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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."


From: Paul Eggert
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Fri, 29 Dec 2006 16:35:39 -0800
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.4 (gnu/linux)

"Seongbae Park" <address@hidden> writes:

> On 12/29/06, Paul Eggert <address@hidden> wrote:
>> -O2 does not currently imply '-ffast-math'ish optimizations even
>> though the C standard would allow it to.
>
> Can you point me to the relevant section/paragraph in C99 standard
> where it allows the implementation to do -ffast-math style optimization ?
> C99 Annex F.8 quite clearly says the implementation can't,
> as long as it claims any conformity to IEC 60559.

This is more of a pedantic standards question than a
real-world programming question, but I'll answer anyway.


C99 does not require implementations to conform to IEC 60559
as specified in C99 Annex F.  It's optional.

Similarly, C99 does not require implementations to conform
to LIA-1 wrapping semantics as specified in C99 Annex H.
That's optional, too.

The cases are not entirely equivalent, as Annex F is
normative but Annex H is informative.  But as far as the
standard is concerned, it's clear that gcc could enable many
non-IEEE optimizations (including some of those enabled by
-ffast-math); that would conform to the minimal standard.




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