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

From: Ian Lance Taylor
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: 24 Mar 2007 17:23:15 -0700
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4

Robert Dewar <address@hidden> writes:

> > You're right, I shouldn't have said "implementation defined."
> > What will happen with -fno-strict-overflow is whatever the processor
> > ISA happens to do when a signed arithmetic operation overflows.  For
> > ordinary machines it will just wrap.
> Given that all ordinary machines wrap, is there really enough
> difference in performance in practice between -fno-strict-overflow
> and -fwrapv to justify adding this vague and rather dubious (given
> it is vague) switch. Saying "whatever the processor does" is not
> enough, since on some processors there are multiple addition
> instructions (e.g. trapping and non-trapping variants)

I believe there is a comprehensible distinction between "compiler will
not assume that signed overflow is undefined behaviour" and "compiler
will cause all arithmetic to wrap around."

In any case, I have no plans to continue working on this.  I described
my work in considerable detail as I did it and the patches have gone
through multiple rounds of review.  I would be happy to see your
concrete proposal.


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