Re: Jitter: new stack primitive (or micro-primitive): quake

[Luca Saiu]

Hello José.

On 2019-11-27 at 10:09 +0100, Jose E. Marchesi wrote:

> Nice name for the instruction :)
>
> I updated the jitter submodule reference to the tag v0.9.204 and added a
> quake instruction to the PVM, along with a rewrite rule for rot;swap ->
> quake.

I have looked at your manually written instructions, and saw several
opportunities for simplification.  It is true that you can achieve
exactly the same performance with rewrite rules (another is
  swap;over -> tuck
), but to me the main motivation is readability; I find some of the
stack juggling in Poke quite difficult to follow.  I am bothered by
  dup rot dup rot
, for example.

If you are not against this kind of change I will prepare a patch, but
please do not accept it just to please me: you should retain full
control over the esthetics in Poke.

This is from my notes file:

--8<---------------cut here---------------start------------->8---
Stack optimizations in Poke
===========================
Compliment José about GCD.  When he showed it to me in Frankfurt I did
not fully appreciate how clever it was.

Still, GCD can be optimized.  The unconditional branch to a conditional
branch is ugly -- even if almost certainly not performance-critical.
%
.macro addo @base_type #unit
  I think the second swap is useless.
%
Replace
  nrot swap
by
  reverse 3
Simpler, more intuitive, and faster.  (it occurs once in .macro ais @etype)
%
Replace
  rot swap
by
  quake
It occurs several times.
%
Replace
  rot drop
by
  slide  [TO IMPLEMENT: ( a b c -- b c ).  POSSIBLY WITH A DEPTH ARGUMENT. ]
It also occurs several times.
%
Replace
  dup rot dup rot                                 12 insns
  ( a b -- b a a b )
by
  ( a b ) tuck ( b a b ) over swap ( b a a b )    11 insns; but "over swap" 
stinks.
or
  ( a b ) tuck ( b a b ) bulge ( b a a b )        7 insns estimated: I have to 
implement bulge
or
  ( a b ) swap over over swap ( b a a b )         14 insns; slower and only 
marginally
                                                            easier to understand
%
Replace
  over swap
  ( a b -- a a b )
with a new operation:
  bulge
  ( a b -- a a b )  [ TO IMPLEMENT.  If slide has an argument this can be the 
same operation. ]
(I want to keep the name "swell" or "pump" for a different micro-primitive,
 with effect ( a b -- a ? b ), "?" meaning unspecified.)
%
In src/pkl-gen.pks, .function array_valmapper
Replace
  swap drop
by
  nip
.
The rest of src/pkl-gen.pks looks fine.
--8<---------------cut here---------------end--------------->8---

Regards,

-- 
Luca Saiu
* My personal web site:  http://ageinghacker.net
* GNU epsilon:           http://www.gnu.org/software/epsilon
* Jitter:                http://ageinghacker.net/projects/jitter

I support everyone's freedom of mocking any opinion or belief, no
matter how deeply held, with open disrespect and the same unrelented
enthusiasm of a toddler who has just learned the word "poo".

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