From: Solar Designer <solar@openwall.com>
To: musl@lists.openwall.com
Subject: Re: tough choice on thread pointer initialization issue
Date: Fri, 10 Feb 2012 15:22:38 +0400 [thread overview]
Message-ID: <20120210112238.GA14906@openwall.com> (raw)
In-Reply-To: <20120210104252.GA14485@openwall.com>
On Fri, Feb 10, 2012 at 02:42:52PM +0400, Solar Designer wrote:
> [...] I just did some testing on
> a Core 2'ish CPU (E5420), and the instruction is very fast. I put 1000
> instances of:
>
> movl %ss,%eax
> testl %eax,%eax
> jz 0
>
> one after another, and this runs in just 1 cycle per the three
> instructions above (so 1000 cycles for the 1000 instances total).
To test my approach to testing this, I tried using:
movw %ss,%ax
followed by 1000 instances of:
movl %eax,%gs
or:
movw %ax,%gs
This executes in about 16.5 cycles per instruction. Indeed, setting a
segment register is slow - but reading it apparently is not, at least
not on this CPU.
Alexander
next prev parent reply other threads:[~2012-02-10 11:22 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-10 2:58 Rich Felker
2012-02-10 7:40 ` Solar Designer
2012-02-10 7:58 ` Rich Felker
2012-02-10 10:42 ` Solar Designer
2012-02-10 11:22 ` Solar Designer [this message]
2012-02-10 17:00 ` Rich Felker
2012-02-10 19:12 ` Solar Designer
2012-02-25 6:56 ` Rich Felker
2012-02-25 13:32 ` Rich Felker
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