From: Richard Guenther <rguenther@suse.de>
To: Paolo Bonzini <bonzini@gnu.org>
Cc: Eric Botcazou <ebotcazou@adacore.com>,
gcc-patches@gcc.gnu.org, Arnaud Charlet <charlet@adacore.com>
Subject: Re: [PATCH] Keep lattice abstract in the SSA propagator, substitue lattice values at defs
Date: Sun, 08 Aug 2010 12:37:00 -0000 [thread overview]
Message-ID: <alpine.LNX.2.00.1008081424560.25856@zhemvz.fhfr.qr> (raw)
In-Reply-To: <alpine.LNX.2.00.1008081410221.25856@zhemvz.fhfr.qr>
On Sun, 8 Aug 2010, Richard Guenther wrote:
> On Sun, 8 Aug 2010, Paolo Bonzini wrote:
>
> > On 08/08/2010 04:44 AM, Richard Guenther wrote:
> > > arithmetic. But - we can do so if reassociating constants only
> > > as in the example above. Because if (L43b_216 - 1) - 2147483648
> > > doesn't overflow then L43b_216 + 2147483647 doesn't either.
> >
> > Huh?
> >
> > a - 1 - 2147483648 is valid if a >= 1.
> >
> > a + 2147483647 is valid if a < 1.
>
> Oh, indeed. Hm. I guess we have to be more careful with
> constant-folding in reassoc ... (and it's one more reason for
> me to pick up no-undefined-overflow again ...).
>
> I guess as I have exposed the issue I have to look into it.
C testcase which already fold messes up (well, exposes bogus
IL to VRP):
extern void abort (void);
int i = 1;
int main()
{
if ((i - 1) - (-__INT_MAX__ - 1) != (-__INT_MAX__ - 1))
abort ();
return 0;
}
Testcase which forwprop breaks (via fold) with -fstrict-overflow:
extern void abort (void);
int i = 1;
int main()
{
int j = i - 1;
j = j - (-__INT_MAX__ - 1);
if (j != (-__INT_MAX__ - 1))
abort ();
return 0;
}
Bah.
Richard.
next prev parent reply other threads:[~2010-08-08 12:28 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-04 12:22 Richard Guenther
2010-08-04 12:26 ` Arnaud Charlet
2010-08-04 12:29 ` Richard Guenther
2010-08-04 12:31 ` Arnaud Charlet
2010-08-04 12:49 ` Richard Guenther
2010-08-04 12:57 ` Eric Botcazou
2010-08-04 12:58 ` Richard Guenther
2010-08-04 13:27 ` Eric Botcazou
2010-08-08 8:01 ` Eric Botcazou
2010-08-08 8:44 ` Richard Guenther
2010-08-08 12:09 ` Paolo Bonzini
2010-08-08 12:12 ` Paolo Bonzini
2010-08-08 12:28 ` Richard Guenther
2010-08-08 12:37 ` Richard Guenther [this message]
2010-08-08 13:36 ` Richard Guenther
2010-08-08 22:46 ` Eric Botcazou
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