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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/104371] [x86] Failure to use optimize
 pxor+pcmpeqb+pmovmskb+cmp 0xFFFF pattern to ptest
Date: Fri, 04 Feb 2022 08:03:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104371

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
  <bb 2> [local count: 1073741824]:
  _2 =3D VIEW_CONVERT_EXPR<__v16qi>(x_3(D));
  _6 =3D _2 =3D=3D { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  _7 =3D VIEW_CONVERT_EXPR<vector(16) signed char>(_6);
  _4 =3D __builtin_ia32_pmovmskb128 (_7);
  _5 =3D _4 =3D=3D 65535;
  return _5;

so likely one reason is the builtin and later UNSPEC for the movemask
operation.

combine does try the following though

Trying 8, 11, 13 -> 14:
    8: r92:V16QI=3Dr89:V16QI=3D=3Dr96:V2DI#0
      REG_DEAD r96:V2DI
      REG_DEAD r89:V16QI
   11: r88:SI=3Dunspec[r92:V16QI] 44
      REG_DEAD r92:V16QI
   13: flags:CCZ=3Dcmp(r88:SI,0xffff)
      REG_DEAD r88:SI
   14: r95:QI=3Dflags:CCZ=3D=3D0
      REG_DEAD flags:CCZ
Failed to match this instruction:
(set (reg:QI 95)
    (eq:QI (unspec:SI [
                (eq:V16QI (reg:V16QI 89)
                    (subreg:V16QI (reg:V2DI 96) 0))
            ] UNSPEC_MOVMSK)
        (const_int 65535 [0xffff])))

of course I have my doubts the pattern is a useful one to optimize.=