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Date: Tue, 29 Sep 2009 17:51:09 +0200
Message-ID: <5b31733c0909290851h30e05edauf5c31a3d18fbfbf9@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT a4b18c6] tcg/x86_64:
	generated dec/inc instead of sub/addwhen possible
From: Filip Navara <filip.navara@gmail.com>
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To: Laurent Desnogues <laurent.desnogues@gmail.com>
Cc: Avi Kivity <avi@redhat.com>, Aurelien Jarno <aurelien@aurel32.net>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On Mon, Sep 28, 2009 at 10:21 AM, Laurent Desnogues
<laurent.desnogues@gmail.com> wrote:
> On Mon, Sep 28, 2009 at 9:51 AM, Avi Kivity <avi@redhat.com> wrote:
>> On 09/27/2009 06:13 PM, Anthony Liguori wrote:
>>>
>>> From: Aurelien Jarno<aurelien@aurel32.net>
>>>
>>
>> Intel recommends not using inc/dec. =A0Since these instructions don't up=
date
>> all of the flags, they cannot be issued in parallel to previous instruct=
ions
>> that do.
>
> That's true, but I think Icache misses will cost you more than
> the false dependencies; =A0due to code size expansion in the
> translation process, optimizing rules should not be obeyed
> blindly. =A0I guess this should be measured.

I vaguely remember measuring it and the results were different on
Pentium IV-based and Pentium M-based processors. On one of them
inc/dec was faster, while on the other it was add/sub. I can't
remember which one had which results though.

Best regards,
Filip Navara