Message-ID: <475FF4A1.4020401@keyaccess.nl>
Date: Wed, 12 Dec 2007 15:48:01 +0100
From: Rene Herman <rene.herman@keyaccess.nl>
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To: linux@horizon.com
CC: Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFT] Port 0x80 I/O speed
References: <20071212112049.17891.qmail@science.horizon.com>
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On 12-12-07 12:20, linux@horizon.com wrote:

> Here are a variety of machines:

Thanks much for all! Collecting all data now...

> With -O2, the cycle counts come out (before division) as
> out: 0xFFFFFFFFFFEA6F4F
> in:  0xFFFFFFFFFCE68BB6
> I think the "A" constraint doesn't work quite the same in
> 64-bit code.  The compiler seems to be using %rdx rather than
> %edx:%eax.

Yes indeed, that tripped me up. Have been using the "=A" locally for a while 
for similar timing tests. Will use a manual "=a" (lo), "=d" (hi) I guess for 
amd64 compatibility from now on.

If I'd care deeply I'd probably categorize this as a backwards compatibility 
bug in GCC though. Things were probably never guaranteed but they certainly 
worked that way...

Rene.
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