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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113326] Optimize vector shift with constant delta on
 shifting-count operand
Date: Thu, 11 Jan 2024 08:10:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113326

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
One more thing:
```
 vect_shift_0 =3D vect_value >> { 0, 1, 2, 3 };
 vect_shift_1 =3D vect_value >> { 4, 5, 6, 7 };
 vect_shift_2 =3D vect_value >> { 8, 9, 10, 11 };
 vect_shift_3 =3D vect_value >> { 12, 13, 14, 15 };
```
vs
```
 vect_shift_0 =3D vect_value >> { 0, 1, 2, 3 };
 vect_shift_1 =3D vect_shift_0 >> { 4, 4, 4, 4 };
 vect_shift_2 =3D vect_shift_0 >> { 8, 8, 8, 8 };
 vect_shift_3 =3D vect_shift_0 >> { 12, 12, 12, 12 };
```

the first has fully independent operations while in the second case, there =
is
one dependent and the are independent operations.

On cores which has many vector units the first one might be faster than the
second one.  So this needs a cost model too.=