[PATCH, tree-ssa] Optimize loop invariant phi defs constants

[Christian Bruel]

Hello,

while checking why a loop snippet like

  for (i = 0; i <= 5000; i++)
    if (b)
      a = 2;
    else
      a = x;

was not optimized in -O2 (unless loop unrolling or loop switching), I
found out that the case was already  partially handled by Richard in
PR43934. So this patch just adds a cost to the phi defs constants to
allow the whole test to be hoisted out of the loop.

Richard, does this seem reasonable and OK for 4.10 ?

bootstrapped/regtested for x86

many thanks

Christian






2014-06-03  Christian Bruel  <christian.br...@st.com>

	PR tree-optimization/43934
	* tree-ssa-loop-im.c (determine_max_movement): Add PHI def constant cost.
2014-06-03  Christian Bruel  <christian.br...@st.com>

	PR tree-optimization/43934
	* gcc.dg/tree-ssa/ssa-lim-8.c: New testcase.

Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-8.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-8.c	(revision 0)
+++ gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-8.c	(working copy)
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-lim1-details" } */
+
+void bar (int);
+void foo (int n, int m)
+{
+  unsigned i;
+  for (i = 0; i < n; ++i)
+    {
+      int x;
+      if (m < 0)
+	x = 1;
+      else
+	x = m;
+      bar (x);
+    }
+}
+
+/* { dg-final { scan-tree-dump-times "Moving PHI node" 1 "lim1"  } } */
+/* { dg-final { cleanup-tree-dump "lim1" } } */
Index: gcc/tree-ssa-loop-im.c
===================================================================
--- gcc/tree-ssa-loop-im.c	(revision 211255)
+++ gcc/tree-ssa-loop-im.c	(working copy)
@@ -719,8 +719,21 @@ determine_max_movement (gimple stmt, bool must_pre
       FOR_EACH_PHI_ARG (use_p, stmt, iter, SSA_OP_USE)
 	{
 	  val = USE_FROM_PTR (use_p);
+
 	  if (TREE_CODE (val) != SSA_NAME)
-	    continue;
+	    {
+	      unsigned cst_cost = 1;
+
+	      gcc_assert (TREE_CODE (val) == INTEGER_CST
+			  || TREE_CODE (val) == REAL_CST
+			  || TREE_CODE (val) == VECTOR_CST
+			  || TREE_CODE (val) == COMPLEX_CST
+			  || TREE_CODE (val) == ADDR_EXPR);
+
+	      min_cost = MIN (min_cost, cst_cost);
+	      total_cost += cst_cost;
+	      continue;
+	    }
 	  if (!add_dependency (val, lim_data, loop, false))
 	    return false;
 	  def_data = get_lim_data (SSA_NAME_DEF_STMT (val));