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From: "rguenther at suse dot de" <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 10:24:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113583

--- Comment #17 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 7 Feb 2024, juzhe.zhong at rivai dot ai wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113583
>=20
> --- Comment #16 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
> The FMA is generated in widening_mul PASS:
>=20
> Before widening_mul (fab1):
>=20
>   _5 =3D 3.33333333333333314829616256247390992939472198486328125e-1 - _4;
>   _6 =3D _5 * 1.229999999999999982236431605997495353221893310546875e-1;
>   _8 =3D _4 + _6;

So this is x + (CST1 - x) * CST2 which we might fold/associate to
x * (1. - CST2) + CST1 * CST2

this looks like something for reassociation (it knows some rules,
like what it does in undistribute_ops_list, I'm not sure if that
comes into play here already, this would be doing the reverse
before).  A match.pd pattern also works, but it wouldn't be
general enough to handle more complicated but similar cases.=