[libre-riscv-dev] fp special functions

[Luke Kenneth Casson Leighton]

there's some other functions necessary for 3D, which, again, are on
the slides: texturisation, pixel, and vector processing: dot-product
and so on.

"4xFP32 ARGB to 1xINT32 ARGB8888" in particular, a "trick" occurred to
me last night that might allow us to use SimpleV in the FP opcode
space.

it's quite unusual, it goes like this:

* FP opcode funct5=NNNNN in the "scalar" space (non-SV-mode) raises an
ILLEGAL instruction.

if however, the following special conditions are met:

* SV's Sub-Vector Len is set to 4 AND
* SV element width is set to 32-bit for the TARGET register AND
* SV element width is set to 32-bit for the SOURCE register

*THEN* that very same opcode is NO LONGER an illegal instruction, it
is a "4xFP32 ARGB to 1xINT32 ARGB8888" instruction.

there is also a Vulkan API function to do "4xFP32 ARGB to 1xINT16
ARGB565".  the conditions for activating this may be:

* SV's Sub-Vector Len is set to 4 AND
* SV element width is set to 16-bit for the TARGET register AND
* SV element width is set to 32-bit for the SOURCE register

if there are any 3xFP32 ARGB instructions needed, it should be clear
that the SV Sub-Vector length should be set to 3.

it is an extremely unusual use of SV: normally the scalar opcodes
exist independently of their "vectorised equivalents".  however it
makes absolutely no sense in this case, because 1xFP32 ARGB to 1xINT32
ARGB8888 is nonsense (impossible).

i believe there may be many other candidate instruction opportunities
that could benefit from this trick.  SLERP for example requires
Quaternions, to specify the points, *and* a 3rd argument (t).
https://en.wikipedia.org/wiki/Slerp

those Quaternions could be selected by setting SV SUBVL=4, at which
point the special *Vector* opcode activates.

thoughts?

l.