Re: [Qemu-devel] Re: branches are expensive

[Avi Kivity <avi@redhat.com>]

Paul Brook wrote:
>> The ratio is quite bad. Do you have any documentation on when Qemu does the
>> chaining and more important, when it does not. For example are
>> unconditional jumps always chained, or only in one direction (forward or
>> backward).
>>     
>
> Direct jumps[1] within the same page are chained (including ). Indirect 
> jumps[2] and direct jumps to a different page are not chained. Chaining jumps 
> between pages would require breaking TB chains every time a TLB flush occurs.
>   

You could optimize interpage direct jumps as follows:

    if (tb->tlb_generation != global_tlb_generation)
        revalidate_interpage_branch();
    asm ("B target_address")

A tlb flush (or switching execution to a different cpu) increments 
global_tlb_generation; revalidate_interpage_branch() sets target_address 
to the slow path which does the tb lookup, and sets tlb_generation = 
global_tlb_generation.  Should compile an unconditional branch to 5 
instructions.

-- 
error compiling committee.c: too many arguments to function