C program compiler presentation

Rigvendra Kumar Vardhan
M.TECH ECE
Pondicherry University
1

 Interpreters (direct execution)
 Assemblers
 Preprocessors
 Text formatters (non-WYSIWYG)
 Analysis tools
CS 540 Spring 2013 GMU 2

 Interpreter
A program that reads a source program
and produces the results of executing
that program
 Compiler
A program that translates a program
from one language (the source) to
another (the target)
3

 Correctness
 Speed (runtime and compile time)
Degrees of optimization
Multiple passes
 Space
 Feedback to user
 Debugging

 Interpreter
Execution engine
Program execution interleaved with
analysis
running = true;
while (running) {
analyze next statement;
execute that statement;
}
May involve repeated analysis of some
statements (loops, functions)
5

 Read and analyze entire program
 Translate to semantically equivalent
program in another language
Presumably easier to execute or more
efficient
Should “improve” the program in some
fashion
 Offline process
Tradeoff: compile time overhead
(preprocessing step) vs execution
performance
6

 Compilers
FORTRAN, C, C++, Java, COBOL, etc.
etc.
Strong need for optimization, etc.
 Interpreters
PERL, Python, awk, sed, sh, csh,
postscript printer, Java VM
Effective if interpreter overhead is low
relative to execution cost of language
statements
7

8
Source code
Compiler
Assembly code
Assembler
Object code
(machine code)
Fully-resolved object
code (machine code)
Linker
Loader
Executable image

 Series of program representations
 Intermediate representations optimized
for program manipulations of various
kinds (checking, optimization)
 Become more machine-specific, less
language-specific as translation proceeds
9

 First approximation
Front end: analysis
 Read source program and understand its structure
and meaning
Back end: synthesis
 Generate equivalent target language program
Source Front End Back End Target
10

 Must recognize legal programs (&
complain about illegal ones)
 Must generate correct code
 Must manage storage of all variables
 Must agree with OS & linker on target
format
11

 Need some sort of Intermediate
Representation (IR)
 Front end maps source into IR
 Back end maps IR to target machine code
12

Scanner
(lexical
analysis)
Parser
(syntax
analysis)
Code
Optimizer
Semantic
Analysis
(IC generator)
Code
Generator
Symbol
Table
Source
language
tokens Syntactic
structure
Intermediate
Language
Target
language
Intermediate
Language

 Lexical Analysis
 Syntax Analysis
 Semantic Analysis
 Runtime environments
 Code Generation
 Code Optimization

15
Source code
(character stream)
Lexical analysis
Parsing
Token stream
Abstract syntax tree
Intermediate Code Generation
Intermediate code
Optimization
Code generation
Intermediate code
Assembly code
Front end
(machine-independent)
Back end
(machine-dependent)

Scanner Parser source tokens IR
 Split into two parts
Scanner: Responsible for converting
character stream to token stream
 Also strips out white space, comments
Parser: Reads token stream; generates IR
 Both of these can be generated
automatically
Source language specified by a formal
grammar
Tools read the grammar and generate
scanner & parser (either table-driven or hard
coded)
16

 Token stream: Each significant lexical
chunk of the program is represented by a
token
Operators & Punctuation: {}[]!+-=*;: …
Keywords: if while return goto
Identifiers: id & actual name
Constants: kind & value; int, floating-point
character, string, …
17

 Input text
// this statement does very little
if (x >= y) y = 42;
 Token Stream
IF LPAREN ID(x) GEQ ID(y)
RPAREN ID(y) BECOMES INT(42) SCOLON
Note: tokens are atomic items, not character
strings
18

 Many different forms
(Engineering tradeoffs)
 Common output from a parser is an
abstract syntax tree
Essential meaning of the program
without the syntactic noise
19

 Token Stream Input  Abstract Syntax Tree
20
IF LPAREN ID(x)
GEQ ID(y) RPAREN
ID(y) BECOMES
INT(42) SCOLON
ifStmt
>=
ID(x) ID(y)
assign
ID(y) INT(42)

 During or (more common) after parsing
Type checking
Check for language requirements like
“declare before use”, type compatibility
Preliminary resource allocation
Collect other information needed by
back end analysis and code generation
21

 Responsibilities
Translate IR into target machine code
Should produce fast, compact code
Should use machine resources
effectively
Registers
Instructions
Memory hierarchy
22

 Typically split into two major parts with
sub phases
“Optimization” – code improvements
May well translate parser IR into
another IR
Code generation
Instruction selection & scheduling
Register allocation
23

 Input
if (x >= y)
y = 42;
 Output
mov eax,[ebp+16]
cmp eax,[ebp-8]
jl L17
mov [ebp-8],42
L17:
24

lda $30,-32($30)
stq $26,0($30)
stq $15,8($30)
bis $30,$30,$15
bis $16,$16,$1
stl $1,16($15)
lds $f1,16($15)
sts $f1,24($15)
ldl $5,24($15)
bis $5,$5,$2
s4addq $2,0,$3
ldl $4,16($15)
mull $4,$3,$2
ldl $3,16($15)
addq $3,1,$4
mull $2,$4,$2
ldl $3,16($15)
addq $3,1,$4
mull $2,$4,$2
stl $2,20($15)
ldl $0,20($15)
br $31,$33
$33:
bis $15,$15,$30
ldq $26,0($30)
ldq $15,8($30)
addq $30,32,$30
ret $31,($26),1
Optimized Code
25
s4addq $16,0,$0
mull $16,$0,$0
addq $16,1,$16
mull $0,$16,$0
mull $0,$16,$0
ret $31,($26),1
Unoptimized Code

26
Source code
(character stream)
Lexical analysis
Parsing
Token stream
Abstract syntax tree
(AST)
Semantic Analysis
if (b == 0) a = b;
if ( b == 0 ) a = b ;
if
==
b 0
=
a b
if
==
int b int 0
=
int a
lvalue
int b
boolean
Decorated AST
;
int ;

Intermediate Code Generation
Optimization
Code generation
27
if
boolean ==
int ;
int b int 0
=
int a
lvalue
int b
CJUMP ==
MEM
+
fp 8
CONST MOVE
0 MEM MEM
fp 4 fp 8
NOP
+ +
CJUMP ==
CONST MOVE
CX NOP
0 DX CX
CMP CX, 0
CMOVZ DX,CX

 Compiler techniques are everywhere
Parsing (little languages, interpreters)
Database engines
AI: domain-specific languages
Text processing
Tex/LaTex -> dvi -> Postscript -> pdf
Hardware: VHDL; model-checking tools
Mathematics (Mathematica, Matlab)
28

 Fascinating blend of theory and
engineering
Direct applications of theory to practice
Parsing, scanning, static analysis
Some very difficult problems (NP-hard or
worse)
Resource allocation, “optimization”,
etc.
Need to come up with good-enough
solutions
29

 Ideas from many parts of CSE
AI: Greedy algorithms, heuristic search
Algorithms: graph algorithms, dynamic
programming, approximation
algorithms
Theory: Grammars DFAs and PDAs,
pattern matching, fixed-point
algorithms
Systems: Allocation & naming,
synchronization, locality
Architecture: pipelines & hierarchy
management, instruction set use
30

 program ::= statement | program
statement
 statement ::= assignStmt | ifStmt
 assignStmt ::= id = expr ;
 ifStmt ::= if ( expr ) stmt
 expr ::= id | int | expr + expr
 Id ::= a | b | c | i | j | k | n | x | y | z
 int ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
31

 There are several syntax notations for
productions in common use; all mean the
same thing
ifStmt ::= if ( expr ) stmt
ifStmt if ( expr ) stmt
<ifStmt> ::= if ( <expr> ) <stmt>
32

program ::= statement | program statement
statement ::= assignStmt | ifStmt
assignStmt ::= id = expr ;
ifStmt ::= if ( expr ) stmt
expr ::= id | int | expr + expr
id ::= a | b | c | i | j | k | n | x | y | z
int ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
a = 1 ;
if ( a + 1 )
b = 2 ;
33
program
program
ID(a) expr
stmt
stmt
assign
int (1)
ifStmt
expr stmt
expr expr
assign
ID(a) int (1) ID(b) expr
int (2)

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