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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109907] Missed optimization for bit extraction (uses
 shift instead of single bit-test)
Date: Sun, 21 May 2023 05:36:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109907

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #14)
> Actually I take back on what is going on those 3. But I will be looking i=
nto
> it.
>=20
>=20
>   x.0_1 =3D (signed long) x_4(D);
>   _2 =3D x.0_1 >> 31;
>   _3 =3D (unsigned char) _2;
>   _5 =3D _3 & 1;

we could pattern match this on the gimple using something like:
(simplify
 (bit_and (convert? (rshift (nop_convert? @1) INTEGER_CST@2)) integer_onep)
 (if (wi::to_wide (@2) =3D=3D TYPE_PRECISION (TREE_TYPE (@0)) - 1)
 (with
  { tree utype =3D unsigned_type_for (type); }
  (convert (rshift (convert:utype @1) @2)))


But that still fails because combine really does not like subregs:
Trying 6 -> 12:
    6: r47:SI=3Dr48:SI 0>>0x1f
      REG_DEAD r48:SI
   12: r24:QI=3Dr47:SI#0
      REG_DEAD r47:SI
Failed to match this instruction:
(set (reg/i:QI 24 r24)
    (subreg:QI (lshiftrt:SI (reg:SI 48)
            (const_int 31 [0x1f])) 0))=