From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 24496 invoked by alias); 10 Apr 2008 12:20:56 -0000 Received: (qmail 24422 invoked by uid 22791); 10 Apr 2008 12:20:55 -0000 X-Spam-Check-By: sourceware.org Received: from web88301.mail.re4.yahoo.com (HELO web88301.mail.re4.yahoo.com) (216.39.53.224) by sourceware.org (qpsmtpd/0.31) with SMTP; Thu, 10 Apr 2008 12:20:36 +0000 Received: (qmail 34730 invoked by uid 60001); 10 Apr 2008 12:20:33 -0000 X-YMail-OSG: sx5UBjMVM1maSR08kgdXfZHXAYsTlW7Bnz51TzYtR6HnkUDE_qnoHCePuagn5bTJXXmyW69iYXsbOH91ITB8ZixIqWcw4SCAcOb69udSH6Dyer9cH2C.2ZiPQsLILDjh_IuMem4xtBeXEDA- Received: from [99.242.109.220] by web88301.mail.re4.yahoo.com via HTTP; Thu, 10 Apr 2008 08:20:33 EDT Date: Thu, 10 Apr 2008 13:32:00 -0000 From: Ted Byers Subject: Re: how to pass params to inline functions by reference or value? To: VM , gcc-help@gcc.gnu.org In-Reply-To: <1207801566.12695.8.camel@vm-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Message-ID: <676074.17361.qm@web88301.mail.re4.yahoo.com> X-IsSubscribed: yes Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-help-owner@gcc.gnu.org X-SW-Source: 2008-04/txt/msg00153.txt.bz2 --- VM wrote: > Hello, > > I'm trying to decide on the best way to pass > parameters to inline > function. For example, the two functions below: > > inline int > sum(atype_t *x) > { > return x->a + x->b; > } > > and > > inline int > sum(atype_t x) > { > return x.a + x.b; > } > > Since the function is inline, my guess is that the > compiler will > generate identical code for both. So there should be > no performance > difference. > > Is this assumption correct? > No! Ignore, for the moment, that you are merely advising the compiler that you want the function inlined. Unless things have changed since last I looked, there is no guarantee that the compiler will inline a function you have asked to be inlined. Your bigger problem is that you have ignored the semantics associated with passing an argument by reference, pointer and value. The first version of your function passes a pointer argument. Always the same size, and since you don't say the pointer, or the object to which it is pointing, is const, any change made to the object within the function remains after the function finishes and is visible to the calling code. In the second, you pass by value, so the copy constructor must be called. And any change made to that object within the function is lost when the function completes, and is never visible to the calling code (unless you return the object, which again will require invoking the copy constructor). I would recommend you base your decisions about whether to pass by reference or by value on what the function needs to do to, or with, the argument object, rather than idle speculation about what the compiler may do IF it inlines your function. If the compiler isn't broken, it will not change the semantics of the function. Therefore, if you pass an object by value, it has to be copied so that IF your function makes changes to it, those changes are not visible to the calling code and cease to exist once the function completes. I would not expect inlining the function to change that. And it gets more complicated. For example, is there polymorphism involved, and if so, which version of the virtual function used do you need in the context of your function? The bottom line is that there is no one best way to pass arguments since what you do depends on what is needed, irrespective of whether or not the function you're analyzing ought to be inlined. HTH Ted