Sender: rich AT phekda DOT freeserve DOT co DOT uk
Message-ID: <3EA649FE.312DD4CF@phekda.freeserve.co.uk>
Date: Wed, 23 Apr 2003 09:08:30 +0100
From: Richard Dawe <rich AT phekda DOT freeserve DOT co DOT uk>
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To: djgpp-workers AT delorie DOT com
Subject: Re: Yet another try on nan in strto{f,d,ld}
References: <200304230720 DOT JAA07123 AT lws256 DOT lu DOT erisoft DOT se>
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Reply-To: djgpp-workers AT delorie DOT com

Hello.

Martin Stromberg wrote:
> 
> Richard said:
[snip]
> > > -fstrict-aliasing'
> > >     Allows the compiler to assume the strictest aliasing rules
> > >     applicable to the language being compiled.  For C (and C++), this
> > >     activates optimizations based on the type of expressions.  In
> > >     particular, an object of one type is assumed never to reside at
> > >     the same address as an object of a different type, unless the
> > >     types are almost the same.  For example, an `unsigned int' can
> > >     alias an `int', but not a `void*' or a `double'.  A character type
> > >     may alias any other type.
> 
> (Sounds to me that "strictly aliased" (gcc term) should be named
> strictly UNaliased (my understanding of it).)

Yes, that would be a clearer name!

The set of strict aliases for something could be a set containing one item
(/the/ only pointer to the thing). Maybe strict aliasing was named by some
mathematicians? ;)

> > Time to get a new mail client, I think. It cut off the last half of my
> > message:
> >
> >      The practice of reading from a different union member than the one
> >      most recently written to (called "type-punning") is common.  Even
> >      with `-fstrict-aliasing', type-punning is allowed, provided the
> >      memory is accessed through the union type.
> 
> Ah. So the day we turn that one on (it isn't on yet, right?) we need
> to add many unions (I suspect we cast pretty much in the source).

gcc 3.3 (well, pre-releases) has a new warning option -Wstrict-aliasing, which
is enabled by -fstrict-aliasing. This is also included by default in -Wall.
-O2 turns on -fstrict-aliasing which turns on -Wstrict-aliasing. So if you use
gcc 3.3, you get the warning.

Bye, Rich =]

-- 
Richard Dawe [ http://www.phekda.freeserve.co.uk/richdawe/ ]