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Home Page Title Page CS432F/CSL 728: Compiler Design July 2004 Page 1 of 100 S. Arun-Kumar Go Back sak@cse.iitd.ernet.in Department of Computer Science and Engineering Full Screen I. I. T. Delhi, Hauz Khas, New

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CS432F/CSL 728:

Compiler Design

July 2004

  • S. Arun-Kumar

sak@cse.iitd.ernet.in Department of Computer Science and Engineering

  • I. I. T. Delhi, Hauz Khas, New Delhi 110 016.

July 30, 2004

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Contents

  • first The Bigpicture last
  • first Lexical Analysis last
  • first Syntax Analysis last

– first Context-free Grammars last – first Ambiguity last – first Shift-Reduce Parsing last – first Parse Trees last

  • first Semantic Analysis last
  • first Synthesized Attributes last
  • first Inherited Attributes last

Contd . . .

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Contents

. . . Contd

  • first Abstract Syntax Trees last
  • first Symbol Tables last
  • first Intermediate Representation last

– IR: Properties – Typical Instruction Set – IR: Generation

  • first Runtime Structure last
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The Big Picture: 1

SCANNER stream of characters stream of tokens

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The Big Picture: 2

SCANNER PARSER stream of characters stream of tokens parse tree

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The Big Picture: 3

SCANNER PARSER SEMANTIC ANALYZER stream of characters stream of tokens parse tree abstract syntax tree

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The Big Picture: 4

SCANNER PARSER SEMANTIC ANALYZER I.R. CODE GENERATOR stream of characters stream of tokens parse tree intermediate representation abstract syntax tree

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The Big Picture: 5

SCANNER PARSER SEMANTIC ANALYZER I.R. CODE GENERATOR OPTIMIZER stream of characters stream of tokens parse tree intermediate representation

  • ptimized

intermediate representation abstract syntax tree

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The Big Picture: 6

SCANNER PARSER SEMANTIC ANALYZER I.R. CODE GENERATOR OPTIMIZER CODE GENERATOR stream of characters stream of tokens parse tree intermediate representation

  • ptimized

intermediate representation target code abstract syntax tree

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The Big Picture: 7

SCANNER PARSER SEMANTIC ANALYZER I.R. CODE GENERATOR OPTIMIZER CODE GENERATOR ERROR− HANDLER SYMBOL TABLE MANAGER stream of characters stream of tokens parse tree intermediate representation

  • ptimized

intermediate representation target code abstract syntax tree

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The Big Picture: 7

SCANNER PARSER SEMANTIC ANALYZER I.R. CODE GENERATOR OPTIMIZER CODE GENERATOR ERROR− HANDLER SYMBOL TABLE MANAGER stream of characters stream of tokens parse tree intermediate representation

  • ptimized

intermediate representation target code abstract syntax tree

Scanner Parser Semantic Analysis Symbol Table IR Optimization Contents

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Scanning: 1

  • Takes a stream of characters and identifies tokens from

the lexemes.

  • Eliminates comments and redundant whitepace.
  • Keeps track of line numbers and column numbers and

passes them as parameters to the other phases to en- able error-reporting to the user.

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Scanning: 2

  • Whitespace:

A sequence of space, tab, newline, carriage-return, form-feed characters etc.

  • Lexeme: A sequence of non-whitespace characters de-

limited by whitespace or special characters (e.g. oper- ators like +, -, *).

  • Examples of lexemes.

– reserved words, keywords, identifiers etc. – Each comment is usually a single lexeme – preprocessor directives

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Scanning: 3

  • Token: A sequence of characters to be treated as a

single unit.

  • Examples of tokens.

– Reserved words (e.g. begin, end, struct, if etc.) – Keywords (integer, true etc.) – Operators (+, &&, ++ etc) – Identifiers (variable names, procedure names, pa- rameter names) – Literal constants (numeric, string, character con- stants etc.) – Punctuation marks (:, , etc.)

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Scanning: 4

  • Identification of tokens is usually done by a Determinis-

tic Finite-state automaton (DFA).

  • The set of tokens of a language is represented by a

large regular expression.

  • This regular expression is fed to a lexical-analyser gen-

erator such as Lex, Flex or ML-Lex.

  • A giant DFA is created by the Lexical analyser genera-

tor.

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Scanning: 5

1 2 3 4 5 6 7 8 9 13 i f a − h , j − z a−e,g−z 0−9,a−z 0−9,a−z 0−9 0−9 − 9 0−9 0−9 ( * 10 11 12 14 * if identifier real real error comment white space error int identifier

  • t

h e r c h a r s any char )

The Big Picture

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Syntax Analysis

Consider the following two languages over an alphabet

A = {a, b}. R = {anbn|n < 100} P = {anbn|n > 0}

  • R may be finitely represented by a regular expression

(even though the actual expression is very long).

  • However, P cannot actually be represented by a regular

expression

  • A regular expression is not powerful enough to repre-

sent languages which require parenthesis matching to arbitrary depths.

  • All high level programming languages require an under-

lying language of expressions which require parenthe- ses to be nested and matched to arbitrary depth.

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CF-Grammars: Definition

A context-free grammar (CFG) G = N, T, P, S consists

  • f
  • a set N of nonterminal symbols,
  • 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 nontermi- nals

  • a start symbol S ∈ N.
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CFG: Example

G = {S}, {a, b}, P, S, where S − → ab and S − → aSb

are the only productions in P. Derivations look like this:

S ⇒ ab S ⇒ aSb ⇒ aabb S ⇒ aSb ⇒ aaSbb ⇒ aaabbb L(G), the language generated by G is {anbn|n > 0}.

Actually can be proved by induction on the length and struc- ture of derivations.

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CFG: Empty word

G = {S}, {a, b}, P, S, where S − → SS | aSb | ε

generates all sequences of matching nested parentheses, including the empty word ε. A leftmost derivation might look like this:

S ⇒ SS ⇒ SSS ⇒ SS ⇒ aSbS ⇒ abS ⇒ abaSb . . .

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 ⇒ . . .

Could be quite confusing!

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CFG: Derivation trees 1

Derivation sequences

  • put an artificial order in which productions are fired.
  • instead look at trees of derivations in which we may think
  • f productions as being fired in parallel.
  • There is then no highlighting in red to determine which

copy of a nonterminal was used to get the next member

  • f the sequence.
  • Of course, generation of the empty word ε must be

shown explicitly in the tree.

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CFG: Derivation trees 2

S S S S S S S S a b a b a b ε ε ε

Derivation tree of abaabb

ε

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CFG: Derivation trees 3

S S S S S S S a b a b ε ε S a b ε

Another Derivation tree of abaabb

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CFG: Derivation trees 4

S S S S S S a b a b ε S a b ε S ε

Yet another Derivation tree of abaabb

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Ambiguity: 1

E → I | C | E+E | E∗E I → y | z C → 4

Consider the sentence y + 4 ∗ z.

E E

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Ambiguity: 2

E → I | C | E+E | E∗E I → y | z C → 4

Consider the sentence y + 4 ∗ z.

E E E * E E E +

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Ambiguity: 3

E → I | C | E+E | E∗E I → y | z C → 4

Consider the sentence y + 4 ∗ z.

E E E E * E + I E E E E + E I *

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Ambiguity: 4

E → I | C | E+E | E∗E I → y | z C → 4

Consider the sentence y + 4 ∗ z.

E E E E * E + I C I z E E E E + E C I y I *

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Ambiguity: 5

E → I | C | E+E | E∗E I → y | z C → 4

Consider the sentence y + 4 ∗ z.

E E E E * E + I y C 4 I z E E E E + E C I y 4 I z *

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Parsing: 0

− / b ( ) a − a a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E

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Parsing: 1

− / b ( ) a − a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E

Shift

a

Principle:

Reduce whenever possible. Shift only when reduce is impossible

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Parsing: 2

− / b ( ) a − a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E D

Reduce by r5

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Parsing: 3

− / b ( ) a − a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E T

Reduce by r4

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Parsing: 4

− / b ( ) a − a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E E

Reduce by r2

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Parsing: 5

− / b ( ) a − a / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E E

Shift

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Parsing: 6

− / b ( ) a − / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E E

Shift

a

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Parsing: 7

− / b ( ) a − / b

  • r1. E

E T r2 E T r3 T T D r4 T D r5 D | | E E D E

Reduce by r5

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Parsing: 8

− / b ( ) a − / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4

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Parsing: 8a

− / b ( ) a / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4

−

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Parsing: 9a

− / b ( ) a / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Reduce by r1

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Parsing: 10a

− / b ( ) a b /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Shift

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Parsing: 11a

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Shift

b

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Parsing: 12a

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T D

Reduce by r5

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Parsing: 13a

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4

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Parsing: 14a

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T E

Reduce by r2 S t u c k ! Get back!

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Parsing: 14b

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T E

Reduce by r2 Get back!

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Parsing: 13b

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4 Get back!

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Parsing: 12b

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T D

Reduce by r5 Get back!

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Parsing: 11b

− / b ( ) a /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Shift

b

Get back!

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Parsing: 10b

− / b ( ) a b /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Shift Get back!

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Parsing: 9b

− / b ( ) a / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Reduce by r1 Get back to where you

  • nce belonged!
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Parsing: 8b

− / b ( ) a / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4

−

Shift instead

  • f reduce here!

Principle: m

  • d

i f i e d

Reduce whenever possible, but but depending upon lookahead Shift−reduce conflict

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Parsing: 8

− / b ( ) a − / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r4

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Parsing: 9

− / b ( ) a − / b

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Shift

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Parsing: 10

− / ( ) a − /

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Shift

b b

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Parsing: 11

− / b ( ) a −

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T / D

Reduce by r5

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Parsing: 12

− / b ( ) a

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T T

Reduce by r3

−

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Parsing: 13

− / b ( ) a

  • r1. E

E T r2 E T r3 T T D r4 D r5 D | | E E T

Reduce by r1

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Parse Trees: 0

−

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D a − / b a

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Parse Trees: 1

a − a / b −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D D

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Parse Trees: 2

a − a / b T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D D

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Parse Trees: 3

a − a / b T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 3a

a − a / b T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 3b

a − a / b T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 4

a − a D / b T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 5

a − a D / b T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 5a

a − a D / b T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 5b

a − a D / b T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 6

a − a D / b D T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 7

a − a D / b D T T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E D

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Parse Trees: 8

a − a D / b D T T T −

  • r1. E

E T r2 E T / r3 T T D b ( ) a D | | E r5 r4 T D E E D

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Parsing: Summary: 1

  • All high-level languages are designed so that they may

be parsed in this fashion with only a single token look- ahead.

  • Parsers for a language can be automatically con-

structed by parger-generators such as Yacc, Bison, ML- Yacc.

  • Shift-reduce conflicts if any, are automatically detected

and reported by the parser-generator.

  • Shift-reduce conflicts may be avoided by suitably

redesigning the context-free grammar.

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Parsing: Summary: 2

  • Very often shift-reduce conflicts may occur because
  • f the prefix problem.

In such cases many parser- generators 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 non- terminal symbol to which the contents of the stack may reduce.

  • A minor reworking of the grammar to avoid redundant

non-terminal symbols will get rid of reduce-reduce con- flicts. 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 gener- ated.

  • The parser also provides a structuring mechanism that

divides the task of code generation into bits and pieces determined by the individual nonterminals and produc- tion rules.

  • However,

contex-free grammars are not powerful enough to represent all computable functions. Exam- ple, the language {anbncn|n > 0}.

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Semantic Analysis: 2

  • There are context-sensitive aspects of a program that

cannot be represented/enforced by a context-free gram- mar definition. Examples include – correspondence between formal and actual param- eters – type consistency between declaration and use. – scope and visibility issues with respect to identifiers in a program.

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Synthesized Attributes:

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n

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Synthesized Attributes: 1

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 3 2 1 Synthesized Attributes

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Synthesized Attributes: 2

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 3 2 1 4 Synthesized Attributes

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Synthesized Attributes: 3

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 3 2 1 Synthesized Attributes

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Synthesized Attributes: 4

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 3 2 1 Synthesized Attributes

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Synthesized Attributes: 5

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 3 2 1 Synthesized Attributes

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Synthesized Attributes: 6

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 3 2 1 Synthesized Attributes

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Synthesized Attributes: 7

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 1 Synthesized Attributes

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Synthesized Attributes: 8

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 1 3 Synthesized Attributes

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Synthesized Attributes: 9

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 3 2 1 3 Synthesized Attributes

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Synthesized Attributes: 10

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 3 3 2 1 3 Synthesized Attributes

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Synthesized Attributes: 11

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 3 3 2 1 3 Synthesized Attributes

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Synthesized Attributes: 12

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 2 3 3 2 1 3 Synthesized Attributes

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Synthesized Attributes: 13

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 2 3 3 2 1 1 3 Synthesized Attributes

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Synthesized Attributes: 14

E T F T T F / / ( n F ( ) E E ) n E T − − T F F n n 4 4 4 4 4 1 1 1 3 2 2 3 3 2 1 1 1 3 Synthesized Attributes

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An Attribute Grammar

E T T F / n F ( ) E E T − T F F n n 4 4 4 4 1 1 1 3 2 2 3 1 1 3

E0 → E1−T ⊲ E0.val := sub(E1.val, T.val) E → T ⊲ E.val := T.val T0 → T1/F ⊲ T0.val := div(T1.val, F.val) T → F ⊲ T.val := F.val F → (E) ⊲ F.val := E.val F → n ⊲ F.val := n.val

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Inherited Attributes: 0

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int ,

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Inherited Attributes: 1

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int ,

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Inherited Attributes: 2

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int int int ,

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Inherited Attributes: 3

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int int int , int

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Inherited Attributes: 4

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int int int , int int int

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Inherited Attributes: 5

T int L z L L x I y I D I , , x D L T I y z int int int int int , int int int int int int

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Inherited Attributes: 6

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int int int , int int int int int int int int

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Inherited Attributes: 7

C-style declarations generating int x, y, z. D → T L T → int | float L → L,I | I I → x | y | z

T int L z L L x I y I D I , , x D L T I y z int int int int int , int int int int int int int int int

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An Attribute Grammar

T int L z L L x I y I D I , int int , int int int int int int int int int

D → TL ⊲ L.in := T.type T → int ⊲ T.type := int.int T → float ⊲ T.type := float.float L0 → L1,I ⊲ L1 := L0.in L → I ⊲ I.in := L.in I → id ⊲ id.type := I.in

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Abstract Syntax: 0

E → E−T | T T → T/F | F F → n | (E)

Suppose we want to evaluate an expression (4 − 1)/2. What we actually want is a tree that looks like this:

4 1 2 −− /

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Evaluation: 0

4 1 2 −− /

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Evaluation: 1

4 1 2 −− /

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Evaluation: 2

2 / 3

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Evaluation: 3

2 / 3

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Evaluation: 4

1

But what we actually get during parsing is a tree that looks like . . .

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Abstract Syntax: 1

. . . THIS!

E T T F / n F ( ) E E T − T F F n n

n n / n −−

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Abstract Syntax: 2

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 re- turns a pointer to it. makeBinaryNode(opr, opd1, opd2) : Creates a node with la- bel 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

E0 → E1−T ⊲ E0.ptr := makeBinaryNode(−, E1.ptr, T.ptr) E → T ⊲ E.ptr := T.ptr T0 → T1/F ⊲ T0.ptr := makeBinaryNode(/, T1.ptr, F.ptr) T → F ⊲ T.ptr := F.ptr F → (E) ⊲ F.ptr := E.ptr F → n ⊲ F.ptr := makeLeaf(n.val) The Big Picture

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Symbol Table:1

  • The store house of context-sensitive and run-time infor-

mation about every identifier in the source program.

  • All accesses relating to an identifier require to first find

the attributes of the identifier from the symbol table

  • Usually organized as a hash table – provides fast ac-

cess.

  • Compiler-generated temporaries may also be stored in

the symbol table

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Symbol Table:2

Attributes stored in a symbol table for each identifier:

  • type
  • size
  • scope/visibility information
  • base address
  • addresses to location of auxiliary symbol tables (in case
  • f records, procedures, classes)
  • address of the location containing the string which ac-

tually 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

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

the compilation process begins.

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Symbol Table:4

Accesses to the symbol table at every stage of the compi- lation process, Scanning: Insertion of new identifiers. Parsing: Access to the symbol table to ensure that an

  • perand exists (declaration before use).

Semantic analysis:

  • Determination of types of identifiers from declara-

tions

  • 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. Optimization: All memory accesses through symbol table Target code: Translation of relative addresses to absolute addresses in terms of word length, word boundary etc. The Big picture

ai.e.relative to a base address that is known only at run-time

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Intermediate Representation

Intermediate representations are important for reasons of portability.

  • (more or less) independent of specific features of the

high-level language.

  • Example. Java byte-code for any high-level language.
  • (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 lo-

cations

  • no commitment to Representational Issues
  • 4. To ensure type-safety
  • 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. Next

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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?

  • most arithmetic and logical operations, load and store

instructions etc.

  • so as to be interpreted easily,
  • the interpreter is fairly small,
  • execution speeds are high,
  • to have fixed length instructions (where each operand

position has a specific meaning). Back to IR Properties:1

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IR: How high can you rise?

  • typed variables,
  • temporary variables instead of registers.
  • array-indexing,
  • random access to record fields,
  • parameter-passing,
  • pointers and pointer management
  • no limits on memory addresses

Back to IR Properties:1

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A typical instruction set: 1

Three address code: A suite of instructions. Each instruc- tion has at most 3 operands.

  • an opcode representing an operation with at most 2
  • perands
  • two operands on which the binary operation is per-

formed

  • a target operand, which accumulates the result of the

(binary) operation. 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)
  • Procedures and parameters
  • Arrays and array-indexing
  • Pointer Referencing and Dereferencing
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A typical instruction set: 2

  • 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)
  • Procedures and parameters
  • Arrays and array-indexing
  • Pointer Referencing and Dereferencing
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A typical instruction set: 2

  • Assignments (LOAD-STORE)
  • Jumps (conditional and unconditional)

– goto L – Unconditional jump, – x relop y goto L – Conditional jump, where relop is a relational operator

  • Procedures and parameters
  • Arrays and array-indexing
  • Pointer Referencing and Dereferencing
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A typical instruction set: 2

  • Assignments (LOAD-STORE)
  • Jumps (conditional and unconditional)
  • Procedures and parameters

– call p n, where n is the number of parameters – return y, return value from a procedures call – param x, parameter declaration

  • Arrays and array-indexing
  • Pointer Referencing and Dereferencing
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A typical instruction set: 2

  • Assignments (LOAD-STORE)
  • Jumps (conditional and unconditional)
  • Procedures and parameters
  • Arrays and array-indexing

– 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

  • Pointer Referencing and Dereferencing
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A typical instruction set: 2

  • Assignments (LOAD-STORE)
  • Jumps (conditional and unconditional)
  • Procedures and parameters
  • 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 Picture

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Pointers

x x *y y @z *x *x @z *z z *z z x := ^y x := *y *x := y *y @y @z *z *z x y y x x y z x y y *y *y *y @z z

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IR: Generation

  • Can be generated by recursive traversal of the abstract

syntax tree.

  • Can be generated by syntax-directed translation as fol-

lows: 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 gen- erated code is stored N.code , which is the actual code segment generated.

  • In addition a global counter for the instructions gener-

ated is maintained as part of the generation process.

  • It is independent of the source language but can ex-

press target machine operations without committing to too much detail.

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IR: Infrastructure

Given an abstract syntax tree T, with T also denoting its root node. T.place address of temporary variable where result of exe- cution 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 gener- ated 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

  • 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

  • blue: Names of relevant procedures used in IR code gen-

eration.

  • Black: All other stuff.
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IR: Example

E → id ⊲ E.place := id.place; E.code := emit() E0 → E1 − E2 ⊲ E0.place := newtemp; E0.code := E1.code || E2.code || emit(E0.place := E1.place − E2.place)

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IR: Example

S → id := E ⊲ S.code := E.code || emit(id.place:=E.place) S0 → while E do S1 ⊲ S0.begin := newlabel; S0.after := newlabel; S0.code := emit(S0.begin:) || E.code || emit(if E.place= 0 goto S0.after) || S1.code || emit(gotoS0.begin) || emit(S0.after:)

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IR: Example

S → id := E ⊲ S.code := E.code || emit(id.place:=E.place) S0 → while E do S1 ⊲ S0.begin := newlabel; S0.after := newlabel; S0.code := emit(S0.begin:) || E.code || emit(if E.place= 0 goto S0.after) || S1.code || emit(gotoS0.begin) || emit(S0.after:)

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IR: Generation

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

  • 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 Globals Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P21

Call P21

Body of P2

Call P21

Locals of P1 Procedure P1 Body of P1

Call P2

Main body

Call P1

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Run-time Structure: 1

Main program Globals Main body Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P2 Procedure P1 Locals of P1 Body of P1 Globals Body of P21

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Run-time Structure: 2

Main program Globals Main body Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P2 Procedure P1 Locals of P1 Body of P1 Globals Formal par of P1 Locals of P1 Return address to Main Dynamic link to Main Static link to Main Body of P21

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Run-time Structure: 3

Main program Globals Main body Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P2 Procedure P1 Locals of P1 Body of P1 Globals Formal par of P1 Locals of P1 Return address to Main Formal par P2 Locals of P2 Return address to last of P1 Dynamic link to Main Dynamic link to last P1 Static link to Main Static link to last P1 Body of P21

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Run-time Structure: 4

Main program Globals Main body Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P2 Procedure P1 Locals of P1 Body of P1 Globals Formal par of P1 Locals of P1 Return address to Main Formal par P2 Locals of P2 Return address to last of P1 Formal par P21 Locals of P21 Return address to last of P2 Dynamic link to Main Dynamic link to last P1 Dynamic link to last P2 Static link to Main Static link to last P1 Static link last P2 Body of P21

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Run-time Structure: 5

Main program Globals Main body Procedure P2 Locals of P2 Procedure P21 Locals of P21 Body of P2 Procedure P1 Locals of P1 Body of P1 Globals Formal par of P1 Locals of P1 Return address to Main Formal par P2 Locals of P2 Return address to last of P1 Formal par P21 Locals of P21 Return address to last of P2 Formal par P21 Locals of P21 Return address to last of P21 Dynamic link to Main Dynamic link to last P1 Dynamic link to last P2 Dynamic link to last P21 Static link to Main Static link to last P1 Static link last P2 Static link to last P2 Body of P21

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