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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: 31 Dec 2006 18:11:50 -0800
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4

address@hidden (Richard Kenner) writes:

> > > > Note that -fwrapv also _enables_ some transformations on signed
> > > > integers that are disabled otherwise.  We for example constant fold
> > > > -CST for -fwrapv while we do not if signed overflow is undefined.
> > > > Would you change those?
> > > 
> > > I don't understand the rationale for not wrapping constant folding when
> > > signed overflow is undefined: what's the harm in "defining" it as wrapping
> > > for that purpose?  If it's undefined, then why does it matter what we
> > > fold it to?  So we might as well fold it to what traditional code expects.
> > 
> > If flag_wrapv is false, we can't do any optimization which may
> > introduce signed overflow when it did not already exist.  
> But how would that happen here?  If we constant-fold something that would
> have overflowed by wrapping, we are ELIMINATING a signed overflow, not
> INTRODUCING one.  Or do I misunderstand what folding we're talking about here? is what led to the patch.


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