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Re: Bug in loop optimize (invalid postinc to preinc transformation)

To: Alexandre Oliva <aoliva at redhat dot com>
Subject: Re: Bug in loop optimize (invalid postinc to preinc transformation)
From: Torbjorn Granlund <tege at swox dot com>
Date: 28 Dec 2000 06:09:29 +0100
Cc: Peter Osterlund <peter dot osterlund at mailbox dot swipnet dot se>,David Korn <dkorn at pixelpower dot com>,"'Denis Chertykov'" <denisc at overta dot ru>, gcc at gcc dot gnu dot org
References: <718D38CAB6E0D011B2C90060970C28A5642545@EXCHANGESERVER><m2snn9qvov.fsf@ppro.localdomain><ork88ldwtr.fsf@guarana.lsd.ic.unicamp.br>

Alexandre Oliva <aoliva@redhat.com> writes:

  On Dec 27, 2000, Peter Osterlund <peter.osterlund@mailbox.swipnet.se> wrote:

  > Compiling without optimization indicates that the compiler is
  > transforming (p++ < x) into (++p < (x+1)), even when not optimizing.
  > This transformation is incorrect because x+1 wraps around.

  Overflow invokes undefined behavior.  Since incrementing p in this
  case involves overflow, I think the transformation is ok, as far as
  undefined behavior goes.

Overflow of *unsigned* types is well-defined.  

-- 
Torbjörn

Follow-Ups:
- Re: Bug in loop optimize (invalid postinc to preinc transformation)
  - From: Alexandre Oliva

References:
- RE: Bug in loop optimize (TREE stage)
  - From: David Korn
- Re: Bug in loop optimize (invalid postinc to preinc transformation)
  - From: Peter Osterlund
- Re: Bug in loop optimize (invalid postinc to preinc transformation)
  - From: Alexandre Oliva

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