[libre-riscv-dev] [Bug 139] Add LD.X and ST.X? Strided

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=139

--- Comment #37 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #36)

> > can you please describe it in pseudocode in a simple loop?
> 
> sure.

ah superb. now i get it.

>
>             let mut sel_field = selector >> (FIELD_SIZE * i);
>             sel_field &= FIELD_MASK;
>             let src = if (sel_field & 0b100) != 0 {

This is the bit where I was talking about "if sel_field != 0b111" (to represent
"masked out" / "ignore").

>                 rs1
>             } else {
>                 rs3
>             };

After this if you have "if rs1 == x0 continue" then swizzle may be implemented
as simply setting rs3 to x0

destsubvl must *not* be a CSR, it can be a fmt3 subencoding.

This does however leave out being able to do setting to 0, 1 or other
constants. I wonder why other ISAs do not have the constants as part of the
ISA.

OH hang on. If SUBVL is to be ignored on rs2 (special treatment) we could also
hypothetically use (set) the predicate on rs2 as the rs1/rs3 sub-element
selector mask.

The only downside of that being that it is limited to 64 bits, so 64/SUBVL is
16 when SUBVL is 4, you can only cover up to 16 long VL.

More special treatment, limited range. Not looking attractive. 3 bits for the
rs2 selector is much better.

L.

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