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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43b7370372dsm35525085e9.11.2025.02.27.13.57.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Feb 2025 13:57:43 -0800 (PST) Date: Thu, 27 Feb 2025 21:57:41 +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: <20250227215741.1c2e382f@pumpkin> In-Reply-To: References: <20250223164217.2139331-1-visitorckw@gmail.com> <20250223164217.2139331-3-visitorckw@gmail.com> <20250226222911.22cb0c18@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-input@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 Thu, 27 Feb 2025 13:05:29 -0500 Yury Norov wrote: > On Wed, Feb 26, 2025 at 10:29:11PM +0000, David Laight wrote: > > 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 > > GCC fails to even understand this. Of course, the __v should be an > __auto_type. But that way GCC fails to understand that case 64 is > a dead code for all smaller type and throws a false-positive > Wshift-count-overflow. This is a known issue, unfixed for 25 years! Just do __v ^= __v >> 16 >> 16 > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210 > > > You do better using a __v32 after the 64bit xor. > > It should be an __auto_type. I already mentioned. So because of that, > we can either do something like this: > > #define parity(val) \ > ({ \ > #ifdef CLANG \ > __auto_type __v = (val); \ > #else /* GCC; because of this and that */ \ > u64 __v = (val); \ > #endif \ > int __ret; \ > > Or simply disable Wshift-count-overflow for GCC. For 64bit values on 32bit it is probably better to do: int p32(unsigned long long x) { unsigned int lo = x; lo ^= x >> 32; lo ^= lo >> 16; lo ^= lo >> 8; lo ^= lo >> 4; return (0x6996 >> (lo & 0xf)) & 1; } That stops the compiler doing 64bit shifts (ok on x86, but probably not elsewhere). It is likely to be reasonably optimal for most 64bit cpu as well. (For x86-64 it probably removes a load of REX prefix.) (It adds an extra instruction to arm because if its barrel shifter.) > > > 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: > > No, I don't. I want to have a sane compiler that does it for me. > > > 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. > > It's still unclear to me that this parity thing is used in hot paths. > If that holds, it's unclear that your hand-made version is better than > what's generated by GCC. I wasn't seriously considering doing that optimisation. Perhaps just hoping is might make a compiler person think :-) David > > Do you have any perf test? > > Thanks, > Yury