[PATCH] tree-optimization/104912 - ensure cost model is checked first

[Richard Biener <rguenther@suse.de>]

The following makes sure that when we build the versioning condition
for vectorization including the cost model check, we check for the
cost model and branch over other versioning checks.  That is what
the cost modeling assumes, since the cost model check is the only
one accounted for in the scalar outside cost.  Currently we emit
all checks as straight-line code combined with bitwise ops which
can result in surprising ordering of checks in the final assembly.

Since loop_version accepts only a single versioning condition
the splitting is done after the fact.

The result is a 1.5% speedup of 416.gamess on x86_64 when compiling
with -Ofast and tuning for generic or skylake.  That's not enough
to recover from the slowdown when vectorizing but it now cuts off
the expensive alias versioning test.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

OK for trunk?

For the rest of the regression my plan is to somehow factor in
the evolution of the number of iterations in the outer loop
(which is {1, +, 1}) to somehow bump the static profitability
estimate and together with the "cheap" cost model check never
execute the vectorized version (well, it is actually never executed,
but only because the alias check fails).

Thanks,
Richard.

2022-03-21  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104912
	* tree-vect-loop-manip.cc (vect_loop_versioning): Split
	the cost model check to a separate BB to make sure it is
	checked first and not combined with other version checks.
---
 gcc/tree-vect-loop-manip.cc | 53 ++++++++++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 3 deletions(-)

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index a7bbc916bbc..8ef333eb31b 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -3445,13 +3445,28 @@ vect_loop_versioning (loop_vec_info loop_vinfo,
 	cond_expr = expr;
     }
 
+  tree cost_name = NULL_TREE;
+  if (cond_expr
+      && !integer_truep (cond_expr)
+      && (version_niter
+	  || version_align
+	  || version_alias
+	  || version_simd_if_cond))
+    cost_name = cond_expr = force_gimple_operand_1 (unshare_expr (cond_expr),
+						    &cond_expr_stmt_list,
+						    is_gimple_val, NULL_TREE);
+
   if (version_niter)
     vect_create_cond_for_niters_checks (loop_vinfo, &cond_expr);
 
   if (cond_expr)
-    cond_expr = force_gimple_operand_1 (unshare_expr (cond_expr),
-					&cond_expr_stmt_list,
-					is_gimple_condexpr, NULL_TREE);
+    {
+      gimple_seq tem = NULL;
+      cond_expr = force_gimple_operand_1 (unshare_expr (cond_expr),
+					  &tem,
+					  is_gimple_condexpr, NULL_TREE);
+      gimple_seq_add_seq (&cond_expr_stmt_list, tem);
+    }
 
   if (version_align)
     vect_create_cond_for_align_checks (loop_vinfo, &cond_expr,
@@ -3654,6 +3669,38 @@ vect_loop_versioning (loop_vec_info loop_vinfo,
       update_ssa (TODO_update_ssa);
     }
 
+  /* Split the cost model check off to a separate BB.  Costing assumes
+     this is the only thing we perform when we enter the scalar loop.  */
+  if (cost_name)
+    {
+      gimple *def = SSA_NAME_DEF_STMT (cost_name);
+      /* All uses of the cost check are 'true' after the check we
+	 are going to insert.  */
+      replace_uses_by (cost_name, boolean_true_node);
+      /* And we're going to build the new single use of it.  */
+      gcond *cond = gimple_build_cond (NE_EXPR, cost_name, boolean_false_node,
+				       NULL_TREE, NULL_TREE);
+      edge e = split_block (gimple_bb (def), def);
+      gimple_stmt_iterator gsi = gsi_for_stmt (def);
+      gsi_insert_after (&gsi, cond, GSI_NEW_STMT);
+      edge true_e, false_e;
+      extract_true_false_edges_from_block (e->dest, &true_e, &false_e);
+      e->flags &= ~EDGE_FALLTHRU;
+      e->flags |= EDGE_TRUE_VALUE;
+      edge e2 = make_edge (e->src, false_e->dest, EDGE_FALSE_VALUE);
+      e->probability = prob;
+      e2->probability = prob.invert ();
+      set_immediate_dominator (CDI_DOMINATORS, false_e->dest, e->src);
+      auto_vec<basic_block, 3> adj;
+      for (basic_block son = first_dom_son (CDI_DOMINATORS, e->dest);
+	   son;
+	   son = next_dom_son (CDI_DOMINATORS, son))
+	if (EDGE_COUNT (son->preds) > 1)
+	  adj.safe_push (son);
+      for (auto son : adj)
+	set_immediate_dominator (CDI_DOMINATORS, son, e->src);
+    }
+
   if (version_niter)
     {
       /* The versioned loop could be infinite, we need to clear existing
-- 
2.34.1