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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109011] missed optimization in presence of
 __builtin_ctz
Date: Mon, 06 Mar 2023 08:23:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109011

--- Comment #15 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Jakub Jelinek from comment #14)
> (In reply to Hongtao.liu from comment #13)
> > It looks like ffs is *just* ctz with defined behavior for zero, so we c=
an
> > handle it exactly the same as ctz in the same pattern match((bitsize - =
.CLZ
> > ((x - 1) & ~x)) or .POPCOUNT ((x - 1) & ~x)) when CLZ_DEFINED_VALUE_AT_=
ZERO
> > 2.
>=20
> No, ffs(x) is ctz(x) + 1 for all x !=3D 0, and 0 for x =3D=3D 0.  But yes=
, we can
> generally handle it similarly.  Let me attach a patch.

Oh, I see.=