11.1 Preparation for Code Generation

To prepare for code generation, the compiler decides where values of variables and expressions will reside during execution. The preferred location is a register since instructions execute faster when the data referred to in operands reside in registers. Ultimate storage is often a memory location, and due to the scarcity of registers, even intermediate results may need to be assigned memory locations also.

Various methods have been developed for good use of registers. One technique is to store all loop variables in registers (until there are no more registers) since statements inside loops may execute more than once.

Another easily implemented technique is to store the variables used the most in registers.

For machines with a stack and a stack pointer, operations involving the stack are generally quicker than those involving an access to (non-stack) memory. Thus, another code generation technique is to push all variables in a procedure onto the stack before the procedure is executed, access them from the stack (and two registers).

Careful assignment of expressions and variables to registers can increase the efficiency of the resulting compiler. In this module, we will generate code as though there were only one available register.