[PATCH] RISC-V: Clarufy the behavior when extensions have superset.

[Jan Beulich]

On 14.11.2024 13:38, Nelson Chu wrote:
> If extensions have superset, then which means the superset includes those
> extensions.  We used to imply those extensions for the superset, and also
> keep the superset since the canonical order form.  For example,
> 
> * m includes zmmul, m implies zmmul
> * b includes zba, zbb and zbs, and b also equals to zba + zbb + zbs, so b
>   implies zba, zbb and zbs
> 
> For all include rules,
> m includes zmmul
> zvbb includes zvkb
> zhinx includes zhinxmin
> zfh includes zfhmin
> 
> For all equal rules,
> a equals to zaamo + zalrsc
> b equals to zba + zbb + zbs
> zks equals to zbkb + zbkc + zbkx + zksed + zksh
> zkn equals to zbkb + zbkc + zbkx + zkne + zknd + zknh
> zk equals to zkn + zkr + zkt
> zvkn equals to zvkb + zvkt + zvkned + zvknhb
> zvks equals to zvkb + zvkt + zvksed + zvksh
> zvkng equals to zvkn + zvkg
> zvknc equals to zvkn + zvbc
> zvksg equals to zvks + zvkg
> zvksc equals to zvks + zvbc
> c equals to zca + zcf (if rv32 and f) + zcd (if d)
> 
> g is a super special case, so skip it here.
> 
> Considering ".option arch, -extension [,...,-extension_n]" removes the
> extension which,
> 1. A includes B;
>    Remove A should also remove B;
>    Remove B is useless since we don't have any extensions to represent A - B
> 2. A equals to B + C + ...;
>    Remove A should also remove B, C and ...;
>    Remove B should also remove A, but keep C and ...;
>    Remove C should also remove A, but keep B and ...;
>    Remove B ansd C should also remove A, but keep ...;

I agree with 2, but that imo contradicts 1: Remove B should also remove A,
as A = B + <unnamed>. The two still aren't mere aliases of one another, as
behavior is different when they're added to the permitted extensions.

(Otherwise, at the very least, a removal that doesn't remove anything
should at least have a diagnostic issued.)

> 3. A implies B;
>    Remove B is useless, since A still imply B;
>    Remove A doesn't affect B, since B may still be implied by others;

I'm afraid I didn't really get what the difference is between "implies"
and "includes". In particular, related to the 2nd sentence, in the
summary at the top I can't spot any case of two distinct extensions
implying the same more narrow one. Imo, if anything this 2nd rule
should be "Remove A doesn't affect B, if B is still implied by another
active one". Plus that (in your or my form) would still be similarly
true for "included".

Jan