[ColorForth] Tail-recursion optimisation idea

[Eric Laforest <ecl@xxxxxxxxxxx>]

Here is an idea I came up with while puttering on a stack computer design.
(The '?;' notation was thought up by someone else, whom I'll name if he
wishes so (I'm not sure if he's on this list...I should ask, but I'm
impatient :) )

Normally, a while-false loop goes like this:
(I can't do colors here, but the notation should be equivalent)

"While TASK leaves a false, execute TASK"

: FOO TASK IF ; THEN FOO ;

Once TASK is done, this takes one cycle for the IF (a JMP0)
and then another for a JMP back to the call to TASK.

: FOO TASK FOO ?;

This would generate a JMP0 to the call to TASK, saving a cycle.
(and implicitely add a RET afterwards)

Better yet:

: TASK ..... TASK ?;

Which collapses this even further by eliminating the call/return overhead.

This variant of tail-recursion optimization is rather limited, but could
be of use where timing is tight or for loops repeated a zillion times.

This could be taken even further by having a 'conditional jump or return'
instruction, such that a flag would cause either a JMP or a RET...but this
is a 'fat' instruction and would probably not map well to hardware.
It's probably also not a good factorization.

Comments?

Eric LaForest


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