[Vm-dev] Re: A trick to speedup #become: (but not becomeForward:)

[Igor Stasenko]

And it works! All become tests are green.
See attached for VM and image side code.

Now most interesting thing. How much speed we gain?

Before:

| x y |

x := 'ab' copy.
y := 'cd' copy.

[ 100 timesRepeat: [ x become: y ] ] timeToRun

1786

After:

| x y |

x := 'ab' copy.
y := 'cd' copy.

[ 1000000 timesRepeat: [ x become: y ] ] timeToRun

82

So,


(1786 / 100)  / (82 / 1000000 ) asFloat 217804.8780487805

217'804 times faster!!!!!!  :)

But of course there are caveats if objects involved are
compiled method(s) or symbols (selectors).

A check is required and invalidate cache in this case.
For symbols it is harder, since symbols are not special objects.
A cheapest way to workaround it is just
override #become: for Symbol class to always use slow version.
(oh.. btw, same could be done for CompiledMethod(s)).

So, instead of putting heavyweight logic into this new primitive, if
may be better to just override #become: for critical
classes.
And since become is symmetric, we may need to use double-dispatch to
check that none of pair is system-critical object,
otherwise just fallback to slow version.

-- 
Best regards,
Igor Stasenko AKA sig.
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