[Libre-soc-isa] [Bug 532] discuss iterative approach for statistical analysis of effectiveness of Compressed PowerISA encoding

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=532

--- Comment #6 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Alexandre Oliva from comment #5)
> -M histogram,raw will get buckets broken down by raw opcode rather than
> aliases. does this get you what you want WRT bc vs beq?

ahh that's it.  star.

(In reply to Alexandre Oliva from comment #4)

> wouldn't fit.  So I don't see what useful information would follow from that
> break-down.  If you see something I don't, please educate me.

parent bug or #213.  can't find it straight away.  ppc64 ABI uses specific regs
for certain purposes.  targetting those makes the process of identifying the 32
bit instructions to replace with direct 16 bit equivalents that much easier.

stack pointer is an obvious candidate.

the idea here is not to go "hey let's make this so fantastically idealistically
complex that we force ourselves to spend 5 manyears on compiler development".

we have not got time to mess about like that.

therefore "what regs are most commonly used" is a good first effort such that a
straight 1-pass non-state-based replacement algorithm can be dropped in with
very little effort and give us a huge drop in code size.

later on heavy optimisation can take place by other people.

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