[PATCH] Add zero-overhead looping for xtensa backend

[Yangfei (Felix)]

> > On Tue, Oct 21, 2014 at 7:20 PM, Yangfei (Felix) <felix.yang@huawei.com>
> wrote:
> > > If the tripcount spill issue is not handled in the pattern, ICE may happen then.
> > > Here reload is trying to spill pseudo 173, but a memory operand is
> > > not allowed
> > in zero_cost_loop_end pattern.
> > > And this is what I am trying to solve.
> >
> > We have full control of the zero_cost_loop_end pattern. Plus, it
> > doesn't actually generate any real code. Edit it so it can take a memory
> operand.
> 
> Here the key point is we need a general purpose register for the "loop"
> instruction.
> If the trip count register is spilled, we don't have a general purpose register then.
> And we cannot use zero-cost looping in this situation.
> And that's why I spilt the zero_cost_loop_end into a normal test and branch.

Also note that the hwloop_pattern_reg interface also expects a general purpose register in the doloop_end pattern.