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Tue, 20 Aug 2024 02:35:58 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Date: Tue, 20 Aug 2024 08:35:37 +0200 From: "Arnd Bergmann" To: "Anshuman Khandual" , linux-kernel@vger.kernel.org Cc: "Ard Biesheuvel" , "Andrew Morton" , "Yury Norov" , "Rasmus Villemoes" , Linux-Arch Message-Id: <8b780eda-bb64-4baf-8e24-501baf8ed8db@app.fastmail.com> In-Reply-To: <17020e56-b0a9-4705-8ee3-c675eca99490@arm.com> References: <20240801071646.682731-1-anshuman.khandual@arm.com> <20240801071646.682731-2-anshuman.khandual@arm.com> <090eb237-10f4-4358-be07-1eb8d30c3ec1@arm.com> <3b219e52-1d2a-4e6d-adff-efbab3e2282d@app.fastmail.com> <17020e56-b0a9-4705-8ee3-c675eca99490@arm.com> Subject: Re: [PATCH V3 1/2] uapi: Define GENMASK_U128 Content-Type: text/plain Content-Transfer-Encoding: 7bit On Tue, Aug 20, 2024, at 03:25, Anshuman Khandual wrote: > On 8/19/24 12:43, Arnd Bergmann wrote: > > Should not the second shift operation warn about the possible > overflow scenario ? But actually it does not. Or the compiler > is too smart in detecting what's happening next in the overall > equation and do the needful while creating the mask below the > highest bit. Not sure about the reasoning behind the compiler warning for one but not the other, but I know that we rely on similar behavior in places like: #define upper_32_bits(n) ((u32)(((n) >> 16) >> 16)) which is intended to return a zero without a compiler warning when passing an 'unsigned long' input on 32-bit architectures. Arnd