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Date: Tue, 09 Jun 2015 11:49:00 -0000
From: Richard Biener <rguenther@suse.de>
To: Marc Glisse <marc.glisse@inria.fr>
cc: Richard Biener <richard.guenther@gmail.com>,     Marek Polacek <polacek@redhat.com>, Jakub Jelinek <jakub@redhat.com>,     GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] Optimize (CST1 << A) == CST2 (PR tree-optimization/66299)
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References: <20150528121545.GE27320@redhat.com> <20150528123436.GM10247@tucnak.redhat.com> <alpine.DEB.2.11.1505282124310.2177@laptop-mg.saclay.inria.fr> <20150608151055.GR2756@redhat.com> <alpine.DEB.2.20.1506081901550.28961@stedding.saclay.inria.fr> <CAFiYyc2vfBzHGHQEkvRujiE2z-brW4J3P0R+TyjQuy44PZj0-A@mail.gmail.com> <alpine.DEB.2.20.1506091305110.9052@stedding.saclay.inria.fr>
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On Tue, 9 Jun 2015, Marc Glisse wrote:

> On Tue, 9 Jun 2015, Richard Biener wrote:
> 
> > > Tweaking it so that (6<<X)==0 becomes X>=31 for TYPE_OVERFLOW_WRAPS and
> > > false for TYPE_OVERFLOW_UNDEFINED is probably more controversial.
> > 
> > Hm, yes.  I think signed overflow != shift amount overflow, so testing the
> > overflow macros for this isn't valid.
> 
> Would it be ok to always turn it to X>=31 then? (the value 31 is conveniently
> already computed in 'cand')

I think so.

Richard.