info loop_
 
   /* Classify all cross-iteration scalar data-flow cycles.
      Cross-iteration cycles caused by virtual phis are analyzed separately.  */
-
   vect_analyze_scalar_cycles (loop_vinfo);
 
   vect_pattern_recog (loop_vinfo);
@@ -1825,6 +1827,9 @@ vect_analyze_loop_2 (loop_vec_info loop_
       return false;
     }
 
+  /* While the rest of the analysis below depends on it in some way.  */
+  fatal = false;
+
   /* Analyze data dependences between the data-refs in the loop
      and adjust the maximum vectorization factor according to
      the dependences.
@@ -2118,7 +2169,8 @@ vect_analyze_loop (struct loop *loop)
          return NULL;
        }
 
-      if (vect_analyze_loop_2 (loop_vinfo))
+      bool fatal = false;
+      if (vect_analyze_loop_2 (loop_vinfo, fatal))
        {
          LOOP_VINFO_VECTORIZABLE_P (loop_vinfo) = 1;
 
@@ -2128,7 +2180,8 @@ vect_analyze_loop (struct loop *loop)
       destroy_loop_vec_info (loop_vinfo, true);
 
       vector_sizes &= ~current_vector_size;
-      if (vector_sizes == 0
+      if (fatal
+         || vector_sizes == 0
          || current_vector_size == 0)
        return NULL;