From: Jakub Jelinek <jakub@redhat.com>
To: Marc Glisse <marc.glisse@inria.fr>
Cc: Joern Wolfgang Rennecke <gnu@amylaar.uk>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: Simplify X * C1 == C2 with undefined overflow
Date: Fri, 7 Aug 2020 23:02:53 +0200 [thread overview]
Message-ID: <20200807210253.GE2363@tucnak> (raw)
In-Reply-To: <alpine.DEB.2.23.453.2008072241090.6903@stedding.saclay.inria.fr>
On Fri, Aug 07, 2020 at 10:57:54PM +0200, Marc Glisse wrote:
> On Fri, 7 Aug 2020, Joern Wolfgang Rennecke wrote:
>
> >
> > On 07/08/20 19:21, Marc Glisse wrote:
> > >
> > > If we are going to handle the wrapping case, we shouldn't limit to
> > > the non-wrapping meaning of multiplicity. 3*X==5 should become
> > > X==1431655767 (for a 32 bit type), etc.
> > >
> > > Do we have an extended gcd somewhere in gcc, to help compute 1431655767?
> > I don't quite see yet how this relates to gcd,
>
> https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm is the most
> common way to compute the modular multiplicative inverse of a number. For 3
> and 2^32, it could tell us that 2863311531*3-2*2^32=1, so modulo 2^32
> 2863311531*3==1, and 3*X==5 is the same as 2863311531*3*X==2863311531*5,
> i.e. X==1431655767.
wi::gcd ?
Jakub
next prev parent reply other threads:[~2020-08-07 21:03 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-07 13:05 Joern Wolfgang Rennecke
2020-08-07 18:21 ` Marc Glisse
2020-08-07 20:30 ` Joern Wolfgang Rennecke
2020-08-07 20:57 ` Marc Glisse
2020-08-07 21:02 ` Jakub Jelinek [this message]
2020-08-07 21:36 ` Marc Glisse
2020-08-08 6:55 ` Jakub Jelinek
2020-08-07 21:58 ` Joern Wolfgang Rennecke
-- strict thread matches above, loose matches on Subject: below --
2020-08-01 7:28 Marc Glisse
2020-08-03 8:51 ` Richard Biener
2020-08-04 15:38 ` Marc Glisse
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