[Vm-dev] postponing tempVector creation

[Clément Bera]

Hello,

Recently I realized that the tempVector is always created at the beginning
of the method. In the case of:

foo
    | temp |
    true ifTrue: [ ^ self ].
    [ temp := 1 ] value.

It means that even if the execution flow goes to the quick return
direction, the tempVector is created even if it is not used. I hacked a bit
the compiler to lazily create the tempVector when needed (access to temp
Vector variable or block closure creation), which means here the tempVector
would be created just before the pushClosure bytecode and not at the
beginning of the method.

I got as result that this specific method was executed 50,000,000 times per
second instead of 21,000,000 times per second. Probably because of some
useless garbage collection avoided.

The problem of this lazy tempVector initialization is that I never know
where I am in the control flow graph. This means than in the case:

bar
    | temp |
    false ifTrue: [ temp := 1 ].
    [ temp := 2 ].

It is hard to detect where to add the extra bytecode for lazy tempVector
initialization, because the first time you need it is in a branch, so if
you create the tempVector in the branch and the execution flow goes in the
other direction then the tempVector will not have been initialized.

Has any of you already investigated in this direction ? Does it worth it to
implement that ?

Thanks for any answers.
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