[Qemu-devel] Re: branches are expensive

[Steffen Liebergeld]

Hi Avi,
Avi Kivity <address@hidden> schrieb:
> Steffen Liebergeld wrote:
>> Hi,
>>
>> while measuring the execution of an ARM guest, I encountered that branches 
>> are
>> extremely expensive in terms of executed host instructions. A single ARM
>> branch takes 148 to 152 host instructions. In my setup host and guest use the
>> ARM instruction set architecture.
>>
>> My question is what makes branches so expensive? What code is run when
>> executing a branch? Other instructions are translated to 14 to 40
>> instructions.
>>   
>
> I'm no tcg guru, but if branches are not chained, you'd need an 
> expensive lookup to find the next translation block.  If branches are 
> chained they'll probably be much faster.

Sure. But how can I get to know how many TBs are chained.

Greetings, Steffen