public inbox for linux-kernel@vger.kernel.org
 help / color / mirror / Atom feed
From: Ingo Molnar <mingo@elte.hu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Maksym Planeta <mcsim.planeta@gmail.com>,
	mingo@redhat.com, kernel-janitors@vger.kernel.org,
	namhyung@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86: page: get_order() optimization
Date: Tue, 29 Mar 2011 09:27:01 +0200	[thread overview]
Message-ID: <20110329072701.GJ27398@elte.hu> (raw)
In-Reply-To: <4D90E5ED.3080604@zytor.com>


* H. Peter Anvin <hpa@zytor.com> wrote:

> On 03/27/2011 01:45 AM, Maksym Planeta wrote:
> > For x86 architecture get_order function can be optimized due to
> > assembler instruction bsr.
> > 
> > This is second version of patch where for constants gcc precompute the
> > result.
> > 
> > Signed-off-by: Maksym Planeta <mcsim.planeta@gmail.com>
> 
> gcc 4.x has an intrinsic, __builtin_clz(), which does the opposite of
> the bsr instruction; specifically:
> 
> 	__builtin_clz(x) ^ 31
> 
> ... generates a bsrl instruction if x is variable.  This tends to
> generate much better code than any assembly hacks.

Indeed, that should work better and should be tried - and it can probably 
propagate the flags result sensibly (which GCC's asm() cannot, unfortunately).

Thanks,

	Ingo

      reply	other threads:[~2011-03-29  7:27 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-03-27  8:45 [PATCH v2] x86: page: get_order() optimization Maksym Planeta
2011-03-27 11:33 ` Ingo Molnar
2011-03-27 16:22   ` Peter Hüwe
2011-03-27 17:15   ` Maksym Planeta
2011-03-28  5:08     ` Ingo Molnar
2011-03-28 19:33       ` Maksym Planeta
2011-03-28 19:44         ` H. Peter Anvin
2011-04-01 14:08           ` Maksym Planeta
2011-03-28 19:47 ` H. Peter Anvin
2011-03-29  7:27   ` Ingo Molnar [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20110329072701.GJ27398@elte.hu \
    --to=mingo@elte.hu \
    --cc=hpa@zytor.com \
    --cc=kernel-janitors@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mcsim.planeta@gmail.com \
    --cc=mingo@redhat.com \
    --cc=namhyung@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox