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From: "gjl 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: Fri, 26 May 2023 16:08:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109907

--- Comment #23 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Thank you so much for looking into this.

For the test case from comment #21 though, the problem is somewhere in tree
optimizations.

> unsigned char lfsr32_mpp_ge0 (unsigned long number)
> {
>   unsigned char b =3D 0;
>   if (number >=3D 0) b--;
>   if (number & (1UL << 29)) b++;
>   if (number & (1UL << 13)) b++;
>=20
>   return b;
> }

The -fdump-tree-optimized dump reads:

;; Function lfsr32_mpp_ge0 (lfsr32_mpp_ge0, funcdef_no=3D0, decl_uid=3D1880,
cgraph_uid=3D1, symbol_order=3D0)

unsigned char lfsr32_mpp_ge0 (long unsigned int number)
{
  unsigned char b;
  long unsigned int _1;
  long unsigned int _2;
  _Bool _3;
  unsigned char _8;
  _Bool _9;
  unsigned char _10;
  unsigned char _11;

  <bb 2> [local count: 1073741824]:
  _1 =3D number_5(D) & 536870912;
  _2 =3D number_5(D) & 8192;
  if (_2 !=3D 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870912]:
  _9 =3D _1 =3D=3D 0;
  _10 =3D (unsigned char) _9;
  _11 =3D -_10;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 536870913]:
  _3 =3D _1 !=3D 0;
  _8 =3D (unsigned char) _3;

  <bb 5> [local count: 1073741824]:
  # b_4 =3D PHI <_11(3), _8(4)>
  return b_4;
}

The ANDs are expanded by expand_binop() and later passes have to deal with =
the
32-bit arithmnetic.  combine finds one combination of andsi3 into
"*sbrx_and_branch<mode>_split" with mode=3Dsi, but apart from that the mess=
 still
lives in asm.=