From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
Nick Desaulniers <ndesaulniers@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, Yury Norov <yury.norov@gmail.com>,
llvm@lists.linux.dev, Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH v1 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation
Date: Mon, 7 Nov 2022 10:38:59 +0100 [thread overview]
Message-ID: <Y2jSMz4GtXGJk6rS@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20221106095106.849154-2-mailhol.vincent@wanadoo.fr>
On Sun, Nov 06, 2022 at 06:51:05PM +0900, Vincent Mailhol wrote:
> The builtin implementation is better for two reasons:
>
> 1/ it saves two instructions on clang (a push and a stack pointer
> decrement) because of a useless tentative to save rax.
I'm thinking this is the same old clang-sucks-at-"rm" constraints and
*really* should not be a reason to change things. Clang should get fixed
already.
> 2/ when used on constant expressions, the compiler is only able to
> fold the builtin version (c.f. [2]).
>
> For those two reasons, replace the assembly implementation by its
> builtin counterpart.
>
> [1] https://elixir.bootlin.com/linux/v6.0/source/include/asm-generic/bitops/builtin-__fls.h
>
> [2] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions")
I would much prefer consistently with 146034fed6ee.
next prev parent reply other threads:[~2022-11-07 9:39 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-06 9:51 [PATCH v1 0/2] x86/asm/bitops: optimize fls functions for constant expressions Vincent Mailhol
2022-11-06 9:51 ` [PATCH v1 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation Vincent Mailhol
2022-11-07 9:38 ` Peter Zijlstra [this message]
2022-11-07 12:19 ` Vincent MAILHOL
2022-11-10 19:20 ` Nick Desaulniers
2022-11-10 19:06 ` Nick Desaulniers
2022-11-06 9:51 ` [PATCH v1 2/2] x86/asm/bitops: Use __builtin_clz*() to evaluate constant expressions Vincent Mailhol
2022-11-10 19:01 ` Nick Desaulniers
2022-11-11 1:57 ` Vincent MAILHOL
2022-11-11 3:36 ` Yury Norov
2022-11-11 7:02 ` Vincent MAILHOL
2022-11-25 2:33 ` [PATCH v2 0/2] x86/asm/bitops: optimize fls functions for " Vincent Mailhol
2022-11-25 2:33 ` [PATCH v2 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation Vincent Mailhol
2022-11-25 2:33 ` [PATCH v2 2/2] x86/asm/bitops: Use __builtin_clz*() to evaluate constant expressions Vincent Mailhol
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