From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id BBF3C6B0253 for ; Sat, 16 Jul 2016 20:45:06 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id 63so298766642pfx.3 for ; Sat, 16 Jul 2016 17:45:06 -0700 (PDT) Received: from out4435.biz.mail.alibaba.com (out4435.biz.mail.alibaba.com. [47.88.44.35]) by mx.google.com with ESMTP id 77si983818pft.11.2016.07.16.17.45.04 for ; Sat, 16 Jul 2016 17:45:05 -0700 (PDT) Message-ID: <578AD67F.9030905@emindsoft.com.cn> Date: Sun, 17 Jul 2016 08:51:11 +0800 From: Chen Gang MIME-Version: 1.0 Subject: Re: [PATCH] mm: migrate: Use bool instead of int for the return value of PageMovable References: <1468079704-5477-1-git-send-email-chengang@emindsoft.com.cn> <20160711002605.GD31817@bbox> <5783F7DE.9020203@emindsoft.com.cn> <20160712074841.GE14586@dhcp22.suse.cz> <57851FC4.4000000@emindsoft.com.cn> <20160713075346.GC28723@dhcp22.suse.cz> In-Reply-To: <20160713075346.GC28723@dhcp22.suse.cz> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Minchan Kim , akpm@linux-foundation.org, vbabka@suse.cz, mgorman@techsingularity.net, gi-oh.kim@profitbricks.com, iamjoonsoo.kim@lge.com, hillf.zj@alibaba-inc.com, rientjes@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chen Gang On 7/13/16 15:53, Michal Hocko wrote: > On Wed 13-07-16 00:50:12, Chen Gang wrote: >> >> >> On 7/12/16 15:48, Michal Hocko wrote: >>> On Tue 12-07-16 03:47:42, Chen Gang wrote: >>> [...] >>>> In our case, the 2 output size are same, but under x86_64, the insns are >>>> different. After uses bool, it uses push/pop instead of branch, for me, >>>> it should be a little better for catching. >>> >>> The code generated for bool version looks much worse. Look at the fast >>> path. Gcc tries to reuse the retq from the fast path in the bool case >>> and so it has to push rbp and rbx on the stack. >>> >>> That being said, gcc doesn't seem to generate a better code for bool so >>> I do not think this is really worth it. >>> >> >> The code below also merge 3 statements into 1 return statement, although >> for me, it is a little more readable, it will generate a little bad code. >> That is the reason why the output looks a little bad. >> >> In our case, for gcc 6.0, using bool instead of int for bool function >> will get the same output under x86_64. > > If the output is same then there is no reason to change it. > For the new version gcc, the output is same. But bool is a little more readable than int for the pure bool function. But for the current widely used gcc version (I guess, gcc-4.8 is still widely used), bool will get a little better output than int for the pure bool function. >> In our case, for gcc 4.8, using bool instead of int for bool function >> will get a little better output under x86_64. > > I had a different impression and the fast path code had more > instructions. But anyway, is there really a strong reason to change > those return values in the first place? Isn't that just a pointless code > churn? > Excuse me, maybe, I do not quite understand your meanings, but I shall try to explain as far as I can understand (welcome additional detail explanation, e.g. "return values" means c code or assembly output code). In the previous reply, I did not mention 3 things directly and clearly (about my 2 mistakes, and the comparation between gcc 6.0 and 4.8): - Mistake 1: "Use one return statement instead of several statements" is not good, the modification may be a little more readable, but it may get a little bad output code by compiler. - Mistake 2: I only notice there is more branches, but did not notice the real execution path (I guess, your "fast path" is about it). - The optimization of upstream gcc 6.0 is better than redhat gcc 4.8: in this case, gcc 6.0 will: generate the same better code for both bool and int for the pure bool function. optimize the first checking branch (no prologue) -- gcc 4.8 need mark 'likely' for it. skip the 'likely' optimization when "use 1 return statement instead of several statements" -- generation a little bad code too. All together, for me: - Only use bool instead of int for pure bool functions' return value will get a little better code - I shall send patch v2, only change bool to int for all Page_XXX, and keep all the other things no touch. Thanks. -- Chen Gang (e??a??) Managing Natural Environments is the Duty of Human Beings. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org