From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-457491-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 9802 invoked by alias); 31 Jul 2014 18:54:18 -0000
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
Received: (qmail 9770 invoked by uid 48); 31 Jul 2014 18:54:15 -0000
From: "avi@cloudius-systems.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61982] New: Optimizer does not eliminate stores to destroyed objects
Date: Thu, 31 Jul 2014 18:54:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords:
X-Bugzilla-Severity: minor
X-Bugzilla-Who: avi@cloudius-systems.com
X-Bugzilla-Status: UNCONFIRMED
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
Message-ID: <bug-61982-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2014-07/txt/msg02082.txt.bz2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61982

            Bug ID: 61982
           Summary: Optimizer does not eliminate stores to destroyed
                    objects
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi@cloudius-systems.com

After a destructor is run, access to the object is forbidden; the object is
turned into a pile of bytes.  Yet the generated code for:


struct X { 
  int i; 
  void clear() { i = 0; } 
}; 

void f(X* x) { 
  x->clear(); 
  x->~X(); 
} 

void g(X* x) {
  x->clear();
  delete x;
}

contains a store for each of f() and g(), stores that should have been
eliminated:

0000000000000000 <f(X*)>:
   0:    c7 07 00 00 00 00        movl   $0x0,(%rdi)
   6:    c3                       retq   
   7:    66 0f 1f 84 00 00 00     nopw   0x0(%rax,%rax,1)
   e:    00 00 

0000000000000010 <g(X*)>:
  10:    c7 07 00 00 00 00        movl   $0x0,(%rdi)
  16:    e9 00 00 00 00           jmpq   1b <g(X*)+0xb>
            17: R_X86_64_PC32    operator delete(void*)-0x4


To be clear, the generated code is correct, but not optimal.