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;

  }