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Date: Wed, 31 Aug 2022 01:51:49 -0700 From: Yury Norov <yury.norov@...il.com> To: Vincent Mailhol <mailhol.vincent@...adoo.fr> Cc: Borislav Petkov <bp@...en8.de>, Nick Desaulniers <ndesaulniers@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, x86@...nel.org, Peter Zijlstra <peterz@...radead.org>, Dave Hansen <dave.hansen@...ux.intel.com>, "H . Peter Anvin" <hpa@...or.com>, Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>, Jan Beulich <JBeulich@...e.com>, Christophe Jaillet <christophe.jaillet@...adoo.fr>, Joe Perches <joe@...ches.com>, Josh Poimboeuf <jpoimboe@...nel.org> Subject: Re: [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions On Wed, Aug 31, 2022 at 04:57:40PM +0900, Vincent Mailhol wrote: > The compilers provide some builtin expression equivalent to the ffs(), > __ffs() and ffz() functions of the kernel. The kernel uses optimized > assembly which produces better code than the builtin > functions. However, such assembly code can not be folded when used > with constant expressions. > > This series relies on __builtin_constant_p to select the optimal solution: > > * use kernel assembly for non constant expressions > > * use compiler's __builtin function for constant expressions. > > > ** Statistics ** > > Patch 1/2 optimizes 26.7% of ffs() calls and patch 2/2 optimizes 27.9% > of __ffs() and ffz() calls (details of the calculation in each patch). Hi Vincent, Can you please add a test for this? We've recently added a very similar test_bitmap_const_eval() in lib/test_bitmap.c. dc34d5036692c ("lib: test_bitmap: add compile-time optimization/evaluations assertions") Would be nice to have something like this for ffs() and ffz() in lib/test_bitops.c. Please keep me in loop in case of new versions. Thanks, Yury
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