Register Allocation

Used	Use Number	Token

An improved register allocation algorithm, which handles the case of running out of registers, is:

1. At the beginning of a statement, mark all registers as not used; set use number to 0.

2. When an operand is loaded into a register, record a pointer to its token in the register table.

3. When a register is requested,
   1. If there is an unused register: mark it used, set its use number to the current use number, increment the use number, and return the register number.

   2. Otherwise, find the register with the smallest use number. Get a temporary data cell. Generate a Store instruction ( spill code) to save the register contents into the temporary. Change the token to indicate the temporary.