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From: "wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/93565] [11/12/13 Regression] Combine
 duplicates instructions
Date: Wed, 03 Apr 2024 16:13:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93565

--- Comment #31 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #29)
> Looking back at this one, I (In reply to Wilco from comment #8)
> > Here is a much simpler example:
> >=20
> > void f (int *p, int y)
> > {
> >   int a =3D y & 14;
> >   *p =3D a | p[a];
> > }
> After r14-9692-g839bc42772ba7af66af3bd16efed4a69511312ae, we now get:
> f:
> .LFB0:
>         .cfi_startproc
>         and     w2, w1, 14
>         mov     x1, x2
>         ldr     w2, [x0, x2, lsl 2]
>         orr     w1, w2, w1
>         str     w1, [x0]
>         ret
>         .cfi_endproc
>=20
> There is an extra move still but the duplicated and is gone. (with
> -frename-registers added, the move is gone as REE is able to remove the z=
ero
> extend but then there is a life range conflict so can't remove the move t=
oo).

Even with the mov it is better since that can be done with zero latency in
rename in most CPUs.

> So maybe this should be closed as fixed for GCC 14 and the cost changes f=
or
> clz reverted.

The ctz costs are correct since it is a 2-instruction sequence - it only ne=
eds
adjusting for CSSC.=