ikarus/lib/how-to-optimize-arg-checks.tex

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2006-12-04 22:05:44 -05:00
Step1: Optimize direct calls:
Suppose we have a case-lambda as follows
(case-lambda
[<formals_0> <code_0>]
[<formals_1> <code_1>]
...
[<formals_n> <code_n>])
First, generate labels for every clause as well as a label for top
(case-lambda <label>
[<formals_0> <label_0> <code0_>]
[<formals_1> <label_1> <code1_>]
...
[<formals_n> <label_n> <coden_>])
Now everywhere there is a call to a closure bound variable:
(funcall x args ...)
We match on the labels of x:
* If a matching label is found, we transform the call to
(dircall <label_i> x args ...)
* If no matching label is found, we transform the call to
(dircall <label> x args ...)
If we match on a case with ". rest" formals, we emit a call
to "list" on the rest arguments because the label will be
inside the code (after the rest-args are constructed).
Step2: Eliminate passing arg-counts:
All direct calls to inner labels do not require passing an argument
count since it will not be used by the procedure.
Step3: Eliminate useless case-lambda cases. (not important)
For every label generated, determine whether it is actually used or
not.
We assume the main label unused until we find:
* A reference to it in a closure that's not a dircall operator.
* A reference to it as the target label to a dircall.
We assume an inner label unused until we find:
* A direct call to it.
* The main label used.
If a case-lambda has unused clauses, they can be eliminated.
If a case-lambda has an unused main label, the dispatch can be
eliminated.