INTERMEDIATE CODE GENERATION

1. INTERMEDIATE CODE
GENERATION
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2. 2Structure of a Compiler
 Front end of a compiler is efficient and can be
automated
 Back end is generally hard to automate and finding the
optimum solution requires exponential time
 Intermediate code generation can affect the
performance of the back end
Instruction
Selection
Instruction
Scheduling
Register
Allocation
Scanner Parser
Semantic
Analysis
Code
Optimization
Intermediate
Code
Generation
IR

3. Overview
 Goal: Generate a Machine Independent
Intermediate Form that is Suitable for Optimization
and Portability
 Facilitates retargeting: enables attaching a back
end for the new machine to an existing front end
 Enables machine-independent code optimization
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4. 4

5. Motivation
 What we have so far...
 A Parser tree
 With all the program information
 Known to be correct
 Well-typed
 Nothing missing
 No ambiguities
 What we need...
 Something “Executable”
 Closer to
 An operations schedule
 Actual machine level
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6. What We Want
 A Representation that
 Is closer to actual machine
 Is easy to manipulate
 Is target neutral (hardware
independent)
 Can be interpreted
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7. Intermediate Languages
 Syntax Tree.
 A syntax tree heiarachical structure of the
source program
 DAG more compact.
 Postfix Notation
 Linearized representation of a syntax tree.
 Edges do not appear explicitly. They can be
recovered.
 Three address code
 Control Flow Graphs (CFGs)
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8. Recall ASTs and DAGs
 Intermediate Forms of a Program:
 ASTs: Abstract Syntax Trees
 DAGs: Directed Acyclic Graphs
 What is the Expression?
assign
a +
* *
b
c
uminusb
c
uminus
assign
a +
*
b
c
uminus
8

9. Representation
 Two Different Forms:
 Linked Data Structure
 Multi-Dimensional Array
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10. 10Abstract Syntax Trees (ASTs)
if (x < y)
x = 5*y + 5*y/3;
else
y = 5;
x = x+y;
Statements
<
AssignStmt
+
*
x
IfStmt
AssignStmt AssignStmt
x x y+ yxy
/
5 y 3*
5 y
5

11. 11Directed Acyclic Graphs
(DAGs)
 Use directed acyclic graphs to represent expressions
 Use a unique node for each n
if (x < y)
x = 5*y + 5*y/3;
else
y = 5;
x = x+y;
Statements
<
AssignStmt
*
IfStmt
AssignStmt AssignStmt
x +y
/
5
3

12. 12
Control Flow Graphs (CFGs)
 Nodes in the control flow graph are basic blocks
 A basic block is a sequence of statements
always entered at the beginning of the
block and exited at the end
 Edges in the control flow graph represent the control flow

13. CFG
if (x < y)
x = 5*y + 5*y/3;
else
y = 5;
x = x+y;
B1
if (x < y) goto B1 else goto B2
x = 5*y + 5*y/3 y = 5
x = x+y
B2
B0
B3
• Each block has a sequence of statements
• No jump from or to the middle of the block
• Once a block starts executing, it will execute till the end
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14. Objective
 Directly Generate Code From AST or DAG as a Side Effect of Parsing
Process.
 Consider Code Below:
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15. Each is Referred to as “3 Address Coding (3AC)”
since there are at Most 3 Addresses per Statement
One for Result and At Most 2 for Operands
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16. The Intermediate Code
Generation Machine
 A Machine with
 Infinite number of temporaries
 Simple instructions
 3-operands
 Branching
 Calls with simple calling convention
 Simple code structure
 Array of instructions
 Labels to define targets of branches.
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17. Temporaries
 The machine has an infinite number of temporaries
 Call them t0, t1, t2, ....
 Temporaries can hold values of any type
 The type of the temporary is derived from the
generation
 Temporaries go out of scope with the function
they are in
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18. What is Three-Address
Coding?
 A simple type of instruction
 3 / 2 Operands x,y,z
 Each operand could be
 A literal
 A variable
 A temporary
 Example
x := y op z
x + y * z
t0 := y * z
t1 := x + t0
x := op z
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19. Types of Three Address
Statements
 Assignment Statements of Form:
 X := Y op Z
 op is a Binary Arithmetic or Logical Operation
 Assignment Instructions of Form:
 X := op Y
 op is Unary Operation such as Unary Minus, Logical
Negative, Shift/Conversion Operations
 Copy Statements of Form:
 X := Y where value of Y assigned to X
 Unconditional Jump of Form:
 goto L which goes to a three address statement labeled
with L
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20. Types of Three Address
Statements
 Conditional Jumps of Form:
 if x relop y goto L
 with relop as relational operators and the goto executed if
the x relop y is true
 Parameter Operations of Form:
 param a (a parameter of function)
 call p, n (call function p with n parameters)
 return y (return value y from function – optional)
 param a
 param b
 param c
 call p, 3
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21. Types of Three Address
Statements
 Indexed Assignments of Form:
 X := Y[i] (Set X to i-th memory location of Y)
 X[i] := Y (Set i-th memory location of X to Y)
 Note the limit of 3 Addresses (X, Y, i)
 Cannot do: x[i] := y[j]; (4 addresses!)
 Address and Pointer Assignments of Form:
 X := & Y (X set to the Address of Y)
 X := * Y (X set to the contents pointed to by Y)
 * X := Y (Contents of X set to Value of Y)
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22. Three Address Code
Representations
 Data structures for representation of TAC can be objects or
records with fields for operator and operands.
Representations include quadruples, triples and indirect
triples.
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23. Quadruples
 In the quadruple representation, there are four
fields for each instruction: op, arg1, arg2, result
 Binary ops have the obvious representation
 Unary ops don’t use arg2
 Operators like param don’t use either arg2 or
result
 Jumps put the target label into result
 The quadruples in Fig (b) implement the three-
address code in (a) for the expression a = b * - c +
b * - c
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24. Quadruples 24

25. Triples
 A triple has only three fields for each instruction:op,arg1,
arg2
 The result of an operation x op y is referred to by its
position.
 Triples are equivalent to signatures of nodes in DAG or
syntax trees.
 Triples and DAGs are equivalent representations only for
expressions; they are not equivalent for control flow.
 Ternary operations like x[i] = y requires two entries in the
triple structure, similarly for x = y[i].
 Moving around an instruction during optimization is a
problem
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26. Representations of a = (b* - c) +
(b* – c)
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27. Indirect Triples
 These consist of a listing of pointers to triples, rather than a listing of
the triples themselves.
 An optimizing compiler can move an instruction by reordering the
instruction list, without affecting the triples themselves.
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28. 28

29. Attribute Grammar for
Assignments
 Concepts:
 Need to Introduce Temporary Variables as
Necessary to Decompose Assignment Statement
 Every Generated Line of Code Must have at Most
3 Addresses!
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30. Declarations
 Stack Utilized during Procedure/Function Calls to
 Allocate Space for Variables
 This now Includes Temporaries for 3AC
 We need to Track
 Name
 Type (Int, real, boolean, etc.)
 Offset (with respect to some relative address)
 Function
 enter (name, type, offset) creates symbol table entry
 offset global initially 0
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31. Storage Layout for Local
Names
 The type and relative address are saved in the symbol-table
entry for the name .
 The width of a type is the number of storage units needed for
objects of that type.
 type
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32. 32Code Generation for Boolean
Expressions
 Two approaches
 Numerical representation
 Implicit representation
 Numerical representation
 Use 1 to represent true, use 0 to represent
false
 For three-address code store this result in a
temporary
 For stack machine code store this result in the
stack

33. Code Generation for Boolean
Expressions
 Implicit representation
 For the boolean expressions which are used
in flow-of-control statements (such as if-
statements, while-statements etc.) boolean
expressions do not have to explicitly
compute a value, they just need to branch
to the right instruction
 Generate code for boolean expressions
which branch to the appropriate instruction
based on the result of the boolean
expression
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34. Generated Code
 Consider: a < b or c <
d and e < f
100: if a< b goto 103
101: t1:=0
102: goto 104
103: t1:=1
104: if c< d goto 107
105: t2:=0
106: goto 108
107: t2 := 1
108: if e< f goto 111
109: t3 := 0
110: goto 112
111: t3:=1
112: t4:=t2 and t3
113: t5:=t1 or t4
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