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CS432F/CSL 728:
Compiler Design
July 2004
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S. Arun-Kumar
sak@cse.iitd.ernet.in
Department of Computer Science and Engineering
I. I. T. Delhi, Hauz Khas, New Delhi 110 016.
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July 30, 2004
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Contents
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• first The Bigpicture last
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• first Lexical Analysis last
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• first Syntax Analysis last
–
–
–
–
Context-free Grammars last
first Ambiguity last
first Shift-Reduce Parsing last
first Parse Trees last
first
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• first Semantic Analysis last
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• first Synthesized Attributes last
• first Inherited Attributes last
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Contents
. . . Contd
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• first Abstract Syntax Trees last
• first Symbol Tables last
• first Intermediate Representation last
– IR: Properties
– Typical Instruction Set
– IR: Generation
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• first Runtime Structure last
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The Big Picture: 1
stream of
characters
SCANNER
stream of
tokens
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The Big Picture: 2
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SCANNER
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PARSER
parse tree
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The Big Picture: 3
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SCANNER
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PARSER
parse tree
SEMANTIC ANALYZER
abstract
syntax tree
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The Big Picture: 4
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SCANNER
stream of
tokens
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PARSER
parse tree
SEMANTIC ANALYZER
abstract
syntax tree
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I.R. CODE GENERATOR
intermediate
representation
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SCANNER
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PARSER
parse tree
SEMANTIC ANALYZER
abstract
syntax tree
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I.R. CODE GENERATOR
intermediate
representation
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OPTIMIZER
optimized
intermediate
representation
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SCANNER
stream of
tokens
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PARSER
parse tree
SEMANTIC ANALYZER
abstract
syntax tree
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I.R. CODE GENERATOR
intermediate
representation
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OPTIMIZER
optimized
intermediate
representation
CODE GENERATOR
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target code
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stream of
characters
SCANNER
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stream of
tokens
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PARSER
parse tree
ERROR−
HANDLER
SEMANTIC ANALYZER
abstract
syntax tree
SYMBOL
TABLE
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MANAGER
I.R. CODE GENERATOR
intermediate
representation
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OPTIMIZER
optimized
intermediate
representation
CODE GENERATOR
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stream of
characters
SCANNER
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stream of
tokens
PARSER
parse tree
ERROR−
HANDLER
SEMANTIC ANALYZER
abstract
syntax tree
SYMBOL
TABLE
MANAGER
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I.R. CODE GENERATOR
intermediate
representation
OPTIMIZER
optimized
intermediate
representation
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CODE GENERATOR
target code
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Scanner Parser
Semantic Analysis Symbol Table
IR
Optimization
Contents
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Scanning: 1
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• Takes a stream of characters and identifies tokens from
the lexemes.
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• Eliminates comments and redundant whitepace.
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• Keeps track of line numbers and column numbers and
passes them as parameters to the other phases to enable error-reporting to the user.
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Scanning: 2
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• Whitespace:
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• Lexeme: A sequence of non-whitespace characters de-
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A sequence of space, tab, newline,
carriage-return, form-feed characters etc.
limited by whitespace or special characters (e.g. operators like +, -, *).
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• Examples of lexemes.
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– reserved words, keywords, identifiers etc.
– Each comment is usually a single lexeme
– preprocessor directives
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• Token: A sequence of characters to be treated as a
single unit.
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• Examples of tokens.
Reserved words (e.g. begin, end, struct, if etc.)
Keywords (integer, true etc.)
Operators (+, &&, ++ etc)
Identifiers (variable names, procedure names, parameter names)
– Literal constants (numeric, string, character constants etc.)
– Punctuation marks (:, , etc.)
–
–
–
–
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Scanning: 4
• Identification of tokens is usually done by a Deterministic Finite-state automaton (DFA).
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• The set of tokens of a language is represented by a
large regular expression.
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• This regular expression is fed to a lexical-analyser generator such as Lex, Flex or ML-Lex.
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• A giant DFA is created by the Lexical analyser generator.
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Scanning: 5
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a−e,g−z
identifier
if
f
2
3
identifier
0−9,a−z
4
h,
j−
a−
other
chars
white
space
0−9
5
0−9
1
14
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0−9
z
i
0−9,a−z
6
real
0−9
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0−9
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8
7
int
any char
(
13
9
error
error
*
10
real
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*
11 )
12
comment
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Syntax Analysis
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Consider the following two languages over an alphabet
A = {a, b}.
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R = {anbn|n < 100}
P = {anbn|n > 0}
• R may be finitely represented by a regular expression
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(even though the actual expression is very long).
• However, P cannot actually be represented by a regular
Page 19 of 100
expression
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• A regular expression is not powerful enough to represent languages which require parenthesis matching to
arbitrary depths.
• All high level programming languages require an underlying language of expressions which require parentheses to be nested and matched to arbitrary depth.
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CF-Grammars:
Definition
A context-free grammar (CFG) G = hN, T , P, Si consists
of
• a set N of nonterminal symbols,
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• a set T of terminal symbols or the alphabet,
• a set P of productions or rewrite rules,
• each production is of the form X −→ α, where
– X ∈ N is a nonterminal and
– α ∈ (N ∪ T )∗ is a string of terminals and nonterminals
• a start symbol S ∈ N .
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CFG: Example
G = h{S}, {a, b}, P, Si, where S −→ ab and S −→ aSb
are the only productions in P .
Derivations look like this:
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S ⇒ ab
Page 21 of 100
S ⇒ aSb ⇒ aabb
S ⇒ aSb ⇒ aaSbb ⇒ aaabbb
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L(G), the language generated by G is {anbn|n > 0}.
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Actually can be proved by induction on the length and structure of derivations.
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CFG: Empty word
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G = h{S}, {a, b}, P, Si, where S −→ SS | aSb | ε
generates all sequences of matching nested parentheses,
including the empty word ε.
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A leftmost derivation might look like this:
S ⇒ SS ⇒ SSS ⇒ SS ⇒ aSbS ⇒ abS ⇒ abaSb . . .
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A rightmost derivation might look like this:
S ⇒ SS ⇒ SSS ⇒ SS ⇒ SaSb ⇒ Sab ⇒ aSbab . . .
Other derivations might look like God alone knows what!
S ⇒ SS ⇒ SSS ⇒ SS ⇒ . . .
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CFG: Derivation trees
1
Derivation sequences
• put an artificial order in which productions are fired.
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• instead look at trees of derivations in which we may think
of productions as being fired in parallel.
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• There is then no highlighting in red to determine which
copy of a nonterminal was used to get the next member
of the sequence.
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• Of course, generation of the empty word ε must be
shown explicitly in the tree.
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CFG: Derivation trees
2
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S
S
S
ε
εε
S
b
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S
S
a
JJ
a
a
S
S
Page 24 of 100
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b
b
Derivation tree of
abaabb
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ε
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CFG: Derivation trees
3
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S
S
S
a
S
b
S
S
a
S
b
ε
a
S
b
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Page 25 of 100
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ε
Another
Derivation tree of
ε
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abaabb
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CFG: Derivation trees
4
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S
S
S
ε
S
S
a
S
b
a
S
b
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Page 26 of 100
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a
S
b
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ε
Yet another
Derivation tree of
abaabb
ε
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Page 27 of 100
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Ambiguity: 1
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E → I | C | E+E | E∗E
I → y | z
C → 4
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Consider the sentence y + 4 ∗ z.
E
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E
Page 28 of 100
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Ambiguity: 2
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E → I | C | E+E | E∗E
I → y | z
C → 4
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Consider the sentence y + 4 ∗ z.
E
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E
Page 29 of 100
E
+
E
E
*
E
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Ambiguity: 3
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E → I | C | E+E | E∗E
I → y | z
C → 4
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Consider the sentence y + 4 ∗ z.
E
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E
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E
E
+
*
I
E
E
E
E
*
+
E
E
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I
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Ambiguity: 4
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E → I | C | E+E | E∗E
I → y | z
C → 4
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Consider the sentence y + 4 ∗ z.
E
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E
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E
E
+
*
I
E
E
E
*
+
E
E
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E
I
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I
y
C
I
C
z
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Ambiguity: 5
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E → I | C | E+E | E∗E
I → y | z
C → 4
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Consider the sentence y + 4 ∗ z.
E
JJ
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E
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E
E
+
*
I
E
E
E
*
+
E
E
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E
I
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I
y
I
C
C
z
z
4
y
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4
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Page 33 of 100
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Parsing: 0
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r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
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D
|
b
|
( E )
a
−
a
/
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b
Page 34 of 100
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Parsing: 1
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r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
( E )
−
a
/
JJ
II
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b
Page 35 of 100
Principle:
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whenever possible.
Shift only when
reduce is
impossible
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a
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Parsing: 2
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r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
( E )
−
a
/
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b
Page 36 of 100
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D
Reduce by r5
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Parsing: 3
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
( E )
−
a
/
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b
Page 37 of 100
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Close
T
Reduce by r4
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Parsing: 4
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
( E )
−
a
/
JJ
II
J
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b
Page 38 of 100
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Close
E
Reduce by r2
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Parsing: 5
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
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( E )
a
/
b
Page 39 of 100
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Close
−
E
Shift
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Parsing: 6
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
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( E )
/
b
Page 40 of 100
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a
Shift
Close
−
E
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Parsing: 7
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
/
b
Page 41 of 100
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D
Reduce by r5
Close
−
E
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Parsing: 8
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
/
b
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T
Reduce by r4
Close
−
E
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Parsing: 8a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
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( E )
/
b
Page 43 of 100
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T
−
E
Reduce by r4
Close
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Parsing: 9a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
/
b
Page 44 of 100
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Close
E
Reduce by r1
Quit
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Parsing: 10a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
b
Page 45 of 100
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Close
/
E
Shift
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Parsing: 11a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 46 of 100
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b
Shift
Close
/
E
Quit
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Parsing: 12a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 47 of 100
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D
Reduce by r5
Close
/
E
Quit
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Parsing: 13a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 48 of 100
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T
Reduce by r4
Close
/
E
Quit
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Parsing: 14a
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 49 of 100
k!
Go Back
St
uc
Full Screen
E
Get back!
Reduce by r2
Close
/
E
Quit
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Parsing: 14b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
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Go Back
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E
Get back!
Reduce by r2
Close
/
E
Quit
•First •Prev •Next •Last •Go Back •Full Screen •Close •Quit
Parsing: 13b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 51 of 100
Go Back
Full Screen
T
Get back!
Reduce by r4
Close
/
E
Quit
•First •Prev •Next •Last •Go Back •Full Screen •Close •Quit
Parsing: 12b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 52 of 100
Go Back
Full Screen
D
Get back!
Reduce by r5
Close
/
E
Quit
•First •Prev •Next •Last •Go Back •Full Screen •Close •Quit
Parsing: 11b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
Page 53 of 100
Go Back
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Get back!
b
Shift
Close
/
E
Quit
•First •Prev •Next •Last •Go Back •Full Screen •Close •Quit
Parsing: 10b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
b
Page 54 of 100
Go Back
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Close
Get back!
/
E
Shift
Quit
•First •Prev •Next •Last •Go Back •Full Screen •Close •Quit
Parsing: 9b
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
J
I
( E )
/
b
Page 55 of 100
Go Back
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Get back to
where you
once belonged!
Close
E
Reduce by r1
Quit
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Parsing: 8b
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
fied
i
d
o
m Principle:
T
Reduce whenever possible, but
but depending upon
D
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lookahead
|
b
|
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/
b
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Shift instead
of reduce here!
Go Back
ce
redu
−
t
f
i
Sh
ict
confl
Full Screen
T
−
E
Reduce by r4
Close
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Parsing: 8
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
/
b
Page 57 of 100
Go Back
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T
Reduce by r4
Close
−
E
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Parsing: 9
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
b
Page 58 of 100
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Shift
/
T
Close
−
E
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Parsing: 10
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
Page 59 of 100
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b
Shift
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/
T
Close
−
E
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Parsing: 11
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
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D
/
Reduce by r5
Full Screen
T
Close
−
E
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Parsing: 12
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
JJ
II
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( E )
Page 61 of 100
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T
−
E
Reduce by r3
Close
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Parsing: 13
Home Page
r1. E
E −
r2 E
T
r3 T
T /
r4 T
D
r5 D
a
T
Title Page
D
|
b
|
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( E )
Page 62 of 100
Go Back
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Close
E
Reduce by r1
Quit
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Parse Trees: 0
r1. E
E −
r2 E
T
r3 T
T /
Home Page
Title Page
T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 63 of 100
Go Back
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a
−
a
/
b
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Parse Trees: 1
r1. E
E −
r2 E
T
r3 T
T /
Home Page
Title Page
T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 64 of 100
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D
Close
a
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a
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b
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Parse Trees: 2
r1. E
E −
r2 E
T
r3 T
T /
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 65 of 100
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T
Full Screen
D
Close
a
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a
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b
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Parse Trees: 3
r1. E
E −
r2 E
T
r3 T
T /
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Title Page
T
r4 T
r5 D
D
D
a
|
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( E )
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Page 66 of 100
E
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T
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D
Close
a
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Parse Trees: 3a
r1. E
E −
r2 E
T
r3 T
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T
r4 T
r5 D
D
D
a
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( E )
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E
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Parse Trees: 3b
r1. E
E −
r2 E
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T /
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T
r4 T
r5 D
D
D
a
|
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( E )
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E
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D
Close
a
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Parse Trees: 4
r1. E
E −
r2 E
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T /
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T
r4 T
r5 D
D
D
a
|
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|
( E )
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E
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Full Screen
D
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a
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Parse Trees: 5
r1. E
E −
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T /
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T
r4 T
r5 D
D
D
a
|
b
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Page 70 of 100
E
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Full Screen
D
D
Close
a
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Parse Trees: 5a
r1. E
E −
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T /
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 71 of 100
E
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T
Full Screen
D
D
Close
a
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a
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Parse Trees: 5b
r1. E
E −
r2 E
T
r3 T
T /
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 72 of 100
E
Go Back
T
T
Full Screen
D
D
Close
a
−
a
/
b
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Parse Trees: 6
r1. E
E −
r2 E
T
r3 T
T /
Home Page
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 73 of 100
E
Go Back
T
T
Full Screen
D
D
D
Close
a
−
a
/
b
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Parse Trees: 7
r1. E
E −
r2 E
T
r3 T
T /
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
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Page 74 of 100
E
T
Go Back
T
T
Full Screen
D
D
D
Close
a
−
a
/
b
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Parse Trees: 8
r1. E
E −
r2 E
T
r3 T
T /
Home Page
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T
r4 T
r5 D
D
D
a
|
b
|
( E )
E
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Page 75 of 100
E
T
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D
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Close
a
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Home Page
Parsing: Summary: 1
Title Page
• All high-level languages are designed so that they may
JJ
II
be parsed in this fashion with only a single token lookahead.
J
I
• Parsers for a language can be automatically constructed by parger-generators such as Yacc, Bison, MLYacc.
• Shift-reduce conflicts if any, are automatically detected
Page 76 of 100
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and reported by the parser-generator.
• Shift-reduce conflicts may be avoided by suitably
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redesigning the context-free grammar.
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Parsing: Summary: 2
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• Very often shift-reduce conflicts may occur because
of the prefix problem. In such cases many parsergenerators resolve the conflict in favour of shifting.
• There is also a possiblility of reduce-reduce conflicts.
This usually happens when there is more than one nonterminal symbol to which the contents of the stack may
reduce.
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II
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Page 77 of 100
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• A minor reworking of the grammar to avoid redundant
non-terminal symbols will get rid of reduce-reduce conflicts.
The Big Picture
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Semantic Analysis: 1
• Every Programming langauge can be used to program
any computable function, assuming of course, it has
– unbounded memory, and
– unbounded time
• The parser of a programming language provides the
framework within which the target code is to be generated.
• The parser also provides a structuring mechanism that
divides the task of code generation into bits and pieces
determined by the individual nonterminals and production rules.
• However, contex-free grammars are not powerful
enough to represent all computable functions. Example, the language {an bn cn |n > 0}.
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Semantic Analysis: 2
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• There are context-sensitive aspects of a program that
cannot be represented/enforced by a context-free grammar definition. Examples include
– correspondence between formal and actual parameters
– type consistency between declaration and use.
– scope and visibility issues with respect to identifiers
in a program.
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Synthesized Attributes:
0
Home Page
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E
T
F
/
(
)
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)
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T
Full Screen
−
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Close
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n
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Synthesized Attributes:
1
Home Page
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E
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/
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)
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−
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4
T
F
3
2
Full Screen
1
Close
F
n
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n
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Synthesized Attributes:
2
Home Page
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E
T
F
/
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(
)
E
E
T
−
Synthesized Attributes
4
T
4
F
3
2
Full Screen
1
Close
F
n
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n
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Synthesized Attributes:
3
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E
T
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)
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3
2
1
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n
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n
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4
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E
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/
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)
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F
3
2
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Synthesized Attributes:
5
Home Page
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E
T
F
/
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Full Screen
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Synthesized Attributes
−
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3
2
1
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1
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E
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n
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E
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n
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Home Page
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E
T
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/
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4
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3
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1
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1
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E
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4
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n
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E
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4
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3
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1
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4
4
4
T
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1
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E
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/
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E
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/
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An Attribute Grammar
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Title Page
T
E
1
T
1
E0 → E1−T B E0.val := sub(E1.val, T.val)
E → T
3
F
2
3
n
2
T0 → T1/F
II
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B E.val := T.val
/
F
JJ
B T0.val := div(T1.val, F.val)
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(
E
4
E
3
)
T
−
4
4
4
T
1
F
1
n
1
T
→ F
B T.val := F.val
Go Back
F
→ (E)
B F.val := E.val
F
n
F
→ n
B F.val := n.val
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Inherited Attributes: 0
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
T
Title Page
JJ
II
J
I
L
I
,
int
L
Page 97 of 100
z
I
L
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,
y
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I
D
L
x
y
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int
Close
x
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,
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Inherited Attributes: 1
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
T
Title Page
JJ
II
J
I
L
I
int
,
int
L
Page 98 of 100
z
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y
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I
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L
x
y
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int
Close
x
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,
int
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Inherited Attributes: 2
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
int
T
Title Page
JJ
II
J
I
L
I
int
,
int
L
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I
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Close
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,
int
int
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Inherited Attributes: 3
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
int
T
L
Title Page
JJ
II
J
I
int
I
int
,
int
L
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z
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y
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I
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L
x
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int
Close
x
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,
int
int
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Inherited Attributes: 4
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
Home Page
T → int | float
I → x | y | z
D
int
T
L
JJ
II
J
I
int
I
int
Title Page
int
,
int
L
int
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y
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I
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x
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int
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int
int
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Inherited Attributes: 5
Home Page
D
Title Page
int
T
L
int
I
int
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int
,
int
L
int
z
I
L
int
int
int
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y
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D
L
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Inherited Attributes: 6
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
int
T
L
L
L
I
x
y
z
I
int
I
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int
int
int
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,
y
T
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int
int
I
L
II
,
int
z
D
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int
I
int
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int
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int
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Inherited Attributes: 7
C -style declarations generating int x, y, z.
D → TL
L → L,I | I
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T → int | float
I → x | y | z
D
int
T
L
L
L
I
x
y
z
int
int
int
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int
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int
I
int
I
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int
int
I
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II
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int
z
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int
I
int
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An Attribute Grammar
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D
int
D → TL
T
L
I
int
int
,
int
L
I
int
I
int
T
→ int
T
→ float B T.type := float.f loat
B T.type := int.int
int
z
L
B L.in := T.type
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int
int
,
int
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int
L0 → L1,I
B L1 := L0.in
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x
int
L → I
B I.in := L.in
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I
→ id
B id.type := I.in
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Abstract Syntax: 0
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E → E−T | T
T → T /F | F
F → n | (E)
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Suppose we want to evaluate an expression (4 − 1)/2.
What we actually want is a tree that looks like this:
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/
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/
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/
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1
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But what we actually get during parsing is a tree that looks
like . . .
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Abstract Syntax: 1
. . . THIS!
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E
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T
T
F
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/
F
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(
)
E
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E
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T
−
n
−−
T
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n
n
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Abstract Syntax: 2
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We use attribute grammar rules to construct the abstract
syntax tree (AST)!.
But in order to do that we first require two procedures for
tree construction.
makeLeaf(literal) : Creates a node with label literal and returns a pointer to it.
makeBinaryNode(opr, opd1, opd2) : Creates a node with label opr (with fields which point to opd1 and opd2) and
returns a pointer to the newly created node.
Now we may associate a synthesized attribute called ptr
with each terminal and nonterminal symbol which points to
the root of the subtree created for it.
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Abstract Syntax: 3
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E0 → E1−T B E0.ptr := makeBinaryN ode(−, E1.ptr, T.ptr)
E → T
T0 → T1/F
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B E.ptr := T.ptr
B T0.ptr := makeBinaryN ode(/, T1.ptr, F.ptr)
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T
→ F
B T.ptr := F.ptr
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F
→ (E)
B F.ptr := E.ptr
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F
→ n
B F.ptr := makeLeaf (n.val)
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Symbol Table:1
• The store house of context-sensitive and run-time information about every identifier in the source program.
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• All accesses relating to an identifier require to first find
the attributes of the identifier from the symbol table
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• Usually organized as a hash table – provides fast access.
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• Compiler-generated temporaries may also be stored in
the symbol table
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Symbol Table:2
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Attributes stored in a symbol table for each identifier:
• type
• size
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• scope/visibility information
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• base address
• addresses to location of auxiliary symbol tables (in case
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of records, procedures, classes)
• address of the location containing the string which actually names the identifier and its length in the string
pool
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Symbol Table:3
• A symbol table exists through out the compilation and
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run-time.
• Major operations required of a symbol table:
– insertion
– search
– deletions are purely logical (depending on scope
and visibility) and not physical
• Keywords are often stored in the symbol table before
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the compilation process begins.
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Symbol Table:4
Accesses to the symbol table at every stage of the compilation process,
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Scanning: Insertion of new identifiers.
Parsing: Access to the symbol table to ensure that an
operand exists (declaration before use).
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Semantic analysis:
• Determination of types of identifiers from declarations
• type checking to ensure that operands are used in
type-valid contexts.
• Checking scope, visibility violations.
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Symbol Table:5
IR generation: . Memory allocation and relativea address
calculation.
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Optimization: All memory accesses through symbol table
Target code: Translation of relative addresses to absolute
addresses in terms of word length, word boundary etc.
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a
i.e.relative to a base address that is known only at run-time
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Intermediate
Representation
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Intermediate representations are important for reasons of
portability.
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• (more or less) independent of specific features of the
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high-level language.
Example. Java byte-code for any high-level language.
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• (more or less) independent of specific features of any
particular target architecture (e.g. number of registers,
memory size)
– number of registers
– memory size
– word length
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IR Properties: 1
1. It is fairly low-level containing instructions common to
all target architectures and assembly languages.
How low can you stoop? . . .
2. It contains some fairly high-level instructions that are
common to most high-level programming languages.
How high can you rise?
3. To ensure portability
• an unbounded number of variables and memory locations
• no commitment to Representational Issues
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4. To ensure type-safety
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• memory locations are also typed according to the
data they may contain,
• no commitment is made regarding word boundaries,
and the structure of individual data items.
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IR: Representation?
• No commitment to word boundaries or byte boundaries
• No commitment to representation of
–
–
–
–
int vs. float,
float vs. double,
packed vs. unpacked,
strings – where and how?.
Back to IR Properties:1
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IR: How low can you
stoop?
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• most arithmetic and logical operations, load and store
instructions etc.
• so as to be interpreted easily,
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• the interpreter is fairly small,
• execution speeds are high,
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• to have fixed length instructions (where each operand
position has a specific meaning).
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IR: How high can you
rise?
• typed variables,
• temporary variables instead of registers.
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• array-indexing,
• random access to record fields,
• parameter-passing,
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• pointers and pointer management
• no limits on memory addresses
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A typical instruction
set: 1
Three address code: A suite of instructions. Each instruction has at most 3 operands.
• an opcode representing an operation with at most 2
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operands
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• two operands on which the binary operation is performed
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• a target operand, which accumulates the result of the
(binary) operation.
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If an operation requires less than 3 operands then one or
more of the operands is made null.
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A typical instruction
set: 2
• Assignments (LOAD-STORE)
• Jumps (conditional and unconditional)
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• Procedures and parameters
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• Arrays and array-indexing
• Pointer Referencing and Dereferencing
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A typical instruction
set: 2
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• Assignments (LOAD-STORE)
– x := y bop z, where bop is a binary operation
– x := uop y, where uop is a unary operation
– x := y, load, store, copy or register transfer
• Jumps (conditional and unconditional)
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• Procedures and parameters
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• Arrays and array-indexing
• Pointer Referencing and Dereferencing
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A typical instruction
set: 2
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• Assignments (LOAD-STORE)
• Jumps (conditional and unconditional)
– goto L – Unconditional jump,
– x relop y goto L – Conditional jump, where
relop is a relational operator
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• Procedures and parameters
• Arrays and array-indexing
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• Pointer Referencing and Dereferencing
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A typical instruction
set: 2
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• Assignments (LOAD-STORE)
• Jumps (conditional and unconditional)
• Procedures and parameters
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– call p n, where n is the number of parameters
– return y, return value from a procedures call
– param x, parameter declaration
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• Arrays and array-indexing
• Pointer Referencing and Dereferencing
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A typical instruction
set: 2
• Assignments (LOAD-STORE)
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• Jumps (conditional and unconditional)
• Procedures and parameters
• Arrays and array-indexing
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– x := a[i] – array indexing for r-value
– a[j] := y – array indexing for l-value
Note: The two opcodes are different depending on
whether l-value or r-value is desired. x and y are always
simple variables
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A typical instruction
set: 2
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• Assignments (LOAD-STORE)
• Jumps (conditional and unconditional)
• Procedures and parameters
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• Arrays and array-indexing
• Pointer Referencing and Dereferencing
– x := ˆy – referencing: set x to point to y
– x := *y – dereferencing: copy contents of location
pointed to by y into x
– *x := y – dereferencing: copy r-value of y into the
location pointed to by x
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Pointers
y
x
*x
x
x
x := ^y
*y
y
@y
@z
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*y
y
x
y
*x
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@z
*z
x := *y
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*z
*z
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z
y
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*y
*y
x
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*x := y
@z
@z
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*z
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IR: Generation
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• Can be generated by recursive traversal of the abstract
syntax tree.
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• Can be generated by syntax-directed translation as follows:
For every non-terminal symbol N in the grammar of the
source language there exist two attributes
N.place , which denotes the address of a temporary
variable where the result of the execution of the generated code is stored
N.code , which is the actual code segment generated.
• In addition a global counter for the instructions gener-
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ated is maintained as part of the generation process.
• It is independent of the source language but can express target machine operations without committing to
too much detail.
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IR: Infrastructure
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Given an abstract syntax tree T, with T also denoting its
root node.
T.place address of temporary variable where result of execution of the T is stored.
newtemp returns a fresh variable name and also installs it in
the symbol table along with relevant information
T.code the actual sequence of instructions generated for
the tree T.
newlabel returns a label to mark an instruction in the generated code which may be the target of a jump.
emit emits an instructions (regarded as a string).
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IR: Infrastructure
Colour and font coding of IR code generation
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• Green: Nodes of the Abstract Syntax Tree
• Brown: Characters and strings of the Intermediate
Representation
• Red: Variables and data structures of the language in
which the IR code generator is written
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• blue: Names of relevant procedures used in IR code generation.
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• Black: All other stuff.
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IR: Example
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E
→ id
B
E.place := id.place;
E.code := emit()
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E0
→ E1 − E2
B
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E0.place := newtemp;
E0.code := E1.code
|| E2.code
|| emit(E0.place := E1.place − E2.place)
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IR: Example
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S
→ id := E
S.code
:= E.code
|| emit(id.place:=E.place)
S0
B
→ while E do S1
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B
Page 138 of 100
S0.begin := newlabel;
S0.af ter := newlabel;
S0.code := emit(S0.begin:)
|| E.code
|| emit(if E.place= 0 goto S0.af ter)
|| S1.code
|| emit(gotoS0.begin)
|| emit(S0.af ter:)
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IR: Example
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S
→ id := E
S.code
:= E.code
|| emit(id.place:=E.place)
S0
B
→ while E do S1
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B
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S0.begin := newlabel;
S0.af ter := newlabel;
S0.code := emit(S0.begin:)
|| E.code
|| emit(if E.place= 0 goto S0.af ter)
|| S1.code
|| emit(gotoS0.begin)
|| emit(S0.af ter:)
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IR: Generation
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While generating the intermediate representation, it is
sometimes necessary to generate jumps into code that has
not been generated as yet (hence the address of the label
is unknown). This usually happens while processing
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• forward jumps
• short-circuit evaluation of boolean expressions
It is usual in such circumstances to either fill up the empty
label entries in a second pass over the the code or through
a process of backpatching (which is the maintenance of
lists of jumps to the same instruction number), wherein the
blank entries are filled in once the sequence number of the
target instruction becomes known.
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A Calling Chain
Main program
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Title Page
Globals
Procedure P2
Locals of P2
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Procedure P21
Locals of P21
Body of P21
Call P21
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Body of P2
Call P21
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Procedure P1
Locals of P1
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Body of P1
Call P2
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Run-time Structure: 1
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Main program
Title Page
Globals
Procedure P2
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Locals of P2
Procedure P21
Locals of P21
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Body of P21
Body of P2
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Procedure P1
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Run-time Structure: 2
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Main program
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Globals
Procedure P2
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Locals of P2
Procedure P21
Locals of P21
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Body of P21
Body of P2
Procedure P1
Locals of P1
Body of P1
Main body
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Return address to Main
Dynamic link to Main
Locals of P1
Static link to Main
Formal par of P1
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Run-time Structure: 3
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Main program
Title Page
Globals
Procedure P2
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Locals of P2
Procedure P21
Locals of P21
Body of P21
Body of P2
Procedure P1
Locals of P1
Body of P1
Main body
Return address to last of P1
Dynamic link to last P1
Locals of P2
Static link to last P1
Formal par P2
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Return address to Main
Dynamic link to Main
Locals of P1
Static link to Main
Formal par of P1
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Run-time Structure: 4
Home Page
Main program
Title Page
Globals
Procedure P2
Locals of P2
Procedure P21
Locals of P21
Body of P21
Body of P2
Procedure P1
Locals of P1
Body of P1
Main body
Return address to last of P2
Dynamic link to last P2
Locals of P21
Static link last P2
Formal par P21
Return address to last of P1
Dynamic link to last P1
Locals of P2
Static link to last P1
Formal par P2
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Return address to Main
Dynamic link to Main
Locals of P1
Static link to Main
Formal par of P1
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Run-time Structure: 5
Main program
Globals
Return address to last of P21
Dynamic link to last P21
Locals of P21
Static link to last P2
Formal par P21
Procedure P2
Locals of P2
Procedure P21
Locals of P21
Body of P21
Body of P2
Procedure P1
Locals of P1
Body of P1
Main body
Return address to last of P2
Dynamic link to last P2
Locals of P21
Static link last P2
Formal par P21
Return address to last of P1
Dynamic link to last P1
Locals of P2
Static link to last P1
Formal par P2
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Return address to Main
Dynamic link to Main
Locals of P1
Static link to Main
Formal par of P1
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