Avoid accounting for non-existent vector loop versioning

vect_analyze_loop_costing uses two profitability thresholds: a runtime
one and a static compile-time one.  The runtime one is simply the point
at which the vector loop is cheaper than the scalar loop, while the
static one also takes into account the cost of choosing between the
scalar and vector loops at runtime.  We compare this static cost against
the expected execution frequency to decide whether it's worth generating
any vector code at all.

However, we never reclaimed the cost of applying the runtime threshold
if it turned out that the vector code can always be used.  And we only
know whether that's true once we've calculated what the runtime
threshold would be.

2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vect_apply_runtime_profitability_check_p):
	New function.
	* tree-vect-loop-manip.c (vect_loop_versioning): Use it.
	* tree-vect-loop.c (vect_analyze_loop_2): Likewise.
	(vect_transform_loop): Likewise.
	(vect_analyze_loop_costing): Don't take the cost of versioning
	into account for the static profitability threshold if it turns
	out that no versioning is needed.

From-SVN: r278124

1 files changed, 25 insertions, 9 deletions

diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index 75ec9e6..83fb848 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -1690,6 +1690,24 @@ vect_analyze_loop_costing (loop_vec_info loop_vinfo)
       return 0;
     }
 
+  /* The static profitablity threshold min_profitable_estimate includes
+     the cost of having to check at runtime whether the scalar loop
+     should be used instead.  If it turns out that we don't need or want
+     such a check, the threshold we should use for the static estimate
+     is simply the point at which the vector loop becomes more profitable
+     than the scalar loop.  */
+  if (min_profitable_estimate > min_profitable_iters
+      && !LOOP_REQUIRES_VERSIONING (loop_vinfo)
+      && !LOOP_VINFO_PEELING_FOR_NITER (loop_vinfo)
+      && !LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo)
+      && !vect_apply_runtime_profitability_check_p (loop_vinfo))
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location, "no need for a runtime"
+			 " choice between the scalar and vector loops\n");
+      min_profitable_estimate = min_profitable_iters;
+    }
+
   HOST_WIDE_INT estimated_niter;
 
   /* If we are vectorizing an epilogue then we know the maximum number of
@@ -2226,8 +2244,7 @@ start_over:
 
       /*  Use the same condition as vect_transform_loop to decide when to use
 	  the cost to determine a versioning threshold.  */
-      if (th >= vect_vf_for_cost (loop_vinfo)
-	  && !LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
+      if (vect_apply_runtime_profitability_check_p (loop_vinfo)
 	  && ordered_p (th, niters_th))
 	niters_th = ordered_max (poly_uint64 (th), niters_th);
 
@@ -8250,14 +8267,13 @@ vect_transform_loop (loop_vec_info loop_vinfo)
      run at least the (estimated) vectorization factor number of times
      checking is pointless, too.  */
   th = LOOP_VINFO_COST_MODEL_THRESHOLD (loop_vinfo);
-  if (th >= vect_vf_for_cost (loop_vinfo)
-      && !LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
+  if (vect_apply_runtime_profitability_check_p (loop_vinfo))
     {
-	if (dump_enabled_p ())
-	  dump_printf_loc (MSG_NOTE, vect_location,
-			   "Profitability threshold is %d loop iterations.\n",
-			   th);
-	check_profitability = true;
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location,
+			 "Profitability threshold is %d loop iterations.\n",
+			 th);
+      check_profitability = true;
     }
 
   /* Make sure there exists a single-predecessor exit bb.  Do this before