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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 07:27:01 +0000	[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

WARNING: multiple messages have this Message-ID (diff)
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: 20+ 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  8:45 ` Maksym Planeta
2011-03-27 11:33 ` Ingo Molnar
2011-03-27 11:33   ` Ingo Molnar
2011-03-27 16:22   ` Peter Hüwe
2011-03-27 16:22     ` Peter Hüwe
2011-03-27 17:15   ` Maksym Planeta
2011-03-27 17:15     ` Maksym Planeta
2011-03-28  5:08     ` Ingo Molnar
2011-03-28  5:08       ` Ingo Molnar
2011-03-28 19:33       ` Maksym Planeta
2011-03-28 19:33         ` Maksym Planeta
2011-03-28 19:44         ` H. Peter Anvin
2011-03-28 19:44           ` H. Peter Anvin
2011-04-01 14:08           ` Maksym Planeta
2011-04-01 14:08             ` Maksym Planeta
2011-03-28 19:47 ` H. Peter Anvin
2011-03-28 19:47   ` H. Peter Anvin
2011-03-29  7:27   ` Ingo Molnar [this message]
2011-03-29  7:27     ` Ingo Molnar

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