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 2FC29385B835 for ; Fri, 17 Apr 2020 01:48:54 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 2FC29385B835 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: sourceware.org; spf=fail smtp.mailfrom=segher@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 03H1mXiP021913; Thu, 16 Apr 2020 20:48:33 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 03H1mVtT021912; Thu, 16 Apr 2020 20:48:31 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 16 Apr 2020 20:48:31 -0500 From: Segher Boessenkool To: Rich Felker Cc: Florian Weimer , musl@lists.openwall.com, libc-alpha@sourceware.org, linuxppc-dev@lists.ozlabs.org, Nicholas Piggin , libc-dev@lists.llvm.org Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2 Message-ID: <20200417014831.GL26902@gate.crashing.org> References: <1586931450.ub4c8cq8dj.astroid@bobo.none> <20200415225539.GL11469@brightrain.aerifal.cx> <87k12gf32r.fsf@mid.deneb.enyo.de> <20200416153509.GT11469@brightrain.aerifal.cx> <87sgh3e613.fsf@mid.deneb.enyo.de> <20200416165257.GY11469@brightrain.aerifal.cx> <87ftd3e1vg.fsf@mid.deneb.enyo.de> <20200416230235.GG26902@gate.crashing.org> <20200417003442.GD11469@brightrain.aerifal.cx> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200417003442.GD11469@brightrain.aerifal.cx> User-Agent: Mutt/1.4.2.3i X-Spam-Status: No, score=-14.7 required=5.0 tests=BAYES_00, JMQ_SPF_NEUTRAL, KAM_DMARC_STATUS, KAM_NUMSUBJECT, RCVD_IN_DNSWL_NONE, TXREP, T_SPF_HELO_PERMERROR, T_SPF_PERMERROR autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 17 Apr 2020 01:48:55 -0000 On Thu, Apr 16, 2020 at 08:34:42PM -0400, Rich Felker wrote: > On Thu, Apr 16, 2020 at 06:02:35PM -0500, Segher Boessenkool wrote: > > On Thu, Apr 16, 2020 at 08:12:19PM +0200, Florian Weimer wrote: > > > > I think my choice would be just making the inline syscall be a single > > > > call insn to an asm source file that out-of-lines the loading of TOC > > > > pointer and call through it or branch based on hwcap so that it's not > > > > repeated all over the place. > > > > > > I don't know how problematic control flow out of an inline asm is on > > > POWER. But this is basically the -moutline-atomics approach. > > > > Control flow out of inline asm (other than with "asm goto") is not > > allowed at all, just like on any other target (and will not work in > > practice, either -- just like on any other target). But the suggestion > > was to use actual assembler code, not inline asm? > > Calling it control flow out of inline asm is something of a misnomer. > The enclosing state is not discarded or altered; the asm statement > exits normally, reaching the next instruction in the enclosing > block/function as soon as the call from the asm statement returns, > with all register/clobber constraints satisfied. Ah. That should always Just Work, then -- our ABIs guarantee you can. > Control flow out of inline asm would be more like longjmp, and it can > be valid -- for instance, you can implement coroutines this way > (assuming you switch stack correctly) or do longjmp this way (jumping > to the location saved by setjmp). But it's not what'd be happening > here. Yeah, you cannot do that in C, not without making assumptions about what machine code the compiler generates. GCC explicitly disallows it, too: 'asm' statements may not perform jumps into other 'asm' statements, only to the listed GOTOLABELS. GCC's optimizers do not know about other jumps; therefore they cannot take account of them when deciding how to optimize. Segher