Virtual Registers

Introduction

Compilers often use named slots to represent locations where a value will be stored. These named slots are virtual in the sense that they may be mapped in the end to a CPU register or a function's stack frame. In our compiler we use the term Register to denote such a virtual slot.

Instructions can define a Register, or use one or more Registers.

Design Considerations

There are several requirements we need to meet when deciding how to represent these virtual registers.

• The first requirement is that every variable declared in a function must get a unique slot. In the source language a variable name may be reused in multiple scopes, but from a compiler's perspective, each such variable is unique, because each has a unique lifetime and type.

Consider this example:

func foo(x: Int)
{
  if (x == 0)
  {
    var i = 0
  }
  if (x == 1)
  {
    var i = 1
  }
}

The variable i has two defined instances, each has a different lifetime. From a compiler's standpoint, each instance of i must be mapped to a distinct virtual register.