Code Generator

Code generator is used to produce the target code for three-address statements. It uses registers to store the operands of the three address statement.

Example:

Consider the three address statement x:= y + z. It can have the following sequence of codes:

MOV x, R0           ADD y, R0  

Register and Address Descriptors:

• A register descriptor contains the track of what is currently in each register. The register descriptors show that all the registers are initially empty.
• An address descriptor is used to store the location where current value of the name can be found at run time.

A code-generation algorithm:

The algorithm takes a sequence of three-address statements as input. For each three address statement of the form a:= b op c perform the various actions. These are as follows:

1. Invoke a function getreg to find out the location L where the result of computation b op c should be stored.
2. Consult the address description for y to determine y’. If the value of y currently in memory and register both then prefer the register y’ . If the value of y is not already in L then generate the instruction MOV y’ , L to place a copy of y in L.
3. Generate the instruction OP z’ , L where z’ is used to show the current location of z. if z is in both then prefer a register to a memory location. Update the address descriptor of x to indicate that x is in location L. If x is in L then update its descriptor and remove x from all other descriptor.
4. If the current value of y or z have no next uses or not live on exit from the block or in register then alter the register descriptor to indicate that after execution of x : = y op z those register will no longer contain y or z.

Generating Code for Assignment Statements:

The assignment statement d:= (a-b) + (a-c) + (a-c) can be translated into the following sequence of three address code:

Code sequence for the example is as follows: