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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 07:48:29 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113583

--- Comment #13 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
Ok. I found the optimized tree:


  _5 =3D 3.33333333333333314829616256247390992939472198486328125e-1 - _4;
  _8 =3D .FMA (_5, 1.229999999999999982236431605997495353221893310546875e-1=
, _4);

Let CST0 =3D 3.33333333333333314829616256247390992939472198486328125e-1,
CST1 =3D 1.229999999999999982236431605997495353221893310546875e-1

The expression is equivalent to the following:

_5 =3D CST0 - _4;
_8 =3D _5 * CST1 + 4;

That is:

_8 =3D (CST0 - _4) * CST1 + 4;

So, We should be able to re-associate it like Clang:

_8 =3D CST0 * CST1 - _4 * CST1 + 4; ---> _8 =3D CST0 * CST1 + _4 * (1 - CST=
1);

Since both CST0 * CST1 and 1 - CST1 can be pre-computed during compilation
time.

Let say CST2 =3D CST0 * CST1, CST3 =3D 1 - CST1, then we can re-associate a=
s Clang:

_8 =3D FMA (_4, CST3, CST2).

Any suggestions for this re-association ?  Is match.pd the right place to d=
o it
?

Thanks.=