From: Filip Navara <filip.navara@gmail.com>
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>
Subject: Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT a4b18c6] tcg/x86_64: generated dec/inc instead of sub/addwhen possible
Date: Tue, 29 Sep 2009 17:51:09 +0200 [thread overview]
Message-ID: <5b31733c0909290851h30e05edauf5c31a3d18fbfbf9@mail.gmail.com> (raw)
In-Reply-To: <761ea48b0909280121r13c14861y4a0a2683ffb79d2b@mail.gmail.com>
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. Since these instructions don't update
>> all of the flags, they cannot be issued in parallel to previous instructions
>> that do.
>
> That's true, but I think Icache misses will cost you more than
> the false dependencies; due to code size expansion in the
> translation process, optimizing rules should not be obeyed
> blindly. I 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
next prev parent reply other threads:[~2009-09-29 16:36 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <200909271613.n8RGDCLo020606@d01av04.pok.ibm.com>
2009-09-28 7:51 ` [Qemu-devel] Re: [Qemu-commits] [COMMIT a4b18c6] tcg/x86_64: generated dec/inc instead of sub/addwhen possible Avi Kivity
2009-09-28 8:21 ` Laurent Desnogues
2009-09-29 15:51 ` Filip Navara [this message]
2009-09-29 15:53 ` Laurent Desnogues
2009-09-29 15:55 ` Filip Navara
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