Re: Optimization techniques

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.compilers
Subject	Re: Optimization techniques
Date	2019-04-28 17:22 +0200
Organization	A noiseless patient Spider
Message-ID	<19-04-036@comp.compilers> (permalink)
References	<19-04-004@comp.compilers> <19-04-012@comp.compilers> <19-04-018@comp.compilers> <19-04-028@comp.compilers>

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On 26/04/2019 21:10, Martin Ward wrote:
> On 23/04/19 08:43, David Brown wrote:
>> But mathematical identities such as associativity and commutativity are
>> valid because signed integer overflow does not happen - thus "a * (b +
>> c)" can be changed to "(a * b) + (a * c)".
>
> If b is large and c has a value close to -b then a * (b + c)
> might be OK while (a * b) will overflow and cause undefined
> behaviour.
>

You are right.  The compiler can go the other way, but not that way.  It
can ignore the possibility of overflows or other errors (like divide by
zero) in the expression you give it, but it can't manipulate things in a
way that might introduce new errors.

Obviously compiler writers are expected to pay more attention to these
details than someone making a Usenet post!

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Optimization techniques "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-04-17 09:42 -0400
  Re: Optimization techniques Hans Aberg <haberg-news@telia.com> - 2019-04-17 18:11 +0200
  Re: Optimization techniques George Neuner <gneuner2@comcast.net> - 2019-04-18 04:07 -0400
    Re: Optimization techniques "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-04-18 11:22 -0400
      Re: Optimization techniques George Neuner <gneuner2@comcast.net> - 2019-04-19 15:48 -0400
    Re: Optimization techniques Hans Aberg <haberg-news@telia.com> - 2019-04-19 00:52 +0200
  Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-04-19 08:49 +0000
    Re: Optimization techniques "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-04-19 11:48 -0400
      Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-23 09:38 +0200
    Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-23 09:18 +0200
  Re: Optimization techniques Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-04-20 00:27 +0200
    Re: Optimization techniques "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-04-19 16:11 -0700
      Re: Optimization techniques Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2019-04-20 19:47 +0200
        Re: Optimization techniques "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2019-04-24 10:16 -0400
    Re: Optimization techniques George Neuner <gneuner2@comcast.net> - 2019-04-20 18:59 -0400
    Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-23 09:43 +0200
      Re: Optimization techniques Martin Ward <martin@gkc.org.uk> - 2019-04-26 20:10 +0100
        Re: Optimization techniques Kaz Kylheku <847-115-0292@kylheku.com> - 2019-04-26 21:11 +0000
        Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-04-28 17:22 +0200
          Re: Optimization techniques Gene Wirchenko <genew@telus.net> - 2019-04-30 18:07 -0700
            Re: Optimization techniques David Brown <david.brown@hesbynett.no> - 2019-05-01 09:03 +0200
    Re: Optimization techniques "Derek M. Jones" <derek@_NOSPAM_knosof.co.uk> - 2019-04-26 13:39 +0100
  Re: Optimization techniques rockbrentwood@gmail.com - 2019-09-26 20:35 -0700

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