Code Generation Algorithm

for each X = Y op Z do

. invoke a function getreg to determine location L where X must be stored. Usually L is a register.

. Consult address descriptor of Y to determine Y'. Prefer a register for Y'. If value of Y not already in L generate

Mov Y', L

. Generate

op Z', L

Again prefer a register for Z. Update address descriptor of X to indicate X is in L. If L is a register update its descriptor to indicate that it contains X and remove X from all other register descriptors.

. If current value of Y and/or Z have no next use and are dead on exit from block and are in registers, change register descriptor to indicate that they no longer contain Y and/or Z.

The code generation algorithm takes as input a sequence of three-address statements constituting a basic block. For each three-address statement of the form x := y op z we perform the following actions:

1. Invoke a function getreg to determine the location L where the result of the computation y op z should be stored. L will usually be a register, but it could also be a memory location. We shall describe getreg shortly.

2. Consult the address descriptor for u to determine y', (one of) the current location(s) of y. Prefer the register for y' if the value of y is currently both in memory and a register. If the value of u is not already in L, generate the instruction MOV y', L to place a copy of y in L.

3. Generate the instruction OP z', L where z' is a current location of z. Again, prefer a register to a memory location if z is in both. Update the address descriptor to indicate that x is in location L. If L is a register, update its descriptor to indicate that it contains the value of x, and remove x from all other register descriptors.

4. If the current values of y and/or y have no next uses, are not live on exit from the block, and are in registers, alter the register descriptor to indicate that, after execution of x := y op z, those registers no longer will contain y and/or z, respectively.