ST20-C1 datasheet(183/205 Pages) STMICROELECTRONICS

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then the total stack requirement is the larger stack requirement of the individual

expressions. This is only the case if care is taken over the order of evaluation. Note

that

‘infinite’ should be taken as meaning greater than any finite depth - because a

function call does not preserve values on the stack.

Let the function

eval( e, r ) evaluate expression e where there are r registers available

to perform the evaluation. Where this expression is an operation on two sub expres-

sions -

e1 op e2 - it is efficiently evaluated by the following algorithm, where op

commutes is true if aop b is equal to bop a and false otherwise:

if (depth(e1) < r AND depth(e2) < r) /* i.e. depth of both expressions is less

than the number of registers available */

{

if (depth(e2) > depth(e1)) {

if (op commutes) {

eval( e2, r ); eval( e1, r-1 ); op

}

else {

eval( e2, r ); eval( e1, r-1 ); rev; op

(*)

}

else if (depth(e2) <= depth(e1)) {

eval( e1, r ); eval( e2, r-1 ); op

}

else if (depth(e1) >= r OR depth(e2) >= r) {

if (depth(e2) >= depth(e1)) {

if (depth(e1) >= r ){

/* i.e. both depths >= r*/

eval( e2, r ); stl temp; eval( e1, r ); ldl temp; op

}

else {

/* i.e. depth(e1) < r AND depth(e2) >=r */

if (op commutes) {

eval( e2, r ); eval( e1, r-1 ); op

}

else {

/* i.e. operation doesn’t commute */

eval( e2, r ); eval( e1, r-1 ); rev; op

}

else if (depth(e2) < depth(e1)) {

if (depth(e2) >= r) {

/* i.e. both depths >= r*/

if (op commutes){

eval( e1, r ); stl temp; eval( e2, r ); ldl temp; op

}

else {

eval( e2, r ); stl temp; eval( e1, r ); ldl temp; op

}

else {

/* i.e. (depth(e2) < r) AND depth(e1) >= r*/

eval( e1, r ); eval( e2, r-1 ); op

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