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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/100711] Miss optimization for pandn
Date: Tue, 25 May 2021 10:09:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100711

--- Comment #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #4)
> > Even w/ canonical RTL, i think a combine splitter is also needed here, =
the
> > canonical RTL only helps combine/forwprop to match more possibility but
> > won't split patterns by itselies.
>=20
> I was wrong, i thought combine only support n->1 combining, but actually
> pass_combine also support 3->2 combining which means a define_split is not
> needed here.

<anything>->1 and <anything>->2, yes.  But note that combine can often split
RTL
without having an explicit define_split; and also note the opposite, combine
does
not always pick the best spot to split, "manual" help (a define_split) can =
be
needed for good results.=