From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-356693-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 21267 invoked by alias); 17 May 2011 15:41:56 -0000
Received: (qmail 21255 invoked by uid 22791); 17 May 2011 15:41:55 -0000
X-SWARE-Spam-Status: No, hits=-2.7 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00
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; Tue, 17 May 2011 15:41:41 +0000
From: "navin.kumar at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/45221] missed optimization with multiple bases and casting
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: navin.kumar at gmail dot com
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: Version
Message-ID: <bug-45221-4-HvXMF1L6cX@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-45221-4@http.gcc.gnu.org/bugzilla/>
References: <bug-45221-4@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
Date: Tue, 17 May 2011 15:49:00 -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
X-SW-Source: 2011-05/txt/msg01360.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45221

Navin Kumar <navin.kumar at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|4.5.0                       |4.6.0

--- Comment #11 from Navin Kumar <navin.kumar at gmail dot com> 2011-05-17 15:18:06 UTC ---
Bumping this to 4.6.0 since the problem still exists.

As a review (since this has been open and not updated for a while), the problem
is very slow performance when dealing with C++'s multiple-inheritance (even
under -O3).  In the example below, the 'simple' fooA function generates slow
code, whereas the equivalent code written with more steps in fooB generates
optimal code.

Base2* fooA(Derived* x)
{
    Base2& y = *x;
    return &y;
}
Base2* fooB(Derived* x) {
    Derived& x2 = *x;
    Base2& y = x2;
    return &y;
}

Both fooA and fooB are funtionally identical.

Yet the assembly generated for fooA is:
    leaq    4(%rdi), %rdx
    xorl    %eax, %eax
    testq   %rdi, %rdi
    cmovne  %rdx, %rax
    ret

and the assembly generated for fooB is:
    leaq    4(%rdi), %rax
    ret