[Bug rtl-optimization/64537] New: Aarch64 redundant sxth instruction gets generated

["vekumar at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64537

            Bug ID: 64537
           Summary: Aarch64 redundant sxth instruction gets generated
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vekumar at gcc dot gnu.org

For the below test case redundant sxth instruction gets generate.

int
adds_shift_ext ( long long a, short b, int c)
{
 long long  d = (a - ((long long)b << 3));

  if (d == 0)
    return a + c;
  else
    return b + d + c;
}


adds_shift_ext:
        sxth    w1, w1  // 3    *extendhisi2_aarch64/1  [length = 4] <==1
        subs    x3, x0, x1, sxth 3      // 11   *subs_extvdi_multp2     [length
= 4] <==2
        beq     .L5     // 12   *condjump       [length = 4]
        add     w0, w1, w2      // 19   *addsi3_aarch64/2       [length = 4]
        add     w0, w0, w3      // 20   *addsi3_aarch64/2       [length = 4]
        ret     // 57   simple_return   [length = 4]
        .p2align 2
.L5:
        add     w0, w2, w0      // 14   *addsi3_aarch64/2       [length = 4]
        ret     // 55   simple_return   [length = 4]

<== 1 is not needed.