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Re: Optimization techniques

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On 2019-04-25, Martin Ward <martin@gkc.org.uk> wrote:
> David Brown <david.brown@hesbynett.no> wrote:
>> And there are undefined behaviours which could be given definitions, but
>> doing so is not actually a positive thing.  The prime example is signed
>> integer overflow.  Since overflowing your integers is almost always an
>> error in the code anyway, there are no benefits in giving it defined
>> behaviour.
>
> This is completely backwards. If signed overflow was given a defined
> behaviour (such as the two's complement result), then compilers for
> CPUs which do not implement two's complement operations would have to
> generate less efficient code (but does anyone still make such a CPU?).
> Any program which guarantees not to overflow would be unaffected. Any
> program which expects a two's complement result could now be relied
> upon to work correctly, and not suddenly produce random results when
> the next version of the compuler comes out!  Any program which
> expected a different result (one's complement anyone?)  would need
> additional code.
>
> With the current situation, anyone wanting to avoid
> undefined behaviour (and don't we all?) has to write code like
> this for any signed operation:
>
> signed int sum;
> if (((si_b > 0) && (si_a > (INT_MAX - si_b))) ||
>      ((si_b < 0) && (si_a < (INT_MIN - si_b)))) {
>    /* Handle error */
> } else {
>    sum = si_a + si_b;
> }

Problem is that even though we have made the overflowing addition
defined behavior, it's often not the right behavior for the program,
which really wanted the arithmetic sum that doesn't fit into the type.

We still need to defend against this situation, and so need a glob of
code.

However, that code becomes a little bit simpler and more elegant: gone
are the INT_MAX and INT_MIN constants, and calculations:

  int sum = si_a + si_b;

  if ((si_a > 0 && si_b > 0 && sum < 0) ||
      (si_a < 0 && si_b < 0 && sum > 0))
      /* Handle error */

Overflow happens when the operands have the same sign, which is opposite
to the result's sign. We can test for that with bit ops:

  if (((si_a ^ s_b) & SIGN_BIT_MASK) == 0) &&
      ((si_a ^ sum) & SIGN_BIT_MASK) != 0))
      /* Handle error */

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Re: Optimization techniques Martin Ward <martin@gkc.org.uk> - 2019-04-25 16:46 +0100
  Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-04-25 23:01 +0000
  Re: Optimization techniques alexfrunews@gmail.com - 2019-04-26 01:33 -0700
    Re: language design and Optimization techniques Martin Ward <martin@gkc.org.uk> - 2019-04-27 11:56 +0100
    Re: Optimization techniques 0xe2.0x9a.0x9b@gmail.com - 2019-04-27 04:56 -0700
      Re: C language andOptimization techniques alexfrunews@gmail.com - 2019-04-27 19:47 -0700
    Re: reliability features and Optimization techniques Bart <bc@freeuk.com> - 2019-04-28 11:58 +0100
      Re: reliability features and Optimization techniques Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2019-04-29 04:33 -0700
    Re: Optimization techniques Gene Wirchenko <genew@telus.net> - 2019-04-30 18:11 -0700
    Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-05-07 16:43 +0200
  Re: Optimization techniques Hans Aberg <haberg-news@telia.com> - 2019-04-27 23:01 +0200
    Re: Optimization techniques, C++ numeric representations David Brown <david.brown@hesbynett.no> - 2019-04-29 17:24 +0200
      Re: Optimization techniques, C++ numeric representations Hans Aberg <haberg-news@telia.com> - 2019-04-30 15:01 +0200

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