Re: Optimization techniques

From	George Neuner <gneuner2@comcast.net>
Newsgroups	comp.compilers
Subject	Re: Optimization techniques
Date	2019-04-20 18:59 -0400
Organization	A noiseless patient Spider
Message-ID	<19-04-015@comp.compilers> (permalink)
References	<19-04-004@comp.compilers> <19-04-012@comp.compilers>

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On Sat, 20 Apr 2019 00:27:11 +0200, Hans-Peter Diettrich
<DrDiettrich1@netscape.net> wrote:

>I'm always a bit sceptic when C/C++ and "safe" occur in the same
>sentence. AFAIR a C compiler is allowed to ignore parentheses when
>reordering expressions, what can lead to numeric instabilities.

By the standard, a C compiler IS required to respect parentheses
within an arithmetic expression. But even fully parenthesized
expressions contain subexpressions whose evaluations can be reordered.

e.g., (a + (b + c)) has to be equivalent to

  t1 = b + c
  t2 = a + t1

but if any of a, b, or c is an expression itself - e.g., a pointer
dereference - the parenthesized expression does not imply any ordering
of *their* individual evaluations.

George

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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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