Translating Boolean Expressions

Abstract

It’s possible to treat Boolean expressions in exactly the same way as arithmetic expressions — generate instructions to evaluate the subnodes, generate an instruction (or a sequence of instructions) to combine the Boolean values. Thus Boolean and and Boolean or operations can be translated using the TranBinOp procedure of chapter 5 with the machine instructions AND and OR. However Boolean expressions more often than not appear as the conditional test in an if or a while statement and, as a result, are used more as sections of program which select between alternative paths of computation than as algebraic expressions which compute a truth value. Most good compilers therefore try to generate ‘jumping’ code for Boolean expressions in these contexts. First of all, however, it is necessary to demonstrate the code fragments which are required when Boolean expressions are regarded as value-calculating mechanisms.