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Boolean expressions

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Boolean expressions

Boolean expressions have two primary purposes. They are used for computing the logical values. They are also used as conditional expression using if-then-else or while-do.

Consider the grammar

The relop is denoted by <, >, <, >.

The AND and OR are left associated. NOT has the higher precedence then AND and lastly OR.

Production rule Semantic actions
E → E1 OR E2 {E.place = newtemp();
Emit (E.place ‘:=’ E1.place ‘OR’ E2.place)
E → E1 + E2 {E.place = newtemp();
Emit (E.place ‘:=’ E1.place ‘AND’ E2.place)
E → NOT E1 {E.place = newtemp();
 Emit (E.place ‘:=’ ‘NOT’ E1.place)
E → (E1) {E.place = E1.place}
E → id relop id2 {E.place = newtemp();
 Emit (‘if’ id1.place relop.op id2.place ‘goto’
 nextstar + 3);
 EMIT (E.place ‘:=’ ‘0’)
 EMIT (‘goto’ nextstat + 2)
 EMIT (E.place ‘:=’ ‘1’)
E → TRUE {E.place := newtemp();
 Emit (E.place ‘:=’ ‘1’)
E → FALSE {E.place := newtemp();
 Emit (E.place ‘:=’ ‘0’)

The EMIT function is used to generate the three address code and the newtemp( ) function is used to generate the temporary variables.

The E → id relop id2 contains the next_state and it gives the index of next three address statements in the output sequence.

Here is the example which generates the three address code using the above translation scheme:

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