From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by sourceware.org (Postfix) with ESMTP id EF5FF3860C32 for ; Wed, 4 Aug 2021 10:18:24 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org EF5FF3860C32 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: sourceware.org; spf=fail smtp.mailfrom=kernel.crashing.org Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 174AHNdO011268; Wed, 4 Aug 2021 05:17:23 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 174AHMQi011267; Wed, 4 Aug 2021 05:17:22 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 4 Aug 2021 05:17:22 -0500 From: Segher Boessenkool To: Prathamesh Kulkarni Cc: GCC Development , Richard Earnshaw Subject: Re: [RFC] Adding a new attribute to function param to mark it as constant Message-ID: <20210804101722.GD1583@gate.crashing.org> References: <20210803215538.GU1583@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.4.2.3i X-Spam-Status: No, score=-5.8 required=5.0 tests=BAYES_00, JMQ_SPF_NEUTRAL, KAM_DMARC_STATUS, TXREP, T_SPF_HELO_PERMERROR, T_SPF_PERMERROR autolearn=no autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gcc@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 04 Aug 2021 10:18:26 -0000 On Wed, Aug 04, 2021 at 03:20:45PM +0530, Prathamesh Kulkarni wrote: > On Wed, 4 Aug 2021 at 03:27, Segher Boessenkool > wrote: > > The Linux kernel has a macro __is_constexpr to test if something is an > > integer constant expression, see . That is a much > > better idea imo. There could be a builtin for that of course, but an > > attribute is less powerful, less usable, less useful. > Hi Segher, > Thanks for the suggestions. I am not sure tho if we could use a macro > similar to __is_constexpr > to check if parameter is constant inside an inline function (which is > the case for intrinsics) ? I said we can make a builtin that returns if its arg is an ICE -- we do not have to do tricky tricks :-) The macro would work fine in an inline function though, or, where do you see potential problems? > For eg: > #define __is_constexpr(x) \ > (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) > > inline int foo(const int x) > { > _Static_assert (__is_constexpr (x)); > return x; > } > > int main() > { > return foo (1); > } > > results in: > foo.c: In function ‘foo’: > foo.c:8:3: error: static assertion failed > 8 | _Static_assert (__is_constexpr (x)); And that is correct, x is *not* an integer constant expression here. Because it is a variable, instead :-) If you do this in a macro it should work though? > Initially we tried to use __Static_assert (__builtin_constant_p (arg)) > for the same purpose but that did not work > because while parsing the intrinsic function, the FE cannot determine > if the arg is indeed a constant. Yes. If you want something like that you need to test very late during compilation whether something is a constant then: it will not be earlier. > I guess the static assertion or __is_constexpr would work only if the > intrinsic were defined as a macro instead of an inline function ? > Or am I misunderstanding ? Both __builtin_constant_p and __is_constexpr will not work in your use case (since a function argument is not a constant, let alone an ICE). It only becomes a constant value later on. The manual (for the former) says: You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC never returns 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and does not return 1 when you pass a constant numeric value to the inline function unless you specify the -O option. An integer constant expression is well-defined whatever the optimisation level is, it is a feature of the language. If some x is an ICE you can do asm ("" :: "n"(x)); and if it is a constant you can do asm ("" :: "i"(x)); (not that that gets you much further, but it might help explorng this). Segher