Compiler chapter 6.ppt course material

1. Chapter six
Intermediate Code Generation
Rift valley University
Harar Campus

2. Overview
 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 machine
model)
 This intermediate code serves as a bridge
between the high-level source code and
the final machine code or another target
language.

3. 3
Overview
 In a compiler, the front end translates source
program into an intermediate representation,
 and the back end generates the target code from
this intermediate representation.
 The use of a machine independent intermediate
code (IC) is:
 retargeting to another machine is facilitated
 the optimization can be done on the machine independent
code

4. 4
Overview
 Intermediate representations span the gap between
the source and target languages:
 closer to target language;
 (more or less) machine independent;
 allows many optimizations to be done in a machine-independent way.
 Implementable via syntax directed translation, so
can be folded into the parsing process.

5. Intermediate Representations
 Decisions in IR design affect the speed and
efficiency of the compiler
 Some important IR properties
 Ease of generation
 Ease of manipulation
 Procedure size
 Level of abstraction
 The importance of different properties varies
between compilers
 Selecting an appropriate IR for a compiler is
critical
5

6. 6
Types of Intermediate Languages
 High Level Representations (e.g., syntax trees):
 closer to the source language
 easy to generate from an input program
 code optimizations may not be straightforward.
 Low Level Representations (e.g., 3-address
code)
 closer to the target machine;
 easier for optimizations, final code generation;

7. Intermediate Code Generation
 Intermediate language can be many different languages, and the
designer of the compiler decides this intermediate language.
 Syntax tree can be used as an intermediate language.
 Postfix notation can be used as an intermediate language.
 Three-address code (Quadraples) can be used as an
intermediate language
 We will use three address to discuss intermediate code
generation.
 Three address are close to machine instructions, but they
are not actual machine instructions.
 Some programming languages have well defined intermediate
languages.
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8. 8
Syntax Trees
A syntax tree shows the structure of a program by abstracting
away irrelevant details from a parse tree.
 Each node represents a computation to be performed;
 The children of the node represents what that computation is
performed on.
 Syntax trees decouple parsing from subsequent
processing.

9. 9
Syntax Trees: Example
Grammar :
E  E + T | T
T  T * F | F
F  ( E ) | id
Input: id + id * id
Parse tree:
Syntax tree:

10. 10
Syntax Trees: Structure
 Expressions:
 leaves: identifiers or constants;
 internal nodes are labeled with operators;
 the children of a node are its operands.
 Statements:
 a node’s label indicates what kind of
statement it is;
 the children correspond to the components
of the statement.

11. Syntax Tree
 While parsing the input, a syntax tree can be constructed.
 A syntax tree (abstract tree) is a condensed form of parse tree
useful for representing language constructs.
 For example, for the string a + b, the parse tree in (a) below can
be represented by the syntax tree shown in (b);
 the keywords (syntactic sugar) that existed in the parse tree will
no longer exist in the syntax tree.
11
E
E
E
+
a b
Parse
tree
+
a b
Abstract
tree

12. Three-Address Code
 A three address code is: x := y op z
where x, y and z are names, constants or compiler-
generated temporaries; op is any operator.
 But we may also use the following notation for three
address code (much better notation because it looks like
a machine code instruction)
 op y, z, x apply operator op to y and z, and store the
result in x.
 We use the term “three-address code” because each
statement usually contains three addresses (two for
operands, one for the result).
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13. Three-Address Code
13
a:= b * -c + b * -c
t1 := - c
t2 := b * t1
t3 := - c
t4 := b * t3
t5 := t2 + t4
a := t5
t1 := - c
t2 := b * t1
t5 := t2 + t2
a := t5

14. Postfix Notation
 Postfix notation is a linear representation of a syntax tree.
 In the postfix notation, any expression can be written
unambiguously without parentheses
 In postfix notation, the operator appears after the operands,
i.e., the operator between operands is taken out & is attached
after operands.
 Example : Translate a ∗ d − (b + c) into Postfix form.
Solution
ad ∗ bc + −

15. Three-Address Code…
 In three-address code:
 Only one operator at the right side of the
assignment is possible, i.e. x + y * z is not
possible.
 Similar to postfix notation, the three address
code is a linear representation of a syntax tree.
 It has been given the name three-address code
because such an instruction usually contains
three addresses (the two operands and the result)
t1 = y * z
t2 = x + t1 15

16. 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

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

18. Example continued…
 Construct quadruple and triple for the following
equation:(a+b) * ( c+ d) - ( a+ b+ c) three Address code:
 a. Quadruple
b. triples
op arg1 arg2 result
+ a b t1
+ c d t2
* t1 t2 t3
+ t1 c t4
- t3 t4 t5 op arg1 arg2
+ a b
+ c d
* (0) (1)
+ (0) c
- (2) (3)