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From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113196] New: [14 Regression] Failure to use ushll{,2}
Date: Tue, 02 Jan 2024 10:06:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113196

            Bug ID: 113196
           Summary: [14 Regression] Failure to use ushll{,2}
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
                CC: tnfchris at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*-*-*

For this testcase, adapted from the one for PR110625:

  int test(unsigned array[4][4]);

  int foo(unsigned short *a, unsigned long n)
  {
    unsigned array[4][4];

    for (unsigned i =3D 0; i < 4; i++, a +=3D 4)
      {
        array[i][0] =3D a[0] << 6;
        array[i][1] =3D a[1] << 6;
        array[i][2] =3D a[2] << 6;
        array[i][3] =3D a[3] << 6;
      }

    return test(array);
  }

GCC now uses:

        mov     x1, x0
        stp     x29, x30, [sp, -80]!
        movi    v30.4s, 0
        mov     x29, sp
        ldp     q0, q29, [x1]
        add     x0, sp, 16
        zip1    v1.8h, v0.8h, v30.8h
        zip1    v31.8h, v29.8h, v30.8h
        zip2    v0.8h, v0.8h, v30.8h
        zip2    v29.8h, v29.8h, v30.8h
        shl     v1.4s, v1.4s, 6
        shl     v31.4s, v31.4s, 6
        shl     v0.4s, v0.4s, 6
        shl     v29.4s, v29.4s, 6
        stp     q1, q0, [sp, 16]
        stp     q31, q29, [sp, 48]
        bl      test(unsigned int (*) [4])
        ldp     x29, x30, [sp], 80
        ret

whereas previously it used USHLL{,2}:

        mov     x1, x0
        stp     x29, x30, [sp, -80]!
        mov     x29, sp
        ldp     q1, q0, [x1]
        add     x0, sp, 16
        ushll   v3.4s, v1.4h, 6
        ushll   v2.4s, v0.4h, 6
        ushll2  v1.4s, v1.8h, 6
        ushll2  v0.4s, v0.8h, 6
        stp     q3, q1, [sp, 16]
        stp     q2, q0, [sp, 48]
        bl      test(unsigned int (*) [4])
        ldp     x29, x30, [sp], 80
        ret

This changed with g:f26f92b534f9, which expanded zero-extensions to ZIPs.  =
The
patch included *ADDW patterns for the new representation, but it looks like
there are several more that should be included for full coverage.

AIUI, the point of lowering to ZIPs during expand was to allow the zero to =
be
hoisted.  An alternative might be to lower during split, but forcibly hoist=
 the
zero by inserting around the FUNCTION_BEG note.  We could then cache the in=
sn
that does that for manual CSE.

Godbolt link: https://godbolt.org/z/vzfnebMhb=