linux-kernel - Re: [PATCH 3/4] x86_64 ia32 syscall audit fast-path

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Date:	Mon, 7 Jul 2008 17:37:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Roland McGrath <roland@...hat.com>
cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <aviro@...hat.com>,
	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86_64 ia32 syscall audit fast-path

On Mon, 7 Jul 2008, Roland McGrath wrote:
>
> Is bt slower than testl?

On many microarchitectures, yes. Especially for a memory operand. 

That said, depending on the size of the constant, 'bt' may be _smaller_ 
than testl (8-bit constant vs 32-bit one). Which can make up for it.

> (I used bt there because I saw it used in entry_64.S for all cases
> of testing for only one bit at a time.

I haven't checked recent CPU's, it may not matter much on ones that 
support 64-bit. But bt with a memop was traditionally quite a bit more 
expensive than 'test'. 

I too am too lazy to check. Once it's in the slow-path, it doesn't much 
matter. We're talking a few cycles here.

		Linus
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