[Qemu-devel] TCG and branches

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi Richard !

So in my discovery of TCG, one thing I noticed is the horrendous amount
of code generated for branches, especially conditional ones.

I have a patch at least to remove a bunch of dead gunk on target-ppc
for non-conditional ones (we still generated the "else" of the
condition even when never branching to it).

However, I wonder if there are ways to do better.

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().

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 ?

Or are there deeper reasons why we really can't link more than 2 ?

With the current implementation, a non-taken branch results in three
branches: One to "skip over" the condition-true case, one to the new TB
and one in the header of that new TB to check for exits. We could
reduce to one in most cases.

Cheers,
Ben.