220 21285 article Path: news.gmane.org!not-for-mail From: Andrew Tomazos Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow inline functions to be evaluated in different contexts Date: Sun, 4 Oct 2015 22:38:45 +0200 Lines: 146 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a11c2b90c21228f05214d6274 X-Trace: ger.gmane.org 1443991129 5449 80.91.229.3 (4 Oct 2015 20:38:49 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 4 Oct 2015 20:38:49 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBD5KHQXXWYPRBVU4Y2YAKGQENPXFIOA@isocpp.org Sun Oct 04 22:38:49 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lb0-f198.google.com ([209.85.217.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Ziq3U-0008Th-3k for gclcip-std-proposals@m.gmane.org; Sun, 04 Oct 2015 22:38:48 +0200 Original-Received: by lbbti1 with SMTP id ti1sf30332741lbb.3 for ; Sun, 04 Oct 2015 13:38:47 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:content-type:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=AleJB1WDABkD7xwj7Fuw2Noi28EbflDqCb2s0UAers0=; b=C+wDSlSdHdwgBhLd4LYto8FjI/CbT09pW8PrGO2zDGQkofF92mifZJiX/oR/PibEWN qTNEWYm42Dq5nW7Lhzi8OsmyEsXJvA2ZFBP0bkpU5o7vJD8WR28Que1PwKAPleN3qqeM XpESRErqYUUpBBHFY21NzcOErIH+zSZ/3arWrfThwX60gaCVhi5Ea+vlOWSVWQcGnItg Di7zVnUC8ykzQxMZpTCTX/BvWWyVEDn0lPQ4qY+FisdpoH0FLTumuUskEu/8BvOc67qs rAEl29lbWPXmexdauUn60Ymfno+Gy0+mlmQ/9Mw+ZbTlRsenQJ5Xh0qEDdtTWE+10FtB 1YDg== X-Gm-Message-State: ALoCoQnfpj6UAazhfu4iBvdGklM3eroqrU2y1f7ggm7cwRpE3EvXQoyhiWMgOtW6Ew5tJ1jbpWwq X-Received: by 10.195.18.100 with SMTP id gl4mr4573768wjd.4.1443991127596; Sun, 04 Oct 2015 13:38:47 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.75.140 with SMTP id c12ls693658wiw.7.gmail; Sun, 04 Oct 2015 13:38:46 -0700 (PDT) X-Received: by 10.180.37.76 with SMTP id w12mr8266261wij.0.1443991126530; Sun, 04 Oct 2015 13:38:46 -0700 (PDT) Original-Received: from mail-wi0-x232.google.com (mail-wi0-x232.google.com. [2a00:1450:400c:c05::232]) by mx.google.com with ESMTPS id p10si26802344wjo.3.2015.10.04.13.38.46 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 04 Oct 2015 13:38:46 -0700 (PDT) Received-SPF: pass (google.com: domain of andrewtomazos@gmail.com designates 2a00:1450:400c:c05::232 as permitted sender) client-ip=2a00:1450:400c:c05::232; Original-Received: by wiclk2 with SMTP id lk2so89683241wic.1 for ; Sun, 04 Oct 2015 13:38:46 -0700 (PDT) X-Received: by 10.180.10.197 with SMTP id k5mr7345682wib.22.1443991126088; Sun, 04 Oct 2015 13:38:46 -0700 (PDT) Original-Received: by 10.28.73.137 with HTTP; Sun, 4 Oct 2015 13:38:45 -0700 (PDT) In-Reply-To: X-Original-Sender: andrewtomazos@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of andrewtomazos@gmail.com designates 2a00:1450:400c:c05::232 as permitted sender) smtp.mailfrom=andrewtomazos@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:21285 Archived-At: --001a11c2b90c21228f05214d6274 Content-Type: text/plain; charset=UTF-8 On Sun, Oct 4, 2015 at 7:48 PM, Jeremy Maitin-Shepard wrote: > On Sunday, October 4, 2015 at 8:07:56 AM UTC-7, Andrew Tomazos wrote: >> >> On Sun, Oct 4, 2015 at 2:52 PM, TONGARI J wrote: >> >>> A more reasonable way is to allow inline functions to be >>> context-agnostic so we can define `min` as: >>> >>> template >>> inline const T& min( const T& a, const T& b ) >>> { >>> return (b < a) ? b : a; >>> } >>> >>> When evaluated in constant context, the restrictions of >>> constant-expression are automatically imposed, >>> >> >> This has been suggested before. You are essentially asking that all >> functions that could be constexpr, be implicitly constexpr. Likewise for >> the other "contexts". >> >> The problem is compile-time. Compiling a constexpr function (for >> example) is expensive. Doing it automatically for all function definitions >> would increase compile times too much. >> >> You have to explicitly mark the functions you want to use in constexpr >> contexts, so the compiler knows it doesn't have to do the extra analysis >> for all the other ones. >> > > I'm not sure I understand this point. I am not really aware of exactly > how constexpr evaluation is handled in compilers, but I assume it comes > down to storing some type of AST when the function is parsed, and then > either evaluating directly based on that or possibly converting it to some > intermediate representation, which could presumably be done in a JIT > fashion when actually required. > I just tested it (gcc 5.2) and observed that a 5-line C++14 constexpr function definition takes 5% longer to compile (~105us) than the same inline function (~100us) (even if it isn't used). This is less than I expected, but still quite significant. It is unclear to me whether this completely highlights the performance concerns about implicit constexpr. Admittedly I am reporting them second-hand. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On S= un, Oct 4, 2015 at 7:48 PM, Jeremy Maitin-Shepard <jeremy@jeremyms.com> wrote:
On Sunday, October 4= , 2015 at 8:07:56 AM UTC-7, Andrew Tomazos wrote:
On Sun, Oct 4, 2015 at 2:52 PM, TONGARI J <tong...@gmail.com> wrote:
A more reasonable way is to allow inlin= e functions to be context-agnostic so we can define `min` as:

template<class T>
inline const T& min( const T& a, const T& b )
{
=C2=A0 =C2=A0 return (b < a) ? b : a;
}
<= br>
When evaluated in constant context, the restrictions of const= ant-expression are automatically imposed,

This has been suggested before.=C2=A0 Yo= u are essentially asking that all functions that could be constexpr, be imp= licitly constexpr.=C2=A0 Likewise for the other "contexts".
=

The problem is compile-time.=C2=A0 Compiling a constexp= r function (for example) is expensive.=C2=A0 Doing it automatically for all= function definitions would increase compile times too much.

=
You have to explicitly mark the functions you want to use in con= stexpr contexts, so the compiler knows it doesn't have to do the extra = analysis for all the other ones.

I&= #39;m not sure I understand this point.=C2=A0 I am not really aware of exac= tly how constexpr evaluation is handled in compilers, but I assume it comes= down to storing some type of AST when the function is parsed, and then eit= her evaluating directly based on that or possibly converting it to some int= ermediate representation, which could presumably be done in a JIT fashion w= hen actually required.

I just tested it (gcc 5.2) and observed that a 5-line C= ++14 constexpr function definition takes 5% longer to compile (~105us) than= the same inline function (~100us) (even if it isn't used).=C2=A0 This = is less than I expected, but still quite significant.

It is unclear to me whether= this completely highlights the performance concerns about implicit constex= pr.=C2=A0 Admittedly I am reporting them second-hand.

--

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