[Libre-soc-isa] [Bug 529] scheme for supporting 16/48-bit instructions on PowerPC LE with full backward compatibility

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=529

--- Comment #11 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
in this comment
https://bugs.libre-soc.org/show_bug.cgi?id=238#c70

we learn that the intention is for 32 bit instructions to look like LE even
when embedded in 48/64 SV Prefixes.

however "looking like ppc64le ABI" is not the same as "can be executed and is
fully conformant with the ppc64le ABI".

can we therefore go through it, with some bare minimum worked examples that
show, step by step, the transition to calling and returning from standard
ppc64le ABI functions?

i have a sneaking suspicion that we may need an instruction that
enables/disables SV on entry and exit from functions, which is going to be a
costly overhead.

secondly: are we overthinking this? the primary motivation is to accelerate
video and 3D fragments.  a full (scalar, standard v3.0 compiled) OS is far down
the list

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