[Libre-soc-isa] [Bug 893] SVP64 proposal to OPF

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Fri Jul 29 23:49:44 BST 2022


--- Comment #20 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Luke Kenneth Casson Leighton from comment #16)

> urr.... https://www.realworldtech.com/forum/?threadid=202688&curpostid=202688
> might as well ask
> https://www.realworldtech.com/forum/?threadid=202688&curpostid=207731

someone called dmcq very kindly replied, prompting me to investigate
the pseudocode, which magically a day later is making sense to me.


the outer-product instructions are definitely power-of-two boundaried,
the "tiles" must be squares. a 128-bit silicon-partner choice would
result in 2x2 64-bit outer-product, a 4x4 32-bit outer-product.

there is no way to stop overwriting of destinations on non-power-two
boundaries, but there *is* a way to stop wasting of CPU cycles on
multiply-by-zero-and-adds, by pre-running some zero-detection instructions
and putting the result of that detection into predicate masks.
there's *two* predicate source masks for that purpose: one for N
one for M.

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