OVERVIEW OF CODE GENERATION

 .. ASTs...-> [ Static Analysis ]
                     |
                     | IR
                     v
              [ Code Generation ]
                     |
                     | Machine Code
                     |
                     v
               Virtual Machine 
                   Execution

The IR (= Intermediate Representation)
    records information from static analysis
    including attributes of identifiers


              
			
            IR TREES

An IR is a tree structure,
   somewhat like an AST
   but in general it's also like an abstract machine code
   with information needed for code generation




Helps in modularizing compilers
and code generation

   WITHOUT IR               WITH IR

Java ------>x86       Java        ->x86  
     \/ |||                \     /     
   C ------>MIPS         C \\   /-->MIPS 
     \\/ ||                 ->IR         
 C++ ------>Sparc      C++ //   \-->Sparc
      \\\/                 /     \     
  C# ------>A1          C#/       \>A1   
                                  
			
          OUR CHOICES FOR AN IR   
                                  
To keep things simple, we will use
   a modified AST type as an IR   
                                  
Parser:                           
   - records structure of programs                      
   - provides placeholders for attributes
       needed for code generation
                                  
Static analysis:                  
   - records attributes of names used
        (in this modified AST)
                                  
			
   GENERAL STRATEGY FOR CODE GENERATION
                                  
Follow the grammar of the ASTs
    by doing a tree walk
    which will generate the IR trees
    (which are just like ASTs with id_use information)
                                  
			
       FOLLOWING THE GRAMMAR
                            
Code resembles the grammar that
   describes the input data (i.e., the ASTs)
                            
When
 - the grammar has a recursion, code generation recurses
 - the grammar has alternatives, use a switch statement
 - for the base cases, generate code
                            
			
         TARGET: CODE SEQUENCES
                            
Need sequences of machine code instructions
       including jumps

Why?
   so can run in VM (or assemble them)