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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Richard Kenner
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Sun, 31 Dec 2006 20:54:50 EST

> This isn't just about old code. If you're saying that old code with
> overflow checking can't be fixed (in a portable manner...), then new
> code will probably use the same tricks. 

I said there's no "good" way, meaning as compact as the current tests.  But
it's certainly easy to test for overflow in a correct and portable manner
that's not TOO inefficient.  I haven't tested it, but this ought to do it and
is only 9 instructions on x86-64:

/* Return 1 IFF a + b will overflow as signed numbers.  Assumes two's
   complement.  */

overflow (int a, int b)
  unsigned int pos_a, pos_b;

  /* If they have different signs, their sum can't overflow.  */
  if ((a ^ b) < 0)
    return false;

  /* Otherwise, sum the non-sign bits as unsigned (this is close to abs,
     but avoids overflow for INT_MIN) and see if that, interpreted as a
     signed number, would be negative.  If so, the sum will overflow.  */
  pos_a = (a & ((1u << (sizeof (int) * HOST_BITS_PER_CHAR - 1)) - 1));
  pos_b = (b & ((1u << (sizeof (int) * HOST_BITS_PER_CHAR - 1)) - 1));
  return (pos_a + pos_b) >> (sizeof (int) * HOST_BITS_PER_CHAR - 1);

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