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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a77b23fd3sm15290695e9.11.2026.04.27.19.04.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Apr 2026 19:04:45 -0700 (PDT) Date: Tue, 28 Apr 2026 03:04:43 +0100 From: David Laight To: Yury Norov Cc: Yury Norov , Rasmus Villemoes , linux-kernel@vger.kernel.org, Frank Binns , Alessio Belle , Brajesh Gupta , Alexandru Dadu , Vincent Mailhol , Matt Coster Subject: Re: [PATCH] bitfield: wire __bf_shf to __builtin_ctzll Message-ID: <20260428030443.521e91e3@pumpkin> In-Reply-To: <20260427225705.418802-1-ynorov@nvidia.com> References: <20260427225705.418802-1-ynorov@nvidia.com> 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 Mon, 27 Apr 2026 18:57:05 -0400 Yury Norov wrote: > __bf_shf() is currently based on built-in ffsll. It's more > straightforward to wire it to __builtin_ctzll, which makes it a pure > rename. > > Worth to notice that __builtin_ffsll() is buggy on GCC before 14.1: > > int main() { > sizeof(struct { > int t : !(__builtin_ffsll(~0ULL) + 1 < 0); > }); > } > > test.c: In function 'main': > test.c:3:21: error: bit-field 't' width not an integer constant > 3 | int t : !(__builtin_ffsll(~0ULL) + 1 < 0); > | ^ > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124699 > > Reported-by: Matt Coster > Closes: https://lore.kernel.org/oe-kbuild-all/202603222211.A2XiR1YU-lkp@intel.com/ > Signed-off-by: Yury Norov > --- > include/linux/bitfield.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h > index cd44013281c7..ddf6d1a70473 100644 > --- a/include/linux/bitfield.h > +++ b/include/linux/bitfield.h > @@ -44,7 +44,7 @@ > * FIELD_MODIFY(REG_FIELD_C, ®, c); > */ > > -#define __bf_shf(x) (__builtin_ffsll(x) - 1) > +#define __bf_shf __builtin_ctzll I'd leave in the (x). David > > #define __scalar_type_to_unsigned_cases(type) \ > unsigned type: (unsigned type)0, \