Question

Consider the grammar rule $E\to E_1-E_2$ for arithmetic expressions. The code generated is targeted to a CPU having a single user register. The subtraction operation requires the first operand to be in the register. If $E_1\text{and}{E}_2$ do not have any common subexpression, in order to get the shortest possible code

• A. $E_2$ should be evaluated first
• B. $E_1$ should be evaluated first
• C. Order of evaluation of $E_1\text{and}{E}_2$ is of no consequence
• D. Evaluation of $E_1\text{and}{E}_2$ should necessarily be interleaved

Answer 1

The correct option is A $E_2$ should be evaluated first

To optimize the solution evaluate the expression $E_2.$ Then we can calculate $E_1$ and finally $E_1$ will be one of operands that will be in register and we can perform subtraction directly. But if we follow the opposite then we have to make move and store operations.