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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: Sun, 31 Dec 2006 22:52:23 -0800
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.4 (gnu/linux)

Bernd Schmidt <address@hidden> writes:

> Paul Eggert wrote:
>> But so far, benchmark scores are the only scores given by the people
>> who oppose having -O2 imply -fwrapv.
> And you expect real-world results will be different because...?

Because of the (admittedly limited) real-world measurements
I've done with gzip and sha512sum, comparing -O2 to
-O2 -fwrapv on x86.  These were mentioned in
and showed that -fwrapv made little difference for those two
cases.  No other real-worldish measurements comparing -O2 to
-O2 -fwrapv has been mentioned in this forum, as far as I know.

> Has an example of code that actually breaks with a new gcc but not an
> old one been posted in this thread yet?

Sure, this whole thread started from one such example:

(Note that the fix proposed there isn't sufficient in
general, as the resulting code still assumes wrapv
elsewhere.)  And here is a bit more discussion:

The code in question has already been fixed in CVS, so
that's not really the issue; the issue is what other gotchas
are lurking out there.

> the burden of proof should be on the people who are surprised
> that interesting things happen when they write in a language that
> isn't C

To repeat myself a bit, let's move beyond rhetoric like "a
language that isn't C".  There's nothing weird or unusual
about assuming wrapv semantics in C programs.  As we've seen
it is common practice dating back to Unix source code in the
1970s, and (if you want to be pedantic) C99 Annex H with
LIA-1 wrapping semantics gives a standard for it.

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