re PR tree-optimization/32044 (final value replacement too aggressive for e.g. targets with no native div/mod insns)

PR tree-optimization/32044
	* tree-scalar-evolution.h (expression_expensive_p): Declare.
	* tree-scalar-evolution.c (expression_expensive_p): New function.
	(scev_const_prop): Avoid introducing expensive expressions.
	* tree-ssa-loop-ivopts.c (may_eliminate_iv): Ditto.

	* gcc.dg/pr34027-1.c: Change outcome.
	* gcc.dg/tree-ssa/pr32044.c: New test.

From-SVN: r142719

gcc/ChangeLog

@@ -1,3 +1,11 @@
+2008-12-12  Zdenek Dvorak  <ook@ucw.cz>
+
+	PR tree-optimization/32044
+	* tree-scalar-evolution.h (expression_expensive_p): Declare.
+	* tree-scalar-evolution.c (expression_expensive_p): New function.
+	(scev_const_prop): Avoid introducing expensive expressions.
+	* tree-ssa-loop-ivopts.c (may_eliminate_iv): Ditto.
+
 2008-12-12  Sebastian Pop  <sebastian.pop@amd.com>
 
 	PR middle-end/38409

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2008-12-12  Zdenek Dvorak  <ook@ucw.cz>
+
+	PR tree-optimization/32044
+	* gcc.dg/pr34027-1.c: Change outcome.
+	* gcc.dg/tree-ssa/pr32044.c: New test.
+
 2008-12-12  Janis Johnson  <janis187@us.ibm.com>
 
 	PR target/11594

gcc/testsuite/gcc.dg/pr34027-1.c

@@ -8,5 +8,9 @@ unsigned long foobar(unsigned long ns)
   return ns;
 }
 
-/* { dg-final { scan-tree-dump "ns % 10000" "optimized" } } */
+/* This test was originally introduced to test that we transform
+   to ns % 10000.  See the discussion of PR 32044 why we do not do
+   that anymore.  */
+/* { dg-final { scan-tree-dump-times "%" 0 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "/" 0 "optimized" } } */
 /* { dg-final { cleanup-tree-dump "optimized" } } */

gcc/testsuite/gcc.dg/tree-ssa/pr32044.c

@@ -0,0 +1,55 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-empty -fdump-tree-final_cleanup" } */
+
+int foo (int n)
+{
+  while (n >= 45)
+    n -= 45;
+
+  return n;
+}
+
+int bar (int n)
+{
+  while (n >= 64)
+    n -= 64;
+
+  return n;
+}
+
+int bla (int n)
+{
+  int i = 0;
+
+  while (n >= 45)
+    {
+      i++;
+      n -= 45;
+    }
+
+  return i;
+}
+
+int baz (int n)
+{
+  int i = 0;
+
+  while (n >= 64)
+    {
+      i++;
+      n -= 64;
+    }
+
+  return i;
+}
+
+/* The loops computing division/modulo by 64 should be eliminated.  */
+/* { dg-final { scan-tree-dump-times "Removing empty loop" 2 "empty" } } */
+
+/* There should be no division/modulo in the final dump (division and modulo
+   by 64 are done using bit operations).  */
+/* { dg-final { scan-tree-dump-times "/" 0 "final_cleanup" } } */
+/* { dg-final { scan-tree-dump-times "%" 0 "final_cleanup" } } */
+
+/* { dg-final { cleanup-tree-dump "empty" } } */
+/* { dg-final { cleanup-tree-dump "final_cleanup" } } */

gcc/tree-scalar-evolution.c

@@ -2805,6 +2805,50 @@ scev_finalize (void)
   scalar_evolution_info = NULL;
 }
 
+/* Returns true if the expression EXPR is considered to be too expensive
+   for scev_const_prop.  */
+
+bool
+expression_expensive_p (tree expr)
+{
+  enum tree_code code;
+
+  if (is_gimple_val (expr))
+    return false;
+
+  code = TREE_CODE (expr);
+  if (code == TRUNC_DIV_EXPR
+      || code == CEIL_DIV_EXPR
+      || code == FLOOR_DIV_EXPR
+      || code == ROUND_DIV_EXPR
+      || code == TRUNC_MOD_EXPR
+      || code == CEIL_MOD_EXPR
+      || code == FLOOR_MOD_EXPR
+      || code == ROUND_MOD_EXPR
+      || code == EXACT_DIV_EXPR)
+    {
+      /* Division by power of two is usually cheap, so we allow it.
+	 Forbid anything else.  */
+      if (!integer_pow2p (TREE_OPERAND (expr, 1)))
+	return true;
+    }
+
+  switch (TREE_CODE_CLASS (code))
+    {
+    case tcc_binary:
+    case tcc_comparison:
+      if (expression_expensive_p (TREE_OPERAND (expr, 1)))
+	return true;
+
+      /* Fallthru.  */
+    case tcc_unary:
+      return expression_expensive_p (TREE_OPERAND (expr, 0));
+
+    default:
+      return true;
+    }
+}
+
 /* Replace ssa names for that scev can prove they are constant by the
    appropriate constants.  Also perform final value replacement in loops,
    in case the replacement expressions are cheap.
@@ -2896,12 +2940,6 @@ scev_const_prop (void)
 	continue;
 
       niter = number_of_latch_executions (loop);
-      /* We used to check here whether the computation of NITER is expensive,
-	 and avoided final value elimination if that is the case.  The problem
-	 is that it is hard to evaluate whether the expression is too
-	 expensive, as we do not know what optimization opportunities the
-	 elimination of the final value may reveal.  Therefore, we now
-	 eliminate the final values of induction variables unconditionally.  */
       if (niter == chrec_dont_know)
 	continue;
 
@@ -2938,7 +2976,15 @@ scev_const_prop (void)
 	      /* Moving the computation from the loop may prolong life range
 		 of some ssa names, which may cause problems if they appear
 		 on abnormal edges.  */
-	      || contains_abnormal_ssa_name_p (def))
+	      || contains_abnormal_ssa_name_p (def)
+	      /* Do not emit expensive expressions.  The rationale is that
+		 when someone writes a code like
+
+		 while (n > 45) n -= 45;
+
+		 he probably knows that n is not large, and does not want it
+		 to be turned into n %= 45.  */
+	      || expression_expensive_p (def))
 	    {
 	      gsi_next (&psi);
 	      continue;

gcc/tree-scalar-evolution.h

@@ -35,6 +35,7 @@ extern void gather_stats_on_scev_database (void);
 extern void scev_analysis (void);
 unsigned int scev_const_prop (void);
 
+bool expression_expensive_p (tree);
 extern bool simple_iv (struct loop *, gimple, tree, affine_iv *, bool);
 
 /* Returns the basic block preceding LOOP or ENTRY_BLOCK_PTR when the

gcc/tree-ssa-loop-ivopts.c

@@ -3844,7 +3844,12 @@ may_eliminate_iv (struct ivopts_data *data,
     return false;
 
   cand_value_at (loop, cand, use->stmt, nit, &bnd);
+
   *bound = aff_combination_to_tree (&bnd);
+  /* It is unlikely that computing the number of iterations using division
+     would be more profitable than keeping the original induction variable.  */
+  if (expression_expensive_p (*bound))
+    return false;
   return true;
 }