Re: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support

[Dave Martin <Dave.Martin@arm.com>]

On Thu, Dec 01, 2016 at 10:21:03AM +0100, Florian Weimer wrote:
> On 11/30/2016 02:56 PM, Dave Martin wrote:
> 
> >If we do have distinct "set process VL" and "set thread VL" interfaces,
> >then my view is that the former should fail if there are already
> >multiple threads, rather than just setting the VL of a single thread or
> >(worse) asynchronously changing the VL of threads other than the
> >caller...
> 
> Yes, looks feasible to me.

OK, I'll try to hack up something along these lines.

> >>>I'm not familiar with resumable functions/executors -- are these in
> >>>the C++ standards yet (not that that would cause me to be familiar
> >>>with them... ;)  Any implementation of coroutines (i.e.,
> >>>cooperative switching) is likely to fall under the "setcontext"
> >>>argument above.
> >>
> >>There are different ways to implement coroutines.  Stack switching (like
> >>setcontext) is obviously impacted by non-uniform register sizes.  But even
> >>the most conservative variant, rather similar to switch-based emulation you
> >>sometimes see in C coroutine implementations, might have trouble restoring
> >>the state if it just cannot restore the saved state due to register size
> >>reductions.
> >
> >Which is not a problem if the variably-sized state is not part of the
> >switched context?
> 
> The VL value is implicitly thread-local data, and the encoded state may have
> an implicit dependency on it, although it does not contain vector registers
> as such.

This doesn't sound like an absolute requirement to me.

If we presume that the SVE registers never need to get saved or
restored, what stops the context data format being VL-independent?

The setcontext()/getcontext() implementation for example will not change
at all for SVE.

> >Because the SVE procedure call standard determines that the SVE
> >registers are caller-save,
> 
> By the way, how is this implemented?  Some of them overlap existing
> callee-saved registers.

Basically, all the *new* state is caller-save.

The Neon/FPSIMD regs V8-V15 are callee-save, so in the SVE view
Zn[bits 127:0] is callee-save for all n = 8..15.

> >they are not live at any external function
> >boundary -- so in cooperative switching it is useless to save/restore
> >this state unless the coroutine framework is defined to have a special
> >procedure call standard.
> 
> It can use the standard calling convention, but it may have selected a
> particular implementation based on the VL value before suspension.

If the save/restore logic doesn't touch SVE, which would its
implementation be VL-dependent?

Cheers
---Dave