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From: "juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113583] Main loop in 519.lbm not vectorized.
Date: Wed, 07 Feb 2024 08:08:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113583

--- Comment #15 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
(In reply to rguenther@suse.de from comment #14)
> On Wed, 7 Feb 2024, juzhe.zhong at rivai dot ai wrote:
>=20
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113583
> >=20
> > --- Comment #13 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
> > Ok. I found the optimized tree:
> >=20
> >=20
> >   _5 =3D 3.33333333333333314829616256247390992939472198486328125e-1 - _=
4;
> >   _8 =3D .FMA (_5, 1.22999999999999998223643160599749535322189331054687=
5e-1, _4);
> >=20
> > Let CST0 =3D 3.33333333333333314829616256247390992939472198486328125e-1,
> > CST1 =3D 1.229999999999999982236431605997495353221893310546875e-1
> >=20
> > The expression is equivalent to the following:
> >=20
> > _5 =3D CST0 - _4;
> > _8 =3D _5 * CST1 + 4;
> >=20
> > That is:
> >=20
> > _8 =3D (CST0 - _4) * CST1 + 4;
> >=20
> > So, We should be able to re-associate it like Clang:
> >=20
> > _8 =3D CST0 * CST1 - _4 * CST1 + 4; ---> _8 =3D CST0 * CST1 + _4 * (1 -=
 CST1);
> >=20
> > Since both CST0 * CST1 and 1 - CST1 can be pre-computed during compilat=
ion
> > time.
> >=20
> > Let say CST2 =3D CST0 * CST1, CST3 =3D 1 - CST1, then we can re-associa=
te as Clang:
> >=20
> > _8 =3D FMA (_4, CST3, CST2).
> >=20
> > Any suggestions for this re-association ?  Is match.pd the right place =
to do it
> > ?
>=20
> You need to look at the IL before we do .FMA forming, specifically=20
> before/after the late reassoc pass.  There pass applying match.pd
> patterns everywhere is forwprop.
>=20
> I also wonder which compilation flags you are using (note clang
> has different defaults for example for -ftrapping-math)

Both GCC and Clang are using   -Ofast -ffast-math.=