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Fri, 27 Mar 2026 05:02:37 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: AUJnWW6OA6Vc Date: Fri, 27 Mar 2026 10:02:17 +0100 From: "Arnd Bergmann" To: "Geert Uytterhoeven" Cc: "Russell King" , "Christian Schrefl" , "Miguel Ojeda" , "Alice Ryhl" , "Ard Biesheuvel" , "Jamie Cunliffe" , "Will Deacon" , "Catalin Marinas" , "Miguel Ojeda" , "Andreas Hindborg" , acourbot@nvidia.com, "Andrew Morton" , "Anton Ivanov" , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , "Boqun Feng" , "Danilo Krummrich" , "David Gow" , "Gary Guo" , "Johannes Berg" , "Justin Stitt" , linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-um@lists.infradead.org, llvm@lists.linux.dev, "Benno Lossin" , "Mark Rutland" , mmaurer@google.com, "Bill Wendling" , "Nathan Chancellor" , "Nick Desaulniers" , "Nicolas Schier" , "Nicolas Schier" , "Peter Zijlstra" , "Richard Weinberger" , rust-for-linux@vger.kernel.org, "Trevor Gross" , "Uladzislau Rezki (Sony)" , "John Paul Adrian Glaubitz" Message-Id: <93439e91-cf81-477b-b880-a813bb01ad7c@app.fastmail.com> In-Reply-To: References: <20260322192159.88138-1-ojeda@kernel.org> <20260323000327.111235-1-ojeda@kernel.org> <9cf5a94c-0f37-446c-b63d-ddac5674d220@gmail.com> <641592e3-2354-4238-9eb8-a1f5c2a13126@app.fastmail.com> Subject: Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO Content-Type: text/plain Content-Transfer-Encoding: 7bit On Fri, Mar 27, 2026, at 08:56, Geert Uytterhoeven wrote: >> >> I noticed a similar issue with m68k-linux, which has a bitfield >> alignment different from anything else on gcc, but uses the normal >> behavior on clang. > > Ugh, I wasn't aware of that. Adrian, did you know? To clarify, this illustrates what I mean here: echo 'struct { short a : 3; short b :15; short c :14; } x; int y = sizeof(x);' | m68k-linux-gcc -xc - -S -o- this produces '4' on m68k-linux-gcc, but '6' everywhere else. I originally thought this was related to this 2009 change in both compilers https://github.com/llvm/llvm-project/commit/830dfccfb2c06a1749938ce8cb78a1d6a295b942 https://gcc.gnu.org/cgit/gcc/commit/?id=2cd36c22d3d16e24f342829a989ac35f2f442656 but that only allowed bitfields to be marked as __attribute__((packed)) in order to get tightly packed fields and return '4' on all architectures, while m68k-linux-gcc apparently has all bitfields implicitly packed unless they are explicitly marked __attribute__((aligned(x))). This behavior is independent of the -malign-int flag. Arnd