[Libre-soc-isa] [Bug 1056] questions and feedback (v2) on OPF RFC ls010

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Sun May 28 12:29:37 BST 2023


--- Comment #17 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Paul Mackerras from comment #4)

> As to the unchanged
> behaviour, it is not clear why that would be so crucially important even if
> it were true.

"catstrophic", not "crucial"


Variable-Length Identification becomes so complex that it catastrophically
damages the product viability of Multi-Issue implementations of the ISA.

two other ISAs have Variable-Length Encoding that is similarly
crippled: Intex x86 and Motorola's 68000.

Intel only by having vast financial resources was able to develop
**SPECULATIVE** Parallel CISC Decoding that after many many cycles
finally allowed the length to be indentified, such that the speculative
undesired decodes could at last be dropped, and only then passed
in to Register&Memory Hazard Management.

As this was so awful Intel then was forced to further implement
realtime JIT translation to internal RISC Micro-code plus cacheing
lookups just to avoid this situation.

> 5th paragraph: You really can't prohibit future architects from doing
> anything. If nothing else, they could just remove this paragraph. At most
> you can say that doing certain things in future is likely to cause
> difficulties for implementators of future versions of the architecture.

nowhere near strong enough.  "result in the entire ISA being eliminated
from product consideration" would be more accurate.  look at the
complexity above. that's what would need to be done.  no "beginner"
team would bother on learning the full details of how to achieve
competitive performance.

> (You
> can't really even be sure about that, because future implementers might
> invent amazing new implementation techniques that would easily overcome any
> difficulty you propose.)

no, they won't.  complexifying length identification is fundamental.
Decode is one of the defining characteristics of an ISA as to what
products it ends up in.

> This paragraph is a bit like Parliament passing a law saying that Parliament
> may not in future pass a law allowing a particular thing. It's futile and
> ineffective.

then the *understanding* is required alongside suitably transparent
(i.e. not in IBM-Confidential files) supporting documentation as
to why it would be catastrophic.

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