PR tree-optimization/32044

From: rakdver

Date: 2008-12-12 21:32:47 +0100

* 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.

cherry picked from svn://gcc.gnu.org/svn/gcc/trunk, rev 142719 and adapted to

apply on gcc 4.3.2

------------------------------------------------------------------------

Index: gcc-4.3.2/gcc/tree-scalar-evolution.c

===================================================================

--- gcc-4.3.2.orig/gcc/tree-scalar-evolution.c	2009-01-28 10:14:37.000000000 +0100

+++ gcc-4.3.2/gcc/tree-scalar-evolution.c	2009-01-28 10:17:50.000000000 +0100

@@ -2716,6 +2716,50 @@

   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.

@@ -2802,12 +2846,6 @@

 	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

-	 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;

@@ -2838,7 +2876,15 @@

 	      /* 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))

 	    continue;

 	  /* Eliminate the PHI node and replace it by a computation outside

Index: gcc-4.3.2/gcc/tree-scalar-evolution.h

===================================================================

--- gcc-4.3.2.orig/gcc/tree-scalar-evolution.h	2009-01-28 10:22:47.000000000 +0100

+++ gcc-4.3.2/gcc/tree-scalar-evolution.h	2009-01-28 10:23:10.000000000 +0100

@@ -35,6 +35,7 @@

 extern void scev_analysis (void);

 unsigned int scev_const_prop (void);

+bool expression_expensive_p (tree);

 extern bool simple_iv (struct loop *, tree, tree, affine_iv *, bool);

 /* Returns the loop of the polynomial chrec CHREC.  */

Index: gcc-4.3.2/gcc/testsuite/gcc.dg/pr34027-1.c

===================================================================

--- gcc-4.3.2.orig/gcc/testsuite/gcc.dg/pr34027-1.c	2009-01-28 10:24:09.000000000 +0100

+++ gcc-4.3.2/gcc/testsuite/gcc.dg/pr34027-1.c	2009-01-28 10:24:43.000000000 +0100

@@ -8,5 +8,9 @@

   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" } } */

Index: gcc-4.3.2/gcc/testsuite/gcc.dg/tree-ssa/pr32044.c

===================================================================

--- /dev/null	1970-01-01 00:00:00.000000000 +0000

+++ gcc-4.3.2/gcc/testsuite/gcc.dg/tree-ssa/pr32044.c	2009-01-28 10:25:50.000000000 +0100

@@ -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" } } */

Index: gcc-4.3.2/gcc/tree-ssa-loop-ivopts.c

===================================================================

--- gcc-4.3.2.orig/gcc/tree-ssa-loop-ivopts.c	2009-01-28 10:26:04.000000000 +0100

+++ gcc-4.3.2/gcc/tree-ssa-loop-ivopts.c	2009-01-28 10:27:09.000000000 +0100

@@ -3778,7 +3778,12 @@

     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;

 }