From: Linus Torvalds <torvalds@linux-foundation.org>
To: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>,
Alexander Viro <aviro@redhat.com>, Eric Paris <eparis@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] x86_64 ia32 syscall audit fast-path
Date: Mon, 7 Jul 2008 17:37:57 -0700 (PDT) [thread overview]
Message-ID: <alpine.LFD.1.10.0807071734540.11076@woody.linux-foundation.org> (raw)
In-Reply-To: <20080708000233.E8EF3154244@magilla.localdomain>
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
next prev parent reply other threads:[~2008-07-08 0:38 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-07-07 6:56 [PATCH 1/4] x86_64: remove bogus optimization in sysret_signal Roland McGrath
2008-07-07 6:57 ` [PATCH 2/4] x86_64 syscall audit fast-path Roland McGrath
2008-07-07 6:57 ` [PATCH 3/4] x86_64 ia32 " Roland McGrath
2008-07-07 16:34 ` Linus Torvalds
2008-07-07 16:39 ` Linus Torvalds
2008-07-08 0:02 ` Roland McGrath
2008-07-08 0:37 ` Linus Torvalds [this message]
2008-07-18 11:36 ` Ingo Molnar
2008-07-21 23:54 ` Roland McGrath
2008-07-24 0:22 ` Linus Torvalds
2008-07-24 0:40 ` Linus Torvalds
2008-07-24 1:04 ` Roland McGrath
2008-07-07 6:58 ` [PATCH 4/4] i386 " Roland McGrath
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