From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-397033-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 31589 invoked by alias); 20 Jul 2012 12:58:10 -0000
Received: (qmail 31579 invoked by uid 22791); 20 Jul 2012 12:58:09 -0000
X-SWARE-Spam-Status: No, hits=-4.3 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00,KHOP_THREADED,TW_CF
X-Spam-Check-By: sourceware.org
Received: from localhost (HELO gcc.gnu.org) (127.0.0.1)    by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 20 Jul 2012 12:57:56 +0000
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default
Date: Fri, 20 Jul 2012 12:58:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Keywords: diagnostic
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields: CC
Message-ID: <bug-54046-4-RJ2ADDFWum@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-54046-4@http.gcc.gnu.org/bugzilla/>
References: <bug-54046-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2012-07/txt/msg01619.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-20 12:57:42 UTC ---
Or:
void foo (void) __attribute__((noreturn));
void bar (void);

struct A
{
  ~A () {}
};

bool
check (int x)
{
  A z;
  switch (x)
    {
    case 0:
      return false;
    default:
      foo ();
      bar ();
    }
}

The problem is related to pass ordering.  EH lowering is done before CFG
cleanups, so if there is code after a noreturn spot (whether throw or noreturn
function, endless loop etc. doesn't matter), when trying to lower try ..
finally EH will see gimple_seq_may_fallthru to be true because of the extra
code after it hiding that it in fact can't fall thru.  We end up then with:
  switch (x) <default: <D.2213>, case 0: <D.2212>>
  <D.2212>:
  D.2216 = 0;
  finally_tmp.0 = 0;
  goto <D.2220>;
  <D.2213>:
  foo ();
  bar ();
  finally_tmp.0 = 1;
  <D.2220>:
  A::~A (&z);
  switch (finally_tmp.0) <default: <D.2223>, default: <D.2223>, case 1:
<D.2221>>

where D.2221 falls thru to exit without returning a value, while D.2223
doesn't.
Then cfg cleanup during cfg pass cleans that up to:
  finally_tmp.0 = 0;
  A::~A (&z);
  switch (finally_tmp.0) <default: <L12>, case 1: <L11>>
after finding out that foo is noreturn and can't fallthru.  But there is no
constant propagation and switch optimization pass that would simplify
switch (0) <default: something; case 1: somethingelse> into
goto something; scheduled before the pass_warn_function_return is scheduled.
As we aren't in SSA form at that point, doing it wouldn't be very easy.