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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf3215066sm6248539f8f.11.2025.01.18.14.11.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 18 Jan 2025 14:11:25 -0800 (PST) Date: Sat, 18 Jan 2025 22:11:23 +0000 From: David Laight To: Linus Torvalds Cc: Guenter Roeck , David Laight , Arnd Bergmann , "linux-kernel@vger.kernel.org" , Jens Axboe , Matthew Wilcox , Christoph Hellwig , Andrew Morton , Andy Shevchenko , Dan Carpenter , "Jason A . Donenfeld" , "pedro.falcato@gmail.com" , Mateusz Guzik , "linux-mm@kvack.org" , Lorenzo Stoakes , intel-xe@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, David Airlie , Simona Vetter , Jani Nikula , Rodrigo Vivi Subject: Re: Buiild error in i915/xe (was: [PATCH next 4/7] minmax.h: Use BUILD_BUG_ON_MSG() for the lo < hi test in clamp()) Message-ID: <20250118221123.5bb65e64@pumpkin> In-Reply-To: References: <34d53778977747f19cce2abb287bb3e6@AcuMS.aculab.com> <20250118170959.3aa56f4d@pumpkin> <29ef57a1-e4dd-4d5d-8726-f1f79c698b66@roeck-us.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) 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-Transfer-Encoding: 7bit On Sat, 18 Jan 2025 13:21:39 -0800 Linus Torvalds wrote: > On Sat, 18 Jan 2025 at 09:49, Guenter Roeck wrote: > > > > No idea why the compiler would know that the values are invalid. > > It's not that the compiler knows tat they are invalid, but I bet what > happens is in scale() (and possibly other places that do similar > checks), which does this: > > WARN_ON(source_min > source_max); > ... > source_val = clamp(source_val, source_min, source_max); > > and the compiler notices that the ordering comparison in the first > WARN_ON() is the same as the one in clamp(), so it basically converts > the logic to > > if (source_min > source_max) { > WARN(..); > /* Do the clamp() knowing that source_min > source_max */ > source_val = clamp(source_val, source_min, source_max); > } else { > /* Do the clamp knowing that source_min <= source_max */ > source_val = clamp(source_val, source_min, source_max); > } > > (obviously I dropped the other WARN_ON in the conversion, it wasn't > relevant for this case). > > And now that first clamp() case is done with source_min > source_max, > and it triggers that build error because that's invalid. > > So the condition is not statically true in the *source* code, but in > the "I have moved code around to combine tests" case it now *is* > statically true as far as the compiler is concerned. Well spotted :-) One option would be to move the WARN_ON() below the clamp() and add an OPTIMISER_HIDE_VAR(source_max) between them. Or do something more sensible than the WARN(). Perhaps return target_min on any such errors? David > > Linus >