Re: Compilers, Aho/Sethi/Ullman, Exercise 2.15

tomazos@gmail.com wrote:


> Consider the following for-statement:
>
> for i:= 1 step 10 - j until 10 * j do j := j + 1
>
> The limit 10 * j and increment 10 - j are to be evaluated once before
> the loop. For example if j = 5 before the loop, we would run through
> ten times and exit.
>
> Construct a syntax-directed translation scheme to translate this
> for-loop into the following stack-machine code.
...
> The parts labeled with ???? are illegal in the stack machine-code. The
> problem is that I need to store the initial evaluation of STEP and
> LIMIT somewhere, and I don't see how this is possible to do using the
> stack. There is also no mention of a way to create and allocate a new
> variable, or anywhere else I could store them.
>
> Any enlightenment on this topic appreciated.


IMO you can assume that according variables have been created,
somewhere, somehow. At least I can't imagine how else this problem could
be solved, on a single-stack machine with the limited instruction set
you mentioned. With more instructions, for stack pointer relative
operands (FORTH: DUP, SWAP, OVER...), it were possible to have the STEP
and LIMIT values on the stack.


DoDi

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