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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43ab37403cfsm55655745e9.1.2025.02.26.14.29.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Feb 2025 14:29:13 -0800 (PST) Date: Wed, 26 Feb 2025 22:29:11 +0000 From: David Laight To: Yury Norov Cc: Kuan-Wei Chiu , tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, jk@ozlabs.org, joel@jms.id.au, eajames@linux.ibm.com, andrzej.hajda@intel.com, neil.armstrong@linaro.org, rfoss@kernel.org, maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@gmail.com, simona@ffwll.ch, dmitry.torokhov@gmail.com, mchehab@kernel.org, awalls@md.metrocast.net, hverkuil@xs4all.nl, miquel.raynal@bootlin.com, richard@nod.at, vigneshr@ti.com, louis.peens@corigine.com, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, pabeni@redhat.com, parthiban.veerasooran@microchip.com, arend.vanspriel@broadcom.com, johannes@sipsolutions.net, gregkh@linuxfoundation.org, jirislaby@kernel.org, akpm@linux-foundation.org, hpa@zytor.com, alistair@popple.id.au, linux@rasmusvillemoes.dk, Laurent.pinchart@ideasonboard.com, jonas@kwiboo.se, jernej.skrabec@gmail.com, kuba@kernel.org, linux-kernel@vger.kernel.org, linux-fsi@lists.ozlabs.org, dri-devel@lists.freedesktop.org, linux-input@vger.kernel.org, linux-media@vger.kernel.org, linux-mtd@lists.infradead.org, oss-drivers@corigine.com, netdev@vger.kernel.org, linux-wireless@vger.kernel.org, brcm80211@lists.linux.dev, brcm80211-dev-list.pdl@broadcom.com, linux-serial@vger.kernel.org, bpf@vger.kernel.org, jserv@ccns.ncku.edu.tw, Yu-Chun Lin Subject: Re: [PATCH 02/17] bitops: Add generic parity calculation for u64 Message-ID: <20250226222911.22cb0c18@pumpkin> In-Reply-To: References: <20250223164217.2139331-1-visitorckw@gmail.com> <20250223164217.2139331-3-visitorckw@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: netdev@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 Mon, 24 Feb 2025 14:27:03 -0500 Yury Norov wrote: .... > +#define parity(val) \ > +({ \ > + u64 __v = (val); \ > + int __ret; \ > + switch (BITS_PER_TYPE(val)) { \ > + case 64: \ > + __v ^= __v >> 32; \ > + fallthrough; \ > + case 32: \ > + __v ^= __v >> 16; \ > + fallthrough; \ > + case 16: \ > + __v ^= __v >> 8; \ > + fallthrough; \ > + case 8: \ > + __v ^= __v >> 4; \ > + __ret = (0x6996 >> (__v & 0xf)) & 1; \ > + break; \ > + default: \ > + BUILD_BUG(); \ > + } \ > + __ret; \ > +}) > + You really don't want to do that! gcc makes a right hash of it for x86 (32bit). See https://www.godbolt.org/z/jG8dv3cvs You do better using a __v32 after the 64bit xor. Even the 64bit version is probably sub-optimal (both gcc and clang). The whole lot ends up being a bit single register dependency chain. You want to do: mov %eax, %edx shrl $n, %eax xor %edx, %eax so that the 'mov' and 'shrl' can happen in the same clock (without relying on the register-register move being optimised out). I dropped in the arm64 for an example of where the magic shift of 6996 just adds an extra instruction. David