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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43796ad015bsm23197171f8f.38.2026.02.15.15.18.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 15 Feb 2026 15:18:18 -0800 (PST) Date: Sun, 15 Feb 2026 23:18:16 +0000 From: David Laight To: Marco Elver Cc: Will Deacon , Peter Zijlstra , 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 , Al Viro Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y Message-ID: <20260215231816.2398e4f5@pumpkin> In-Reply-To: References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-4-elver@google.com> <20260202192923.0707e463@pumpkin> <20260204131400.GI2995752@noisy.programming.kicks-ass.net> <20260206182650.6c21b0ff@pumpkin> <20260215221656.68b2fc1d@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sun, 15 Feb 2026 23:43:23 +0100 Marco Elver wrote: > On Sun, 15 Feb 2026 at 23:16, David Laight wrote: > > > > On Sun, 15 Feb 2026 22:55:44 +0100 > > Marco Elver wrote: > > > > > On Fri, 6 Feb 2026 at 19:26, David Laight wrote: > > > > On Fri, 6 Feb 2026 16:09:35 +0100 > > > > Marco Elver wrote: > > > > > > > > > On Wed, 4 Feb 2026 at 15:15, Will Deacon wrote: > > > > > > > > > > > > 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 looks like an interesting option. > > > > > > > > > > > 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. > > > > > > > > > > From that discussion a month ago and this one, it seems we need > > > > > something to fix __unqual_scalar_typeof(). > > > > > > > > > > What's the way forward? > > > > > > > > > > 1. Bump minimum GCC version to 8.4. Replace __unqual_scalar_typeof() > > > > > for old compilers with the better unqual_non_array hack? > > > > > > > > > > 2. Leave __unqual_scalar_typeof() as-is. The patch "compiler: Use > > > > > __typeof_unqual__() for __unqual_scalar_typeof()" will fix the codegen > > > > > issues for new compilers. Doesn't fix not dropping 'const' for old > > > > > compilers for non-scalar types, and requires localized workarounds > > > > > (like this patch here). > > > > > > > > > > Either way we need a fix for this arm64 LTO version to fix the > > > > > context-analysis "see through" the inline asm (how this patch series > > > > > started). > > > > > > > > > > Option #1 needs a lot more due-diligence and testing that it all works > > > > > for all compilers and configs (opening Pandora's Box :-)). For option > > > > > #2 we just need these patches here to at least fix the acute issue > > > > > with this arm64 LTO version. > > > > > > > > Option 3. > > > > > > > > Look are where/why they are used and change the code to do it differently. > > > > Don't forget the similar __unsigned_scalar_typeof() in bitfield.h. > > > > (I posted a patch that nuked that one not long ago - used sizeof instead.) > > > > > > > > The one in minmax_array (in minmax.h) is particularly pointless. > > > > The value 'suffers' integer promotion as soon as it is used, nothing > > > > wrong with 'auto _x = x + 0' there. > > > > That will work elsewhere. > > > > > > Agreed that getting rid of __unqual_scalar_typeof() in favor of 'auto' > > > where possible is the way to go. > > > > > > Unfortunately I spent the last week occasionally glancing at this > > > arm64 READ_ONCE problem, and could not come up with something that > > > avoids using typeof_unqual() or __unqual_scalar_typeof(). I'm inclined > > > to go with the unqual_non_array hack, but not make this available as a > > > macro for general use - we have too many of these horrid macros, don't > > > want to add more to this hack pile. > > > > Agreed, having to do such things inside what are already horrid 'functions' > > is one thing, but when they get used in 'normal' code it is silly. > > > > Have you checked whether sizes other than 1, 2, 4 and 8 are ever used? > > There aren't any in an x86-64 allmodconfig build and it used to be an error. > > Even if there are handful, having to use a different define wouldn't > > really be an issue. > > Removing that support would make READ_ONCE() easier to write/understand > > and (hopefully) compile faster - there is a measurable cost for the > > 'size check' in the x86-64 build, the arm LTO expansion must be significant. > > I found e.g. xen_get_runstate_snapshot_cpu_delta() uses the >8 byte > case via __READ_ONCE(). READ_ONCE() itself is already restricted to <= > 8 bytes (due to that static assert), but that itself uses the > __READ_ONCE() helper which these patches were touching. One thing that might reduce the cost of that static_assert is to move the error_function out of it - defining that in every expansion can't help. A few places do that, but it really needs a helper - say: #define compiletime_assert_fn(fn, msg) \ __noreturn extern void fn(void) __compiletime_assert(msg) > > We could invert the game: have READ_ONCE() which just deals with <= 8 > bytes. And __READ_ONCE() which uses READ_ONCE() if <= 8 bytes, and the > non-atomic case if >8 bytes. However, I fear the static size check > won't go away because the asm-generic version of __READ_ONCE() happily > works on any type (it's just a volatile cast+deref) - I don't know how > we'd enforce the size limit otherwise. That should probably be a NON_ATOMIC_READ_ONCE() that doesn't 'fall-back'. David