From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 32D2BE9539B for ; Wed, 4 Feb 2026 13:14:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=bJ/9e2DA8SBPV6XQ0gOXCK1heDCNhcbD1ElCKLLyP5Y=; b=Kno+HnVplDZesuiejIUHT2OfdD MCpcs9UoCvTGbOYAyZWH8tuwfRmbvch0xvXKo9nPqBKJbmTqq8+Ml2xkfce0tA1DQoysWb0nmJKZc +l1ZrhYq1swsE/oHezX6x+L3DAFdoP9jhZaCPhT68/ESDWFBIsGpnEGVjSFInGBUu7J4MVnyB8U57 okgiaPYL0+Fs4CQL2c6rGJnOhTpDCTArgc+awGr1Otq22M6yCfJU3xbCHw+J3iXNIOAwUIcvOCB5/ 4aoEiksSX+wEZz9T2XxgFrG/Q6seBogrrvJ2fNrIVq6wk21s8UcFTxpT2gilU+JdquQkYJ6T0oUTm EFg+9uAw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vnchp-00000008U0a-44m9; Wed, 04 Feb 2026 13:14:05 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vncho-00000008U0T-0pB0 for linux-arm-kernel@bombadil.infradead.org; Wed, 04 Feb 2026 13:14:04 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=bJ/9e2DA8SBPV6XQ0gOXCK1heDCNhcbD1ElCKLLyP5Y=; b=WFAAd/9yNOh7RV0efTlEPqxbue x8Q0Sctd+c4ROAGh4fPza2KWnZ6NSspi1NkxcWZMYEKdD+djm3RkQBGSvB0S1ZC1LO/hqlLylBf6I neHjzbqbnMWfGwDrxCUOQFuhRfjtX4+V4EXokqRhSiDhtjkJz+dZQi1PMf3ATfaDJm/jxy7k/W9W3 2uuwOfK7dXsmIx1S+wxWUhcw0y3Gz3xiuQ7GeYK9kulncMPLXxkmzR95F0N2LZhlR5mBlBdHZAKRf ok4TLbfXUGC6naV7ncg9uCSyEK6b+J2YO+17bNs4rnhlb+f/dPHOi9Joymio581Xd05yFd48PtjSP l9QC++HA==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vnchl-000000026tM-0NwX; Wed, 04 Feb 2026 13:14:01 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 73BA5300E8B; Wed, 04 Feb 2026 14:14:00 +0100 (CET) Date: Wed, 4 Feb 2026 14:14:00 +0100 From: Peter Zijlstra To: Marco Elver Cc: Will Deacon , David Laight , Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, Catalin Marinas , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kernel test robot , Boqun Feng , Linus Torvalds Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y Message-ID: <20260204131400.GI2995752@noisy.programming.kicks-ass.net> References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-4-elver@google.com> <20260202192923.0707e463@pumpkin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Feb 04, 2026 at 11:46:02AM +0100, Marco Elver wrote: > On Tue, 3 Feb 2026 at 12:47, Will Deacon wrote: > [...] > > > > What does GCC do with this? :/ > > > > > > GCC currently doesn't see it, LTO is clang only. > > > > LTO is just one way that a compiler could end up breaking dependency > > chains, so I really want to maintain the option to enable this path for > > GCC in case we run into problems caused by other optimisations in future. > > It will work for GCC, but only from GCC 11. Before that __auto_type > does not drop qualifiers: > https://godbolt.org/z/sc5bcnzKd (switch to GCC 11 to see it compile) > > So to summarize, all supported Clang versions deal with __auto_type > correctly for the fallback; GCC from version 11 does (kernel currently > supports GCC 8 and above). From GCC 14 and Clang 19 we have > __typeof_unqual__. > > I really don't see another way forward; there's no other good way to > solve this issue. I would advise against pessimizing new compilers and > features because maybe one day we might still want to enable this > version of READ_ONCE() for GCC 8-10. > > Should we one day choose to enable this READ_ONCE() version for GCC, > we will (a) either have bumped the minimum GCC version to 11+, or (b) > we can only do so from GCC 11. At this point GCC 11 was released 5 > years ago! There is, from this thread: https://lkml.kernel.org/r/20260111182010.GH3634291@ZenIV another trick to strip qualifiers: #define unqual_non_array(T) __typeof__(((T(*)(void))0)()) which will work from GCC-8.4 onwards. Arguably, it should be possible to raise the minimum from 8 to 8.4 (IMO). But yes; in general I think it is fine to have 'old' compilers generate suboptimal code.