On 2014/05/02 09:17:31, jbramley wrote:
On 2014/05/01 18:29:11, rmcilroy wrote:
> > Since we added UseScratchRegisterScope, I don't think this is true.
> > It used to be important so we could do `Push(..., ip0, ...)`, but we
> > don't do that any more unless we first acquire ip0 (making it
> > unavailable). It might be worth checking that now, since it would
> > greatly simplify this function.
>
> Yes I had hoped that was true and tried it originally, however,
> unfortunately we still reach here with no remaining temp registers in
> LCodeGen::PushSafepointRegistersScope.
Ok, fair enough.
> Sure I understand your concern. We are not yet sure whether this CL
> will be necessary (it has a very big impact currently, but there may
> be other ways around the issue. I will investigate whether this can
> be gated on the platform if we decide to land this.
There's something else I considered when we first learned about the
stack pointer problem: We could set csp to the V8 stack limit, then
leave it there. We'd have to move it for calls into C code, and restore
it afterwards, but normal 'Push' calls could just ignore it. I didn't do
this to start with because there are some complications:
- Can the (real) stack limit move? If so, we'd need to update csp
accordingly. If it's only C calls that can move it, we could reload
the stack limit on return to generated code.
- There needs to be enough valid below the VM stack limit to allow
signal handlers to run. I didn't know if we could guarantee this.
Yes I did consider this, but the signal handler issue was what made me
avoid it.
I don't think it would be safe to do this in general given that we don't
know
how much space a signal handler might need, so I would prefer to do it this
way
(it also gives us the benefit of ensuring we don't write below csp (modulo
the
16 byte alignment buffer) when running on the simulator.
https://codereview.chromium.org/264773004/
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