From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Monjalon Subject: Re: [PATCH] eal/common: better likely() and unlikely() Date: Sat, 13 Jan 2018 23:24:39 +0100 Message-ID: <216561584.1QX0fu7NSy@xps> References: <1511129764-23123-1-git-send-email-Aleksey.Baulin@gmail.com> <5180253.XvhpJrJZVt@xps> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: dev@dpdk.org, "Wiles, Keith" To: Aleksey Baulin Return-path: Received: from out1-smtp.messagingengine.com (out1-smtp.messagingengine.com [66.111.4.25]) by dpdk.org (Postfix) with ESMTP id A8306A496 for ; Sat, 13 Jan 2018 23:25:10 +0100 (CET) In-Reply-To: List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" Hi, I moved your top-post below and did some comments inline. More opinions are welcome. 13/01/2018 23:05, Aleksey Baulin: > On Fri, Jan 12, 2018 at 6:35 PM, Thomas Monjalon > wrote: > > 21/11/2017 08:05, Aleksey Baulin: > > > On Mon, Nov 20, 2017 at 4:36 PM, Wiles, Keith > > wrote: > > > > > On Nov 19, 2017, at 4:16 PM, Aleksey Baulin < > > aleksey.baulin@gmail.com> > > > > wrote: > > > > > -#define unlikely(x) __builtin_expect((x),0) > > > > > +#define unlikely(x) __builtin_expect(!!(x), 0) > > > > > > > > I have not looked at the generated code, but does this add some extra > > > > instruction now to do the !!(x) ? > > > > > > Sorry for late response. Jim had given the correct answer already. > > > You won't get an extra instruction with compiler optimization turned on. > > > > So this patch is adding an instruction in not optimized binary. > > I don't understand the benefit. > > Is it just to avoid to make pointer comparison explicit? > > likely(pointer != NULL) looks better than likely(pointer). > > This is an interesting question. Perhaps, even a philosophical one. :-) > > 'likely(pointer)' is a perfectly legal statement in C language, as well as > a concise one as > compared to a more explicit (and redundant/superfluous) 'likely(pointer != > NULL)'. If you > _require_ this kind of explicitness in cases like this in the code style, > then I have no > argument here. However, I don't see that anywhere. It is stated here: http://dpdk.org/doc/guides/contributing/coding_style.html#null-pointers > There're other cases of explicitness, with the most widespread being a > series of logical and > compare operations in one statement. For instance, 'if (a > b && a < c)'. > Explicitness would > require writing it like this: 'if ((a > b) && (a < c))'. I've seen cases on > this list where that was > frowned upon as it's totally unnecessary due to C operator precedence > rules, even though > those statements, I think, looked better to their authors (actually, they > do to me). Granted, > it didn't lead to compiler errors, which is the case with the current > implementation of 'likely()'. > > So, my answer to the question is yes, it's to avoid making pointer > comparison explicit. I would > add though, that it is to avoid making a perfectly legal C statement an > illegal one, as with the > way the current macro is constructed, compiler emits an error when DPDK is > built. I write in C > for many years with the most time spent in kernels, Linux and not, and I > find it unnatural to > always add a redundant '!= NULL' just to satisfy the current macro > implementation. I would > have to accept that though if it's a requirement clearly stated somewhere > like a code style. > > As for an extra instruction, I believe that everything in DPDK distribution > is compiled with > optimization. So the execution speed in not a concern here. Perhaps there > are cases where > it's compiled without optimization, like debugging, but then I believe it's > a non-issue. Yes you're right about optimization. But can we be 100% sure that it is always well optimized? > Hope my explanations shed some more light on this patch. :-) If we can be sure that there is no cost on optimized code, I am not against this patch. It may be especially useful when not complying to the DPDK coding rules, like in applications.