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 AE8953857C40; Thu, 18 Nov 2021 08:48:04 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org AE8953857C40
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103278] [12 Regression] Recent change to
 cddce inhibits switch optimization
Date: Thu, 18 Nov 2021 08:48:04 +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: normal
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: ASSIGNED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: rguenth at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 12.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: cc
Message-ID: <bug-103278-4-esQyZzOUJC@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-103278-4@http.gcc.gnu.org/bugzilla/>
References: <bug-103278-4@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, 18 Nov 2021 08:48:04 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103278

Richard Biener <rguenth at gcc dot gnu.org> changed:

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

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the main issue is that while CD-DCE tries to undo the factorization by
running cleanup CFG, this process has the choice between two forwarder bloc=
ks
to remove - one pre-existing to disambiguate the two edges from the last te=
st
into the PHI
and the one we created.  But CFG cleanup simply picks the first candidate
which, in some cases is not the newly created one.  That results in differe=
nt
handling
of if-to-switch cluster processing.  In particular the ->m_has_forward_bb
handling seems important.  If we do

diff --git a/gcc/gimple-if-to-switch.cc b/gcc/gimple-if-to-switch.cc
index 16fabef7ca0..157c5f6f10b 100644
--- a/gcc/gimple-if-to-switch.cc
+++ b/gcc/gimple-if-to-switch.cc
@@ -219,8 +219,7 @@ if_chain::is_beneficial ()
     {
       simple_cluster *right =3D static_cast<simple_cluster *> (clusters[i]=
);
       tree type =3D TREE_TYPE (left->get_low ());
-      if (!left->m_has_forward_bb
-         && !right->m_has_forward_bb
+      if (left->m_has_forward_bb =3D=3D right->m_has_forward_bb
          && left->m_case_bb =3D=3D right->m_case_bb)
        {
          if (wi::eq_p (wi::to_wide (right->get_low ()) - wi::to_wide

which seems more natural then even with the original IL we don't get any
if-to-switch as we seem to fail JT building because the number of clusters
is then just 4 which is lower than case_values_threshold () which is 5.

If we supply --param case-values-threshold=3D4 to the testcase it is optimi=
zed
(that overrides the target default) with the result

  switch (aChar_10(D)) <default: <L7> [INV], case 9 ... 10: <L6> [INV], case
12: <L6> [INV], case 13: <L6> [INV], case 32: <L6> [INV], case 48: <L6> [IN=
V]>

  <bb 3> :
<L6>:

  <bb 4> :
  # iftmp.0_9 =3D PHI <1(3), 0(2)>
<L7>:
  return iftmp.0_9;

compared to the following before the CD-DCE change which looks clearly worse
(but even the above has unmerged case 12 and 13?!)

  switch (aChar_10(D)) <default: <L10> [INV], case 9 ... 10: <L6> [INV], ca=
se
12: <L6> [INV], case 13: <L7> [INV], case 32: <L7> [INV], case 48: <L8> [IN=
V]>

  <bb 3> :
<L6>:
  goto <bb 6>; [100.00%]

  <bb 4> :
<L7>:
  goto <bb 6>; [100.00%]

  <bb 5> :
<L8>:

  <bb 6> :
  # iftmp.0_9 =3D PHI <1(4), 0(2), 1(3), 1(5)>
<L10>:
  return iftmp.0_9;

so this looks like a testcase issue to me.=