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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/48580] missed optimization: integer overflow checks
Date: Sun, 15 Aug 2021 11:45:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D48580

--- Comment #22 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
For the original testcase in comment #0 we produce (in GCC 11+):
        movl    %edi, %eax
        mull    %esi
        seto    %dl
        xorl    %r8d, %r8d
        movzbl  %dl, %edx
        testl   %eax, %eax
        jle     .L1
        testl   %edx, %edx
        sete    %r8b
.L1:
        movl    %r8d, %eax
        ret

------- CUT ----
I have a patch which I think improves the code even more.

The gimple level looks like this correctly:
  x.0_1 =3D (unsigned int) x_6(D);
  y.1_2 =3D (unsigned int) y_7(D);
  _11 =3D .MUL_OVERFLOW (x.0_1, y.1_2);
  tmp_8 =3D REALPART_EXPR <_11>;
  tmp.3_3 =3D (int) tmp_8;
  if (tmp.3_3 > 0)
    goto <bb 3>; [59.00%]
  else
    goto <bb 4>; [41.00%]

  <bb 3> [local count: 633507680]:
  _12 =3D IMAGPART_EXPR <_11>;
  _10 =3D _12 =3D=3D 0;

  <bb 4> [local count: 1073741824]:
  # iftmp.2_5 =3D PHI <_10(3), 0(2)>

Notice no divide.  The _12 =3D=3D 0 part really should just _12 ^ 1.

After my patch (which I need to finish up) we get:
        movl    %edi, %eax
        mull    %esi
        seto    %dl
        xorl    %r8d, %r8d
        movzbl  %dl, %edx
        xorl    $1, %edx
        testl   %eax, %eax
        cmovg   %edx, %r8d
        movl    %r8d, %eax
        ret
Which should be exactly what you wanted or very close.
There looks to be a few micro-optimizations needed still really.=