Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT a4b18c6] tcg/x86_64: generated dec/inc instead of sub/addwhen possible

[Filip Navara]

On Mon, Sep 28, 2009 at 10:21 AM, Laurent Desnogues
<address@hidden> wrote:
> On Mon, Sep 28, 2009 at 9:51 AM, Avi Kivity <address@hidden> wrote:
>> On 09/27/2009 06:13 PM, Anthony Liguori wrote:
>>>
>>> From: Aurelien Jarno<address@hidden>
>>>
>>
>> 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