[Libre-soc-dev] opcode allocation pressure

[Jacob Lifshay]

On Thu, Sep 1, 2022, 02:01 Luke Kenneth Casson Leighton via Libre-soc-dev <
libre-soc-dev at lists.libre-soc.org> wrote:

> but it is *not the whole story* because the GF ops have
> not been allocated, nor rounding converts, nor 3D textures,
> and additionally there are the Transcendentals
> https://libre-soc.org/openpower/transcendentals/
>
> fortunately the Ts can fit into EXT063 and EXT059 naturally.
>

looking over the transcendentals encodings, they both have 5 bits set to
`///`, which iirc is don't care bits, this seems like a waste, we should
have those be an `EO` expanded opcode field (where RA would go for
single-input instructions) or just part of XO (for 2-in instructions),
reducing the number of XO values used by 32x.

iirc `EO` is already used like that for some X-form instructions, e.g.:
VSX Scalar Negate Quad-Precision X-form
xsnegqp VRT,VRB
fields:
PO RT  EO RB  XO
63 VRT 16 VRB 804 /
0  6   11 16  21  31

Jacob

>