Reusing Register Contents

Used	Contents

Many instructions can be eliminated by reusing variable values that are already in registers:[We assume that there are no aliases for variables.]

1. Initially, set the contents of each register to NULL.

2. When a simple variable is loaded, set the contents of the register to point to its symbol table entry.

3. When a register is requested, if possible choose an unused register that has no contents marked.

4. When a variable is to be loaded, if it is contained in an unused register, just mark the register used. This saves a Load instruction.

5. When a register is changed by an operation, set its contents to NULL.

6. When a value is stored into a variable, set the contents of any register whose contents is that variable to NULL. Then mark the register from which it was stored as containing that variable.

7. When a Label is encountered, set the contents of all registers to NULL.

8. The condition code contents can be reused also.