From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DB9CA28504D; Wed, 4 Feb 2026 14:15:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770214558; cv=none; b=qg4cAKBQGveJglFvMVkVbE3gypLNexPawfK01p6K/cjGsnRhj3MEbjJjk+GQLdV6Dg8wH3I5ZKflSXlVv5yHO21zywY5loITm0OZokV1lryUOHZFz8z03vOUYjbmr25sA/Q2++Vefe2Wwjyke6UqNpO5lBtwVz4IZGVLu3g0pq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770214558; c=relaxed/simple; bh=E1jdxwYVfm2leYwVacUOhzPIAuFdaG8WtIXWBNAtcUA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XAd2LrYyvey2SNmxeSuZsbQpmagGYD9CAFtS5DvKZd8s2M3HFA33blZ3/jrFeY5axsfjF9e5qBkdVH1Af4ISYnrFM1Vtv3ItZP7Lc14t4rYmdYUcaYDD74g1YNuDQthd1uETJWtj1OMijQJ2W287uRe6o3r2SP+pO/u+eyF8D+Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IEIGMLgZ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IEIGMLgZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 69DE9C4CEF7; Wed, 4 Feb 2026 14:15:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770214558; bh=E1jdxwYVfm2leYwVacUOhzPIAuFdaG8WtIXWBNAtcUA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=IEIGMLgZkHXkFgAdvwA8FEmf5l8TTj4oqyicVJQQbsnC/PBzaP9Pg5TjOmtwn7N52 p3Ff9aSftwE2aL1zZEcxyIeUJG7pBaGmHH3pS3H2o3nPmM5xovo4ttN/Q2rPbHJAlf WHOIGLKLG2RgfjYgDGKq6Dw+6Z0ul/20rGH3v4vIr9kX38dHhWEfy1JnRhdeBWSEoq /DvLYOR8jF9JfisiuA7zKSamB8M3X7RqKPGy/jYMB98MQW75D8BeHebA/CBJdLim8p Rq/WAmYpirMcBNY5UXSeni2aZ+ei8kWPsIQTihd3k50/1nYlxPzR7fF/8XXUlBMdnP Zii2AGtbOCkeA== Date: Wed, 4 Feb 2026 14:15:52 +0000 From: Will Deacon To: Peter Zijlstra Cc: Marco Elver , 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: References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-4-elver@google.com> <20260202192923.0707e463@pumpkin> <20260204131400.GI2995752@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260204131400.GI2995752@noisy.programming.kicks-ass.net> On Wed, Feb 04, 2026 at 02:14:00PM +0100, Peter Zijlstra wrote: > 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). That sounds reasonable to me but I'm not usually the one to push back on raising the minimum compiler version! > But yes; in general I think it is fine to have 'old' compilers generate > suboptimal code. I'm absolutely fine with the codegen being terrible for ancient toolchains as long as it's correct. Will