From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 96FB33858430; Thu, 31 Mar 2022 18:03:44 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 96FB33858430
From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/105119] New: the division in x / (1 << y) is
 optimized away when x has unsigned type, but not when it's signed
Date: Thu, 31 Mar 2022 18:03:44 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ppalka at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-105119-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 31 Mar 2022 18:03:44 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105119

            Bug ID: 105119
           Summary: the division in x / (1 << y) is optimized away when x
                    has unsigned type, but not when it's signed
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

For the following on x86_64 with -O2

int f(int x, int y) {
  return x / (1 << y);
}

GCC generates an idiv instruction:

        movl    %esi, %ecx
        movl    $1, %edx
        movl    %edi, %eax
        sall    %cl, %edx
        movl    %edx, %ecx
        cltd
        idivl   %ecx
        ret

But I believe this is equivalent to the division-less

int g(int x, int y) {
  return (x + (x < 0) * ((1 << y) - 1)) >> y;
}

(which basically generalizes the existing x / (1 << y) -> x >> y transforma=
tion
that we perform for unsigned x).  For this latter function, we generate

        movl    %esi, %ecx
        movl    $1, %eax
        xorl    %edx, %edx
        sall    %cl, %eax
        subl    $1, %eax
        testl   %edi, %edi
        cmovns  %edx, %eax
        addl    %edi, %eax
        sarl    %cl, %eax
        ret

which seems to be significantly faster according to some rough benchmarks.=