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Then A's preference class will be targeted on register 2, and B's on register 3 (since these are the only uses of A and B within ); this will cause T1 and T2 to have the same respective targets, and at the outer level an attempt will be made to perform the addition in register 2 and the multiplication in register 3.

This general scheme will produce much better code than a scheme which says that all LAMBDA expressions must, like the function FOO, take their arguments in certain registers.

Note too that no code whatsoever is generated for the variable bindings as such; the fact that we assign names to the results of the expressions (+ X Y) and (* Z W) rather than writing

(FOO 1 (* Z W) (+ X Y))

makes no difference at all, which is as it should be.

Thus, compiler temporaries and simple user variables are treated on a completely equal basis.

This idea was used in [Johnsson 75], but without any explanation of why such equal treatment is justified.

Here we have some indication that there is conceptually no difference between a user variable and a compiler-generated temporary.

This claim will be made more explicit later in the discussion of continuation-passing.

Names are merely a convenient textual device for indicating the various places in a program where a computed quantity is referred to.

If we could, say, draw arrows instead, as in a data flow diagram, we would not need to write names.

In any case, names are eliminated at compile time, and so by run time the distinction between user names and the compiler's generated names has been lost.

Thus, at the low leve, we may view LAMBDA as a renaming operation which has more to do with the internal workings of the compiler (or the interpreter), and with a notation for indicating where quantities are referred to, than with the semantics as such of the computation to be performed by the program.

1.4. An Example: Compiling a Simple Function

(DEFINE FACT

(LAMBDA (N)

(LABELS ((FACT1

(LAMBDA (M A)

(IF (= M 0) A

(FACT1 (- M 1)

(* MA))))))

(FACT1 N 1))))

Let us step through a complete compilation process for this function, based on the ideas we have seen.

(This scenario is intended only to exemplify certain ideas, and does not reflect entirely accurately the targeting and preferencing techniques described in [Wulf 75] and [Johnsson 75].)

First, let us assign names to all the intermediate quantities (temporaries) which will arise: