[Libre-soc-dev] results of phone call about simple-v prefix

[Richard Wilbur]

> On Nov 28, 2020, at 01:19, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> i will be sad in a Top Gear Jeremy Clarkson way to see SV-P64 unable
> to do single instruction self-contained predicated full arbitrary
> vectorisation of all and any scalar instructions.
> 
> my favourite is full twin-predicated vectorised LD and ST.  SV-P64
> LD/ST is the "dream" bucketlist instruction that every ISA wishes they
> had then regrets efforts to jam it into a scalar ISA.  ARM, POWER,
> MIPS, x86, they've all tried it on and had to backpedal.
> 
> SV-P64 LD/ST is *accident and incidental* provision of a
> context-switch and function call "dream instruction" that can place
> context-sensitive parts of a full and entire register file onto and
> off of the stack in *one* instruction.
> 
> the predicate register acts as the mask that indicates which registers
> are relevant to the function call and need pushing or popping on the
> stack.  likewise for context-switching.
> 
> and it wasn't even planned to be deliberately added, it just happened
> to fall out of the prefixing!

I think I feel your pain, Luke!  That is a particularly beautiful thing to have in your architecture—and particularly heart-breaking to lose.  Especially in light of context-switching efficiency and the hoops other architectures have jumped through to achieve it!  I am thinking of SPARC with register windows (also on i960, IA_64, 29k) into a built-in register stack[*] versus separate push/pop per register.

Looks like I need to read up in the list archives to understand why we would consider dropping those instructions.  (Hints of where to start reading gratefully accepted.)

Richard

Reference:
[*]  https://en.wikipedia.org/wiki/SPARC