Chapter 6 intermediate code generation

Chapter 6
Intermediate Code Generation

Outline
Variants of Syntax Trees
Three-address code
Types and declarations
Translation of expressions
Type checking
Control flow
Backpatching

Introduction
Intermediate code is the interface between front end
and back end in a compiler
Ideally the details of source language are confined to
the front end and the details of target machines to the
back end (a m*n model)
In this chapter we study intermediate
representations, static type checking and
intermediate code generation
Parser Static
Checker
Intermediate
Code Generator
Code
Generator
Front end Back end

Variants of syntax trees
It is sometimes beneficial to crate a DAG instead of
tree for Expressions.
This way we can easily show the common sub-expressions
and then use that knowledge during code
generation
Example: a+a*(b-c)+(b-c)*d
+
+ *
*
-
b c
a
d

SDD for creating DAG’s
Production Semantic Rules
1) E -> E1+T
2) E -> E1-T
3) E -> T
4) T -> (E)
5) T -> id
6) T -> num
E.node= new Node(‘+’, E1.node,T.node)
E.node= new Node(‘-’, E1.node,T.node)
E.node = T.node
T.node = E.node
T.node = new Leaf(id, id.entry)
T.node = new Leaf(num, num.val)
Example:
1)p1=Leaf(id, entry-a)
2)P2=Leaf(id, entry-a)=p1
3)p3=Leaf(id, entry-b)
4)p4=Leaf(id, entry-c)
5)p5=Node(‘-’,p3,p4)
6)p6=Node(‘*’,p1,p5)
7)p7=Node(‘+’,p1,p6)
8) p8=Leaf(id,entry-b)=p3
9) p9=Leaf(id,entry-c)=p4
10) p10=Node(‘-’,p3,p4)=p5
11) p11=Leaf(id,entry-d)
12) p12=Node(‘*’,p5,p11)
13) p13=Node(‘+’,p7,p12)

Value-number method for
constructing DAG’s
=
+
i 10
Algorithm
Search the array for a node M with label op, left child l
and right child r
If there is such a node, return the value number M
If not create in the array a new node N with label op,
left child l, and right child r and return its value
We may use a hash table
id To entry for i
num 10
+ 1 2
3 1 3

Three address code
In a three address code there is at most one operator
at the right side of an instruction
Example:
+
+ *
*
-
b c
a
d
t1 = b – c
t2 = a * t1
t3 = a + t2
t4 = t1 * d
t5 = t3 + t4

Forms of three address
instructions
x = y op z
x = op y
x = y
goto L
if x goto L and ifFalse x goto L
if x relop y goto L
Procedure calls using:
 param x
 call p,n
 y = call p,n
x = y[i] and x[i] = y
x = &y and x = *y and *x =y

Example
do i = i+1; while (a[i] < v);
L: t1 = i + 1
i = t1
t2 = i * 8
t3 = a[t2]
if t3 < v goto L
Symbolic labels
100: t1 = i + 1
101: i = t1
102: t2 = i * 8
103: t3 = a[t2]
104: if t3 < v goto 100
Position numbers

Data structures for three
address codes
Quadruples
Has four fields: op, arg1, arg2 and result
Triples
Temporaries are not used and instead references to
instructions are made
Indirect triples
In addition to triples we use a list of pointers to triples

Example
b * minus c + b * minus c
Three address code
t1 = minus c
t2 = b * t1
t3 = minus c
t4 = b * t3
t5 = t2 + t4
a = t5
Quadruples
op arg1 arg2 result
minus
* minus c t3
*
+
=
c t1
b t1 t2
b t3 t4
t2 t4 t5
t5 a
Triples
op arg1 arg2
minus
* minus c
*
+
=
c
b (0)
b (2)
(1) (3)
a
(4)
012
345
Indirect Triples
op arg1 arg2
minus
* minus c
*
+
=
c
b (0)
b (2)
(1) (3)
a
(4)
012
345
op
(0)
(1)
(2)
(3)
(4)
(5)
35
36
37
38
39
40

Type Expressions
Example: int[2][3]
array(2,array(3,integer))
 A basic type is a type expression
 A type name is a type expression
 A type expression can be formed by applying the array type
constructor to a number and a type expression.
 A record is a data structure with named field
 A type expression can be formed by using the type constructor  for
function types
 If s and t are type expressions, then their Cartesian product s*t is a
type expression
 Type expressions may contain variables whose values are type
expressions

Type Equivalence
They are the same basic type.
They are formed by applying the same constructor to
structurally equivalent types.
One is a type name that denotes the other.

Storage Layout for Local Names
Computing types and their widths

Storage Layout for Local Names
Syntax-directed translation of array types

Sequences of Declarations

Actions at the end:


Fields in Records and Classes



Translation of Expressions and
Statements
We discussed how to find the types and offset of
variables
We have therefore necessary preparations to discuss
about translation to intermediate code
We also discuss the type checking

Three-address code for expressions

Addressing Array Elements
Layouts for a two-dimensional array:

Semantic actions for array reference

Translation of Array References
Nonterminal L has three synthesized
attributes:
L.addr
L.array
L.type

Conversions between primitive
types in Java

Introducing type conversions into
expression evaluation

Abstract syntax tree for the
function definition
fun length(x) =
if null(x) then 0 else length(tl(x)+1)
This is a polymorphic function
in ML language

Inferring a type for the function length

Unification algorithm
boolean unify (Node m, Node n) {
s = find(m); t = find(n);
if ( s = t ) return true;
else if ( nodes s and t represent the same basic type ) return true;
else if (s is an op-node with children s1 and s2 and
t is an op-node with children t1 and t2) {
union(s , t) ;
return unify(s1, t1) and unify(s2, t2);
}
else if s or t represents a variable {
union(s, t) ;
return true;
}
else return false;
}

Control Flow
boolean expressions are often used to:
Alter the flow of control.
Compute logical values.

Generating three-address code for booleans

translation of a simple if-statement



Backpatching
Previous codes for Boolean expressions insert symbolic labels for
jumps
It therefore needs a separate pass to set them to appropriate addresses
We can use a technique named backpatching to avoid this
We assume we save instructions into an array and labels will be
indices in the array
For nonterminal B we use two attributes B.truelist and B.falselist
together with following functions:
makelist(i): create a new list containing only I, an index into the array
of instructions
Merge(p1,p2): concatenates the lists pointed by p1 and p2 and returns a
pointer to the concatenated list
Backpatch(p,i): inserts i as the target label for each of the instruction
on the list pointed to by p

Backpatching for Boolean Expressions



Backpatching for Boolean Expressions
Annotated parse tree for x < 100 || x > 200 && x ! = y

Translation of a switch-statement

Readings
Chapter 6 of the book

Chapter 6 intermediate code generation

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