[Libre-soc-isa] [Bug 924] potential major opcode allocation for SVP64

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=924

--- Comment #20 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #19)
> (In reply to Jacob Lifshay from comment #18)
> > note from meeting: bits 30-31 need to be available for 32-bit po9
> > instructions for Rc
> 
> appreciated - unless there is a really *really* good reason i am
> not going to worry about it.  first reason: breaking up the 24-bits
> of SVRM starts to interfere with XO (in other encodings).

that isn't a problem because all the cases where SVRM exists at all have no
overlap with the cases where the prefix has XO form.

> second, there is precedent for Rc being in bit 21

this is likely confusing and takes extra gates to have Rc in an unusual
location compared to nearly every other scalar 32-bit instruction. additionally
it is likely more work to change Rc in every libre-soc scalar instruction. both
of these combined with XO form not actually being a problem as demonstrated
below i think meets the bar of being worth changing.

example encoding scheme (just complete enough to illustrate my point):
space saving measures: EXT2xx is half the size, XO is much larger in fli than
usual
prefix:
|0  |6   |11  |13 |16  |21  |22  |28 |29 |30      |31  |32 |
| 9 | SVRM[0:21]                 | 0 | 0 | SVRM[22:23] | - | SVP64 ext0xx
| 9 | SVRM[0:21]                 | 0 | 1 | SVRM[22:23] | - | SVP64Single ext0xx
| 9 | SVRM[0:21]                 | 1 | 0 | SVRM[22:23] | 0 | SVP64 ext2xx
| 9 | SVRM[0:21]                 | 1 | 0 | SVRM[22:23] | 1 | SVP64Single ext2xx
| 9 | -                          | 1 | 1 | -               | scalar 32/64-bit
| 9 | RT | RA     | RB | OE | XO | 1 | 1 | XO     | Rc |   | XO-form 32-bit
| 9 | RT | RA     | RB | XO      | 1 | 1 | XO     | Rc |   | X-form 32-bit
| 9 | RS | RA     | sh | XO      | 1 | 1 | sh     | Rc |   | XS-form 32-bit
| 9 |FRT |mode| XO               | 1 | 1 | XO          |imm| fli 64-bit

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