From: Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
To: Borislav Petkov <bp@alien8.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H . Peter Anvin" <hpa@zytor.com>,
Nathan Chancellor <nathan@kernel.org>, Tom Rix <trix@redhat.com>,
linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
David Howells <dhowells@redhat.com>,
Jan Beulich <JBeulich@suse.com>,
Christophe Jaillet <christophe.jaillet@wanadoo.fr>,
Joe Perches <joe@perches.com>,
Josh Poimboeuf <jpoimboe@kernel.org>
Subject: Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions
Date: Fri, 12 Aug 2022 20:55:11 +0900 [thread overview]
Message-ID: <CAMZ6Rq+k-5PKesdW7z9DdDMUNSKQR-c1qmQADvJ6VKZA2CM8nw@mail.gmail.com> (raw)
In-Reply-To: <YvUZVYxbOMcZtR5G@zn.tnic>
Hi Borislav,
On Thu. 11 Aug 2022 at 23:59, Borislav Petkov <bp@alien8.de> wrote:
> On Sun, Jul 24, 2022 at 12:15:20AM +0900, Vincent Mailhol wrote:
> > For x86_64, the current ffs() implementation does not produce
> > optimized code when called with a constant expression. On the
> > contrary, the __builtin_ffs() function of both GCC and clang is able
> > to simplify the expression into a single instruction.
> >
> > * Example *
> >
> > Let's consider two dummy functions foo() and bar() as below:
> >
> > | #include <linux/bitops.h>
> > | #define CONST 0x01000000
>
> Those code examples you can simply indent with two spaces.
>
> > In both examples, we clearly see the benefit of using __builtin_ffs()
>
> Who's "we"?
>
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
>
> > instead of the kernel's asm implementation for constant expressions.
> >
> > However, for non constant expressions, the ffs() asm version of the
> > kernel remains better for x86_64 because, contrary to GCC, it doesn't
> > emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is
> > able optimize out the CMOV call).
> >
> > This patch uses the __builtin_constant_p() to select between the
>
> Avoid having "This patch" or "This commit" in the commit message. It is
> tautologically useless.
>
> Also, do
>
> $ git grep 'This patch' Documentation/process
>
> for more details.
>
> > kernel's ffs() and the __builtin_ffs() depending on whether the
> > argument is constant or not.
>
> In general, you don't have to say what the patch does - that should be
> visible from the diff. The more important part is the *why*. And that
> you do.
>
> Rest looks ok.
Thank you for the review!
I addressed all your comments and sent a v5:
https://lore.kernel.org/all/20220812114438.1574-1-mailhol.vincent@wanadoo.fr/
Yours sincerely,
Vincent Mailhol
next prev parent reply other threads:[~2022-08-12 11:55 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-11 16:03 [PATCH v2 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions Vincent Mailhol
2022-05-11 16:03 ` [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-11 20:56 ` Christophe JAILLET
2022-05-11 23:30 ` Vincent MAILHOL
2022-05-11 21:35 ` Nick Desaulniers
2022-05-11 23:48 ` Vincent MAILHOL
2022-05-11 16:03 ` [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-11 22:20 ` Nick Desaulniers
2022-05-11 23:23 ` Vincent MAILHOL
2022-05-12 0:03 ` [PATCH v3 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-05-12 0:03 ` [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-12 0:28 ` Nick Desaulniers
2022-05-12 1:18 ` Vincent MAILHOL
2022-05-12 0:03 ` [PATCH v3 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-12 0:19 ` Nick Desaulniers
2022-05-12 1:18 ` [PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-05-12 1:18 ` [PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-12 3:02 ` Joe Perches
2022-05-12 4:29 ` Vincent MAILHOL
2022-05-12 1:18 ` [PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-23 9:22 ` [PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent MAILHOL
2022-06-25 7:26 ` [RESEND PATCH " Vincent Mailhol
2022-06-25 7:26 ` [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-06-25 7:26 ` [RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-07-23 15:15 ` [RESEND PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-07-23 15:15 ` [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-08-11 14:59 ` Borislav Petkov
2022-08-12 11:55 ` Vincent MAILHOL [this message]
2022-07-23 15:15 ` [RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-07-29 11:24 ` [RESEND PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent MAILHOL
2022-07-29 12:22 ` Borislav Petkov
2022-07-29 13:50 ` Vincent MAILHOL
2022-08-12 11:44 ` [PATCH v5 " Vincent Mailhol
2022-08-12 11:44 ` [PATCH v5 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-08-12 11:44 ` [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-08-23 16:23 ` Borislav Petkov
2022-08-23 17:12 ` Nick Desaulniers
2022-08-23 17:43 ` Borislav Petkov
2022-08-23 20:31 ` Vincent MAILHOL
2022-08-24 8:43 ` Borislav Petkov
2022-08-24 12:10 ` Vincent MAILHOL
2022-08-24 13:24 ` Borislav Petkov
2022-08-26 21:32 ` Vincent MAILHOL
2022-09-07 4:06 ` Borislav Petkov
2022-09-07 5:35 ` Vincent MAILHOL
2022-09-07 8:50 ` Borislav Petkov
2022-08-31 7:57 ` [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-08-31 7:57 ` [PATCH v6 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-08-31 7:57 ` [PATCH v6 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-08-31 8:51 ` [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Yury Norov
2022-09-01 3:49 ` Yury Norov
2022-09-01 10:30 ` Vincent MAILHOL
2022-09-01 14:19 ` Yury Norov
2022-09-01 17:06 ` Nick Desaulniers
2022-09-02 5:34 ` Borislav Petkov
2022-09-02 0:41 ` Vincent MAILHOL
2022-09-02 1:19 ` Vincent MAILHOL
2022-09-05 0:37 ` [PATCH v7 " Vincent Mailhol
2022-09-05 0:37 ` [PATCH v7 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-09-05 0:37 ` [PATCH v7 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-09-06 18:26 ` [PATCH v7 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Nick Desaulniers
2022-09-07 7:04 ` Nick Desaulniers
2022-09-07 7:49 ` Vincent MAILHOL
2022-09-07 9:09 ` [PATCH v8 " Vincent Mailhol
2022-09-07 9:09 ` [PATCH v8 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-09-07 9:09 ` [PATCH v8 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-09-20 20:00 ` [tip: x86/asm] x86/asm/bitops: Use __builtin_ctzl() " tip-bot2 for Vincent Mailhol
2022-09-20 20:00 ` [tip: x86/asm] x86/asm/bitops: Use __builtin_ffs() " tip-bot2 for Vincent Mailhol
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