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From: Kaz Kylheku <847-115-0292@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: Optimization techniques and undefined behavior
Date: Thu, 2 May 2019 17:18:06 +0000 (UTC)
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References: <72d208c9-169f-155c-5e73-9ca74f78e390@gkc.org.uk> <19-04-021@comp.compilers> <19-04-024@comp.compilers> <19-04-038@comp.compilers>
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Keywords: optimize, errors, C
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On 2019-04-28, David Brown <david.brown@hesbynett.no> wrote:
> On 26/04/2019 04:26, Kaz Kylheku wrote:
>> On 2019-04-25, David Brown <david.brown@hesbynett.no> wrote:
>>> It knows that "x * 2 / 2" is "x".  It knows that "x + 1 > x" is true.
>>
>> Also, we could have it so that multiplication wraps, and yet in the same
>> breath allow algebraic reductions like this anyway.
>>
>> Basically it can be so that, if overflow takes place in the abstract
>> semantics of "x * 2 / 2", the result can be either the truncated
>> version, or just x, if the algebraic reduction took place.
>>
>
> I am afraid I don't understand you here.  Are you saying that you could
> define the language's arithmetic so that, given "x * 2 / 2", the
> compiler could return /either/ "x" or the result you would get from
> wrapped overflow calculations?  You are defining two possible "correct"
> answers?

I'm fairly comfortable with this, because it maps to the familiar ISO C
concept of "unspecified behavior".

For instance, this program can produce one of the results -12, -6, -2
or 4, based on which of the six permutations over the order in which the
three functions are called.

  #include <stdio.h>

  int x = 1;

  int add2(void)
  {
    x += 2;
    return 0;
  }

  int mul4(void)
  {
    x *= 4;
    return 0;
  }

  int neg(void)
  {
    x = -x;
    return 0;
  }

  void dummy(int a, int b, int c)
  {
  }

  int main()
  {
    dummy(add2(), mul4(), neg());
    printf("%d\n", x);
  }

Unspecified behavior isn't exactly wonderful, but it's better than
undefined behavior.