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

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

On Mon, Dec 05, 2016 at 11:44:38AM +0100, Florian Weimer wrote:
> On 12/01/2016 11:30 AM, Dave Martin wrote:
> 
> >>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?
> 
> I'm concerned the suspended computation has code which has been selected to
> fit a particular VL value.
> 
> > If the save/restore logic doesn't touch SVE, which would its
> > implementation be VL-dependent?
> 
> Because it has been optimized for a certain vector length?

I'll respond to these via Szabolcs' reply.

> >>>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.
> 
> Are the extension parts of registers v8 to v15 used for argument passing?

No -- the idea is to be directly compatible with the existing PCS.

> If not, we should be able to use the existing dynamic linker trampoline.

Yes, I believe so.

Cheers
---Dave