Code Generation

Mooly Sagiv

html://www.cs.tau.ac.il/~msagiv/courses/wcc03.html

Chapter 4.2

Basic Compiler Phases

Code generation issues

• Code selection

• Register allocation

• Instruction ordering

Simplifications

• Consider small parts of AST at time– One expression at the time

• Target machine simplifications– Ignore certain instructions

• Use simplifying conventions

Overall Structure

Outline

• Partial evaluation in the nutshell• Simple code generation for expressions (4.2.4)

– Pure stack machine

– Pure register machine

• Code generation of basic blocks (4.2.5)• Automatic generation of code generators (4.2.6)• Register Allocation by Graph Coloring (4.2.7)

Next w

eek

Partial Evaluation

• Partially interpret static parts in a program

• Generates an equivalent program

Partial EvaluatorProgram Program’

Input 1Input 2

Example

int pow(int n, int e)

{

if (e==0)

return 1;

else return n * pow(n, e-1);

}

e=4

int pow4(int n)

{

return n * n * n *n;

}

Example2

Bool match(string, regexp)

{

switch(regexp) {

….

}

}

regexp=a b*

Partial Evaluation Generalizes Compilation

Partial EvaluatorInterpreter Program

AST ProgramInput

But ….

Simple Code Generation

• Fixed translation for each node type

• Translates one expression at the time

• Local decisions only

• Works well for simple machine model– Stack machines (PDP 11, VAX)– Register machines (IBM 360/370)

• Can be applied to modern machines

Simple Stack Machine

BP

SP

Stack

Stack Machine Instructions

Example

p := p + 5Push_Local #p

Push_Const 5

Add_Top2

Store_Local #p

Simple Stack Machine

BP

SP

Push_Local #p

Push_Const 5

Add_Top2

Store_Local #p7 BP+5

Simple Stack Machine

BP

SPPush_Local #p

Push_Const 5

Add_Top2

Store_Local #p

7

BP+57

Simple Stack Machine

BP

SPPush_Local #p

Push_Const 5

Add_Top2

Store_Local #p

7

BP+57

5

Simple Stack Machine

BP

SP Push_Local #p

Push_Const 5

Add_Top2

Store_Local #p

12

BP+57

Simple Stack Machine

BP

SP

Push_Local #p

Push_Const 5

Add_Top2

Store_Local #pBP+512

Register Machine

• Fixed set of registers

• Load and store from/to memory

• Arithmetic operations on register only

Register Machine Instructions

Example

p := p + 5Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P

Simple Register Machine

Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P7 x770

memory

R1 R2

Simple Register Machine

Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P7 x770

memory

7

R1 R2

Simple Register Machine

Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P7 x770

memory

57

R1 R2

Simple Register Machine

Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P7 x770

memory

512

R1 R2

Simple Register Machine

Load_Mem p, R1

Load_Const 5, R2

Add_Reg R2, R1

Store_Reg R1, P12 x770

memory

512

R1 R2

Simple Code Generation for Stack Machine

• Tree rewritings

• Bottom up AST traversal

Abstract Syntax Trees for Stack Machine Instructions

Example

-

*

*

b b 4 *

a c

Push_Local #b Push_Local #b

Mult_Top2

Push_Constant 4

Push_Local #a Push_Local #c

Mult_Top2

Mult_Top2

Subt_Top2

Bottom-Up Code Generation