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Re: [PATCH] Kill TREE_OVERFLOW during gimplification


> 2008-10-16  Jakub Jelinek  <jakub@redhat.com>
>
> 	* gimplify.c (gimplify_expr): Drop TREE_OVERFLOW from
> 	INTEGER_CSTs.

#include <limits.h>

extern void abort (void);

static __attribute__((noinline)) void foo(int a)
{
  int b = (a - 1) + INT_MIN;

  if (b != INT_MIN)
    abort ();
}

int main (void)
{
  foo (1);
  return 0;
}

now aborts at -O2.


fold_binary reassociates the RHS expression for 'b' into 'a + 2147483647' with 
TREE_OVERFLOW set on the constant.  Then the gimplifier clears it.  Finally 
forwprop propagates the result into 'b != INT_MIN' and fold_comparison 
computes that the result is always true:

  /* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 +- C1.  */
  if ((TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
      && (TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
	  && !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
	  && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg1)))
      && (TREE_CODE (arg1) == INTEGER_CST
	  && !TREE_OVERFLOW (arg1)))

Before the change it would have punted because of TREE_OVERFLOW.

-- 
Eric Botcazou


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