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Mon, 27 Apr 2026 13:04:16 +0000 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Mon, 27 Apr 2026 14:04:15 +0100 Message-Id: Subject: Re: [PATCH V13 5/7] rust: Make __udivdi3() and __umoddi3() panic From: "Gary Guo" To: "David Laight" , "Gary Guo" Cc: "Mukesh Kumar Chaurasiya" , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , "Link Mauve" X-Mailer: aerc 0.21.0 References: <20260424054742.45832-1-mkchauras@gmail.com> <20260424054742.45832-6-mkchauras@gmail.com> <20260426230408.489c68c3@pumpkin> In-Reply-To: <20260426230408.489c68c3@pumpkin> X-ClientProxiedBy: LO6P123CA0007.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:338::9) To LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:488::16) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LOVP265MB8871:EE_|CW1P265MB9152:EE_ X-MS-Office365-Filtering-Correlation-Id: e7bcf125-2b74-49a4-5bfd-08dea45d7c71 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|10070799003|1800799024|22082099003|56012099003|18002099003; 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It feels like a better fi= x than >> >> stubbing things out. >> >>=20 >> >> Best, >> >> Gary >> >> =20 >> > The issue is not coming from libcore itself. It's the driver that's >> > causing this. =20 >>=20 >> Sorry. I quoted the wrong part. I was asking if compiling libcore with O= 2 gets >> rid of its use of the builtins, as that's what the change this commit is= for. >>=20 >> Formatting of u64 will be needed, so we should make sure that these work= s as >> intended. > > This code (from nolibc) will convert u64 to ascii in any base: > > [snip] > > Not hard to do without any divides at all. I mean, the exact same logic is when LLVM lowers code where divisor is cons= tant. It just that it decides that the multiply-by-inverse lowering shouldn't be = done with `Os`. In this case libcore is provided by Rust (think it as freestanding headers)= , that code is not part of kernel. And currently they just use constant-divis= or divide and have this task performed by LLVM. We could ask Rust to explicitl= y use multiply-by-inverse to avoid generating __udivdi3 in the future, but even i= f we do that it won't be available to already-released Rust compilers.=20 In the mean time we need to workaround this. Best, Gary