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 E0C013896C14; Tue,  3 Aug 2021 09:25:00 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org E0C013896C14
From: "marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101701] GCC optimization and code generation
 for if-else chains vs ternary chains vs a switch
Date: Tue, 03 Aug 2021 09:25:00 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: marxin at gcc dot gnu.org
X-Bugzilla-Status: NEW
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: cc
Message-ID: <bug-101701-4-RcWLZ78lhN@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-101701-4@http.gcc.gnu.org/bugzilla/>
References: <bug-101701-4@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: Tue, 03 Aug 2021 09:25:01 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101701

Martin Li=C5=A1ka <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #5 from Martin Li=C5=A1ka <marxin at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> So the problem here is iftoswitch should ignore if it is profitable to
> convert the ifs to a switch statement (unless there are only two ifs) and
> then allow the switchlower pass to lower the switch again.

This was my original intention, but Jakub wanted to make the if-to-switch
transformation
only conditionally based on the ability to make a jump table or a bit-test.
Reason is that
some optimization passes work only on series of gimple conditions.

>=20
> So x86 we get:
> ;; Canonical GIMPLE case clusters: 1 2 3 4=20
> For foo
> and then for bar:
> ;; Canonical GIMPLE case clusters: 1 2 3 4=20
> ;; BT can be built: BT(values:4 comparisons:8 range:4 density: 200.00%):1=
-4=20
> /app/example.cpp:10:21: optimized: Condition chain with 4 BBs transformed
> into a switch statement.

In my case, it's not transformed as we do JT at least for 5 cases:

unsigned int
default_case_values_threshold (void)
{
  return (targetm.have_casesi () ? 4 : 5);
}

That's why you see a different codegen on ARM target.=