On 1 March 2017 at 09:50, oussema ben khedher <oussemake...@yahoo.fr> wrote: > I am trying to understand the block chaining of QEMU. And I have > a question about the branch direction of each executed TBs. > Suppose the TB#1 has been executed now, and find the next_tb (TB#2). > Then we know the direction is TB#1--->TB#2 and store the chain > in code cache for speedup the execution next time. So, next time > qemu find the next TB (which is TB#1), then TB#1--->TB#2 will be > executed in code cache. > But I think that there are two branch direction of a TB.(branch > taken/not taken) My question is how qemu ensure the direction is > TB#1--->TB#2 next time?
Every TB has two outbound links (though one may be unused for an unconditional branch). When we chain a TB together we only chain the link that we took. So if we do a "branch taken" exit from TB1 and find that it goes to TB2, then we link TB1's branch-taken code path directly to TB2. But that doesn't affect the branch-not-taken codepath. Later if we do a branch-not-taken exit from TB1 we may be able to chain it to a different TB. (In fact the outbound links are just numbered 0 and 1, and there's no requirement to use them for taken and not-taken in any particular order.) thanks -- PMM
