[Libre-soc-dev] LD/ST Elwidth Overrides

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Sep 1 18:27:08 BST 2021

crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

On Wed, Sep 1, 2021 at 6:23 PM Jacob Lifshay <programmerjake at gmail.com> wrote:

> I realized that it would be more useful to let the src elwidth override
> apply to the index register for lbzx (and all ld/st indexed instructions)

there's a special detection for Vector Indexed Mode: RA.isvec is set.
when that occurs there's nothing stopping us from treating it entirely
differently from unitstride and elstride modes.

> instead of using the src elwidth to change the memory access size, which is
> redundant:
> https://libre-soc.org/irclog/%23libre-soc.2021-09-01.log.html#t2021-09-01T17:54:16
> Note that how I'm proposing is exactly how gather/scatter works on x86 and
> Arm and probably waay more...it's also useful for doing webassembly
> load/stores in one instruction, rather than 3 (explained in irclog link).

yeah i don't have a problem with that, at all.  what do we "lose" by
enabling Indexed mode to have the elwidth override on RA?
(and, what are the 3 instructions that would otherwise be needed?)

we need to make a full-disclosure comparison, in other words.


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