1. Compilers and Language Translation
Gordon College

2. What’s a compiler? All computers only understand machine language
Therefore, high-level language instructions must be translated into machine language prior to execution
This is
a program ……

3. What’s a compiler?
Compiler A piece of system software that translates high-level languages into machine language
program.c
while (c!='x') {
if (c == 'a' || c == 'e' || c == 'i')
printf("Congrats!");
else
if (c!='x')
printf("You Loser!"); }
Congrats!
prog
Compiler ……
gcc -o prog program.c

4. Assembler (a kind of compiler)
LOAD X Assembly
(opcode table)
(symbol table)
Machine Language
One-to-one translation

5. Compiler (high-level language translator)
a = b + c - d;
LOAD B
ADD C
SUBTRACT D
STORE A
…….
One-to-many translation

6. Goals of a compiler Code produced must be correct A = (B+C)-(D+E);
Possible translation:
LOAD B
ADD C
STORE B
LOAD D
ADD E
STORE D
SUBTRACT D
STORE A
Is this correct?
No - STORE B and STORE D changes the values of variables B and D which is the high-level language does not intend

7. Goals of a compiler
Code produced should be reasonably efficient and concise
Compute the sum - 2x1+ 2x2+ 2x3+ 2x4+…. 2x50000
sum = 0.0
for(i=0;i<50000;i++) {
sum = sum + (2.0 * x[i]);
Optimizing compiler:
sum = 0.0
for(i=0;i<50000;i++) {
sum = sum + x[i];
sum = sum * 2.0; 49,999 less instructions

8. General Structure of a Compiler

9. The Compilation Process
Phase I: Lexical analysis
Compiler examines the individual characters in the source program and groups them into syntactical units called tokens
Phase II: Parsing
The sequence of tokens formed by the scanner is checked to see whether it is syntactically correct
Scanner
Source
code
Groups of
tokens
Parser
correct
Groups of
tokens
not correct

10. The Compilation Process
Phase III: Semantic analysis and code generation
The compiler analyzes the meaning of the high-level language statement and generates the machine language instructions to carry out these actions
Code
Generator
Groups of
tokens
Machine
language

11. The Compilation Process
Phase IV: Code optimization
The compiler takes the generated code and sees whether it can be made more efficient
Code
Optimizer
Machine
language
Machine
language

12. Overall Execution Sequence on a High-Level Language Program

13. The Compilation Process
Source program
Original high-level language program
Object program
Machine language translation of the source program

14. Phase I: Lexical Analysis
Lexical analyzer
The program that performs lexical analysis
More commonly called a scanner
Job of lexical analyzer
Group input characters into tokens
Tokens: Syntactical units that are treated as single, indivisible entities for the purposes of translation
Classify tokens according to their type

15. Phase I: Lexical Analysis
Program statement
sum = sum + a[i];
Digital perspective:
tab,s,u,m,blank,=,blank,s,u,m,blank,+,blank,a,[,i,],;
Tokenized:
sum,=,sum,+,a[i],;

16. Phase I: Lexical Analysis
Typical Token Classifications
TOKEN TYPE CLASSIFICATION NUMBER
Symbol 1
Number 2
= 3
+ 4
; 6
== 7
If 8
Else 9
( 10
) 11 [ ] …

17. Phase I: Lexical Analysis
Lexical Analysis Process 1. Discard blanks, tabs, etc. - look for beginning of token. 2. Put characters together 3. Repeat step 2 until end of token 4. Classify and save token 5. Repeat steps 1-4 until end of statement 6. Repeat steps 1-5 until end of source code
sum 1
= 3
+ 4
a 1
[ 12
i 1
] 13
; 6
Scanner
sum=sum+a[i];

18. Phase I: Lexical Analysis
Input to a scanner - A high-level language statement from the source program
Scanner’s output - A list of all the tokens in that statement - The classification number of each token found
sum 1
= 3
+ 4
a 1
[ 12
i 1
] 13
; 6
Scanner
sum=sum+a[i];

19. Phase II: Parsing Parsing phase
A compiler determines whether the tokens recognized by the scanner are a syntactically legal statement
Performed by a parser

20. Phase II: Parsing Output of a parser
A parse tree, if such a tree exists
An error message, if a parse tree cannot be constructed
Successful construction of a parse tree is proof that the statement is correctly formed

21. Example
High-level language statement: a = b + c

22. Grammars, Languages, and BNF
Syntax
The grammatical structure of the language
The parser must be given the syntax of the language
BNF (Backus-Naur Form) Most widely used notation for representing the syntax of a programming language
literal_expression ::= integer_literal | float_literal
| string | character

23. Grammars, Languages, and BNF
In BNF
The syntax of a language is specified as a set of rules (also called productions)
A grammar
The entire collection of rules for a language
Structure of an individual BNF rule
left-hand side ::= “definition”

24. Grammars, Languages, and BNF
BNF rules use two types of objects on the right-hand side of a production
Terminals
The actual tokens of the language
Never appear on the left-hand side of a BNF rule
Nonterminals
Intermediate grammatical categories used to help explain and organize the language
Must appear on the left-hand side of one or more rules

25. Grammars, Languages, and BNF
Goal symbol
The highest-level nonterminal
The nonterminal object that the parser is trying to produce as it builds the parse tree
All nonterminals are written inside angle brackets
Java BNF

26. BNF Example
<postal-address> ::= <name-part> <street-address> <zip-part>
<name-part> ::= <personal-part> <last-name> <opt-jr-part> <EOL>
| <personal-part> <name-part>
<personal-part> ::= <first-name> | <initial> "."
<street-address> ::= <opt-apt-num> <house-num> <street-name> <EOL>
<zip-part> ::= <town-name> "," <state-code> <ZIP-code> <EOL>
<opt-jr-part> ::= "Sr." | "Jr." | <roman-numeral> | ""
Identify the following:
Goal symbol, terminals, nonterminals, a individual rule
Is this a legal postal address?
Steve Moses Sr.
215 Rose Ave.
Everywhere, NC 43563

27. Parsing Concepts and Techniques
Fundamental rule of parsing:
By repeated applications of the rules of the grammar-
If the parser can convert the sequence of input tokens into the goal symbol the sequence of tokens is a syntactically valid statement of the language else the sequence of tokens is not a syntactically valid statement of the language

28. Parsing Example Is the following http address legal:
<httpaddress> ::= <hostport> [ / <path> ] [ ? <search> ]
<hostport> ::= <host> [ : <port> ]
<host> ::= <hostname> | <hostnumber>
<hostname> ::= <ialpha> [ . <hostname> ]
<hostnumber> ::= <digits> . <digits> . <digits> . <digits>
<port> ::= <digits>
<path> ::= <void> | <xpalphas> [ / <path> ]
<search> ::= <xalphas> [ + <search> ]
<xalpha> ::= <alpha> | <digit> | <safe> | <extra> | <escape>
<xalphas> ::= <xalpha> [ <xalphas> ]
<xpalpha> ::= <xalpha> | +
<xpalphas> ::= <xpalpha> [ <xpalpha> ]
<ialpha> ::= <alpha> [ <xalphas> ]
<alpha> ::= a | b | … | z | A | B | … | Z
<digit> ::= 0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<safe> ::= $ | - | _ | . | & | ~
<extra> ::= ! | * | " | ' | ( | ) | : | ; | , | <space>
<escape> ::= % <hex> <hex>
<hex> ::= <digit> | a | b | c | d | e | f | A | B | C | D | E | F
<digits> ::= <digit> [ <digits> ]
<void> ::=

29. Parsing Concepts and Techniques
Look-ahead parsing algorithms - intelligent parsers
One of the biggest problems in building a compiler is designing a grammar that:
Includes every valid statement that we want to be in the language
Excludes every invalid statement that we do not want to be in the language

30. Parsing Concepts and Techniques
Another problem in constructing a compiler: Designing a grammar that is not ambiguous
An ambiguous grammar allows the construction of two or more distinct parse trees for the same statement NOT GOOD - multiple interpretations

31. Phase III: Semantics and Code Generation
Semantic analysis
The compiler makes a first pass over the parse tree to determine whether all branches of the tree are semantically valid
If they are valid the compiler can generate machine language instructions else there is a semantic error; machine language instructions are not generated

32. Phase III: Semantics and Code Generation
Semantic analysis
Syntactically correct, but semantically incorrect example:
sum = a + b;
int a; double sum; data type mismatch char b;
Semantic records
a integer
sum double
b char

33. Phase III: Semantics and Code Generation
Semantic analysis
Parse tree
b char
a integer
<expression> + <expression>
Semantic record
Semantic record
<expression>
temp ?
Semantic record

34. Phase III: Semantics and Code Generation
Semantic analysis
Parse tree
b integer
a integer
<expression> + <expression>
Semantic record
Semantic record
<expression>
temp integer
Semantic record

35. Phase III: Semantics and Code Generation
Compiler makes a second pass over the parse tree to produce the translated code

36. Phase IV: Code Optimization
Two types of optimization
Local
Global
Local optimization
The compiler looks at a very small block of instructions and tries to determine how it can improve the efficiency of this local code block
Relatively easy; included as part of most compilers:

37. Phase IV: Code Optimization
Examples of possible local optimizations
Constant evaluation x = > x = 2
Strength reduction x = x * 2 ---> x = x + x
Eliminating unnecessary operations

38. Phase IV: Code Optimization
Global optimization
The compiler looks at large segments of the program to decide how to improve performance
Much more difficult; usually omitted from all but the most sophisticated and expensive production-level “optimizing compilers”
Optimization cannot make an inefficient algorithm efficient - “only makes an efficient algorithm more efficient”

39. Summary
A compiler is a piece of system software that translates high-level languages into machine language
Goals of a compiler: Correctness and the production of efficient and concise code
Source program: High-level language program

40. Summary
Object program: The machine language translation of the source program
Phases of the compilation process
Phase I: Lexical analysis
Phase II: Parsing
Phase III: Semantic analysis and code generation
Phase IV: Code optimization