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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: