Re: [Qemu-devel] TCG and branches

[Richard Henderson]

On 07/31/2016 05:23 AM, Benjamin Herrenschmidt wrote:
> The first obvious thing that comes to mind is to avoid stopping the
> TB on a non-taken conditional branch. The reason that can't be done
> today, from my limited understanding of things, is because we only
> support index 0 and 1 today for gen_goto_tb().

Correct.

> Now I haven't completely figured out how TB linkage works under the
> hood but do you know of any fundamental reason why we have that limit ?
>
> Could we, for example, have a limit of, for example, 8 and only break
> the TB after a branch if we have less than 2 left ?

We do borrow low bits of a pointer (to a TranslationBlock) in implementing 
exit_tb.  See TB_EXIT_MASK and the large block comment just above its 
definition in tcg/tcg.h.

However, increasing the alignment of TranslationBlock ought to be trivial, 
giving you as many bits as required.  With that, I see no reason in principal 
that this wouldn't work.

Such a change would need some measurement to see how often this occurs, and how 
much this helps the actual runtime performance.


r~