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Re: [PATCH]: Fix bug preventing some LICM in PRE







Daniel Berlin <dberlin@dberlin.org> wrote on 17/01/2005 16:12:16:

> >
> > We have a patch that allows the vectorizer to overcome such invariant
phis
>
> What exactly are you going to vectorize if PRE has removed the only
> computation in the loop?

the store of an invariant value

> Also, the phi is no more or less invariant than the original computation,

> so i'm not quite sure why it matters to the vectorizer :)
>

exactly - that's why we have a trivial patch that tells the vectorizer to
ignore such phis (right now we check all phis and make sure their evolution
in the loop is "simple"; so far we haven't regarded "no evolution" as
simple evolution).

> >
> > By the way, we mantioned this PRE issue in PR18181, which prompted the
> > introduction of these testcases.
> >
> >> ....
> >>
> >> is completely loop invariant out of both loops, and the fixed PRE will
> >> happily move it.
> >>
> >> I've simply added -fno-tree-pre to the default flags for the
vectorizer
> >> tests for now, since this is supposed to be testing vectorization, not
> >> general optimization.
> >
> > Well, we want to check how the vectorizer behaves in a normal -O2
> > compilation (users will not often use -fno-tree-pre) so I would prefer
> > removing the -fno-tree-pre and xfail these testcases.
>
> Okay, if that's what you want to do, please do that.
>
> >
> >>  This will let vectorizer people test that we
> >> vectorize whatever they like.
> >>
> >
> > If we keep the -fno-tree-pre in vect.exp we have to remove the xfail
from
> > vect-85.c, because without PRE vect-85.c gets vectorized (we get an
XPASS
> > with your patch, on i686-pc-linux-gnu and powerpc-darwin).
> >
> > I prefer that we remove the -fno-tree-pre from vect.exp and xfail
testcases
> > vect-[86.87.88].c for now, until we incorporate the patch that ignores
> > invariant phis in the vectorizer.
> >
> > would that be OK for mainline?
>
> I think i can approve that, since it's more or less a reversion of what i

> did.
> Though you should probably wait a day or so and see if anyone objects.
>

ok, thanks

dorit


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