From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
"Borislav Petkov" <bp@suse.de>, "H . Peter Anvin" <hpa@zytor.com>,
"Andy Lutomirski" <luto@amacapital.net>,
"Brian Gerst" <brgerst@gmail.com>,
"Denys Vlasenko" <dvlasenk@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@kernel.org>, "Jiri Kosina" <jkosina@suse.cz>
Subject: Re: [PATCH] x86/hweight: Don't clobber %rdi
Date: Mon, 8 Aug 2016 20:55:09 +0200 [thread overview]
Message-ID: <20160808185509.GG511@nazgul.tnic> (raw)
In-Reply-To: <CA+55aFzC7O16BASN08yKr2XBaBnQbsWWMhweUk_ErB-Mm9Jm=A@mail.gmail.com>
On Mon, Aug 08, 2016 at 11:45:58AM -0700, Linus Torvalds wrote:
> That would work (although the clobbered registers have a different
> syntax than the in/out registers), but it would be wrong, in my
> opinion.
>
> We want the actual POPCNT instruction to be the common case, and that
> instruction does *not* clobber any other registers than the output.
Ok, that's a good point. We would be punishing the common case.
> So I think it's much better to just say: "the __sw_hweight functions
> should have the same semantics as popcnt" (although without the eflags
> rules that we don't care about).
>
> There's nothing wrong with keeping it as assembly language - it's not
> like it's a maintenance headache once it is written.
Yap, makes a whole lotta sense to me.
Thanks!
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
--
prev parent reply other threads:[~2016-08-08 18:55 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-08 17:35 [PATCH] x86/hweight: Don't clobber %rdi ville.syrjala
2016-08-08 17:58 ` Linus Torvalds
2016-08-08 18:32 ` Peter Zijlstra
2016-08-08 18:53 ` Jiri Kosina
2016-08-08 19:17 ` Borislav Petkov
2016-08-08 18:04 ` Borislav Petkov
2016-08-08 18:21 ` Linus Torvalds
2016-08-08 18:37 ` Borislav Petkov
2016-08-08 18:45 ` Linus Torvalds
2016-08-08 18:55 ` Borislav Petkov [this message]
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