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Languages and Compilers
(SProg og Oversættere)
Bent Thomsen
Department of Computer Science
Aalborg University
With acknowledgement to Norm Hutchinson whose slides this lecture is based on.
1
What is this course about?
• Programming Language Design
– Concepts and Paradigms
– Ideas and philosophy
– Syntax and Semantics
• Compiler Construction
– Tools and Techniques
– Implementations
– The nuts and bolts
2
Tools and Techniques
• Front-end: Syntax analysis
– How to build a Scanner and Lexer
• By hand in Java
• Using Tools
– JavaCC
– SableCC
– Lex and Yacc (JLex and JavaCUP)
– (lg and pg – compiler tools for .Net)
• Middle-part: Contextual Analysis
• Back-end: Code Generation
– Target Machines
• TAM
• JVM
• (.Net CLR)
3
Today’s lecture
• Two topics
– Treating Compilers and Interpreters as black-boxes
• Tombstone- or T- diagrams
– A first look inside the black-box
• Your guided tour
4
Terminology
Q: Which programming languages play a role in this picture?
input
source program
Translator
is expressed in the
source language
output
object program
is expressed in the
target language
is expressed in the
implementation language
A: All of them!
5
Tombstone Diagrams
What are they?
– diagrams consisting out of a set of “puzzle pieces” we can use
to reason about language processors and programs
– different kinds of pieces
– combination rules (not all diagrams are “well formed”)
Program P implemented in L
P
L
Machine implemented in hardware
M
Translator implemented in L
S -> T
L
Language interpreter in L
M
L
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Tombstone diagrams: Combination rules
P
M
M
P
L
M
OK!
P
S
P
T
S -> T
M
OK!
M OK!
OK!
WRONG!
P
L
WRONG!
S -> T
M
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Compilation
Example: Compilation of C programs on an x86 machine
Tetris
C
C -> x86
x86
x86
Tetris
x86
Tetris
x86
x86
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What is Tetris?
Tetris® The World's Most Popular
Video Game Since its commercial
introduction in 1987, Tetris® has
been established as the largest
selling and most recognized global
brand in the history of the interactive
game software industry. Simple,
entertaining, and yet challenging,
Tetris® can be found on more than
60 platforms. Over 65 million
Tetris® units have been sold
worldwide to date.
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Cross compilation
Example: A C “cross compiler” from x86 to PPC
A cross compiler is a compiler which runs on one machine (the host
machine) but emits code for another machine (the target machine).
Tetris
C
C -> PPC
x86
x86
Tetris
PPC
download
Tetris
PPC
PPC
Host ≠ Target
Q: Are cross compilers useful? Why would/could we use them?
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Two Stage Compilation
A two-stage translator is a composition of two translators. The
output of the first translator is provided as input to the second
translator.
Tetris
Tetris
Tetris
Java Java->JVM JVM JVM->x86 x86
x86
x86
x86
x86
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Compiling a Compiler
Observation: A compiler is a program!
Therefore it can be provided as input to a language processor.
Example: compiling a compiler.
Java->x86
Java->x86
C -> x86
x86
C
x86
x86
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Interpreters
An interpreter is a language processor implemented in software, i.e.
as a program.
Terminology: abstract (or virtual) machine versus real machine
Example: The Java Virtual Machine
Tetris
JVM
JVM
x86
x86
Q: Why are abstract machines useful?
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Interpreters
Q: Why are abstract machines useful?
1) Abstract machines provide better platform independence
Tetris
JVM
JVM
x86
x86
Tetris
JVM
JVM
PPC
PPC
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Interpreters
Q: Why are abstract machines useful?
2) Abstract machines are useful for testing and debugging.
Example: Testing the “Ultima” processor using hardware emulation
P
Ultima
Ultima
x86
x86

P
Ultima
Ultima
Functional equivalence
Note: we don’t have to implement Ultima emulator in x86 we can
use a high-level language and compile it.
15
Interpreters versus Compilers
Q: What are the tradeoffs between compilation and interpretation?
Compilers typically offer more advantages when
– programs are deployed in a production setting
– programs are “repetitive”
– the instructions of the programming language are complex
Interpreters typically are a better choice when
–
–
–
–
we are in a development/testing/debugging stage
programs are run once and then discarded
the instructions of the language are simple
the execution speed is overshadowed by other factors
• e.g. on a web server where communications costs are much higher than
execution speed
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Interpretive Compilers
Why?
A tradeoff between fast(er) compilation and a reasonable runtime
performance.
How?
Use an “intermediate language”
• more high-level than machine code => easier to compile to
• more low-level than source language => easy to implement as an
interpreter
Example: A “Java Development Kit” for machine M
Java->JVM
M
JVM
M
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Interpretive Compilers
Example: Here is how we use our “Java Development Kit” to run a
Java program P
P
Java
javac
P
Java->JVM JVM
M
M
java
P
JVM
JVM
M
M
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Portable Compilers
Example: Two different “Java Development Kits”
Kit 1:
Java->JVM
M
JVM
M
Java->JVM
JVM
JVM
M
Kit 2:
Q: Which one is “more portable”?
19
Portable Compilers
In the previous example we have seen that portability is
not an “all or nothing” kind of deal.
It is useful to talk about a “degree of portability” as the
percentage of code that needs to be re-written when
moving to a dissimilar machine.
In practice 100% portability is as good as impossible.
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Example: a “portable” compiler kit
Portable Compiler Kit:
Java->JVM
Java
Java->JVM
JVM
JVM
Java
Q: Suppose we want to run this kit on some machine M. How could
we go about realizing that goal? (with the least amount of effort)
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Example: a “portable” compiler kit
Java->JVM
Java
Java->JVM
JVM
JVM
Java
Q: Suppose we want to run this kit on some machine M. How could
we go about realizing that goal? (with the least amount of effort)
JVM
Java
reimplement
JVM
C
C->M
M
M
JVM
M
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Example: a “portable” compiler kit
This is what we have now:
Java->JVM
Java
Java->JVM
JVM
JVM
Java
JVM
M
Now, how do we run our Tetris program?
Tetris
Tetris
Java Java->JVM JVM
JVM
JVM
M
M
Tetris
JVM
JVM
M
M
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Bootstrapping
Remember our “portable compiler kit”:
Java->JVM
Java
Java->JVM
JVM
JVM
Java
JVM
M
We haven’t used this yet!
Java->JVM
Java
Same language!
Q: What can we do with a compiler written in
itself? Is that useful at all?
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Bootstrapping
Java->JVM
Java
Same language!
Q: What can we do with a compiler written in
itself? Is that useful at all?
• By implementing the compiler in (a subset of) its own language, we
become less dependent on the target platform => more portable
implementation.
• But… “chicken and egg problem”? How do to get around that?
=> BOOTSTRAPPING: requires some work to make the first “egg”.
There are many possible variations on how to bootstrap a compiler
written in its own language.
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Bootstrapping an Interpretive Compiler to
Generate M code
Our “portable compiler kit”:
Java->JVM
Java
Java->JVM
JVM
JVM
Java
JVM
M
Goal we want to get a “completely native” Java compiler on machine M
P
P
Java->M
Java
M
M
M
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Bootstrapping an Interpretive Compiler to
Generate M code
Idea: we will build a two-stage Java -> M compiler.
P
Java
P
Java->JVM JVM
M
M
M
We will make this by
compiling
Java->JVM
JVM
JVM->M
M
M
P
M
To get this we implement
JVM->M
Java
and compile it
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Bootstrapping an Interpretive Compiler to
Generate M code
Step 1: implement
JVM->M
Java
Step 2: compile it
JVM->M
JVM->M
Java Java->JVM JVM
JVM
JVM
M
M
Step 3: compile this
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Bootstrapping an Interpretive Compiler to
Generate M code
Step 3: “Self compile” the JVM (in JVM) compiler
JVM->M
JVM->M
JVM JVM->M
M
JVM
JVM
M
M
This is the second
stage of our
compiler!
Step 4: use this to compile the Java compiler
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Bootstrapping an Interpretive Compiler to
Generate M code
Step 4: Compile the Java->JVM compiler into machine code
Java->JVM
Java->JVM
JVM JVM->M
M
M
M
The first stage of
our compiler!
We are DONE!
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Full Bootstrap
A full bootstrap is necessary when we are building a new compiler
from scratch.
Example:
We want to implement an Ada compiler for machine M. We don’t
currently have access to any Ada compiler (not on M, nor on any
other machine).
Idea: Ada is very large, we will implement the compiler in a subset of
Ada and bootstrap it from a subset of Ada compiler in another
language. (e.g. C)
v1
Step 1: build a compiler for Ada-S
Ada-S ->M
in another language
C
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Full Bootstrap
Step 1a: build a compiler (v1) for Ada-S in another language.
v1
Ada-S ->M
C
Step 1b: Compile v1 compiler on M
v1
v1
Ada-S ->M
Ada-S->M
C->M
C
M
M
This compiler can be used for
M
bootstrapping on machine M but we
do not want to rely on it permanently!
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Full Bootstrap
Step 2a: Implement v2 of Ada-S compiler in Ada-S
v2
Ada-S ->M
Q: Is it hard to rewrite the compiler in Ada-S?
Ada-S
Step 2b: Compile v2 compiler with v1 compiler
v2
v2
v1 Ada-S->M
Ada-S ->M
M
Ada-S Ada-S ->M
M
We are now no longer dependent
M
on the availability of a C compiler!
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Full Bootstrap
Step 3a: Build a full Ada compiler in Ada-S
v3
Ada->M
Ada-S
Step 3b: Compile with v2 compiler
v3
v3
v2
Ada->M
Ada->M
M
Ada-S Ada-S ->M
M
M
From this point on we can maintain the compiler in Ada.
Subsequent versions v4,v5,... of the compiler in Ada and compile
each with the the previous version.
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Half Bootstrap
We discussed full bootstrap which is required when we have no
access to a compiler for our language at all.
Q: What if we have access to an compiler for our language on a
different machine HM but want to develop one for TM ?
We have:
Ada->HM
HM
We want:
Ada->HM
Ada
Ada->TM
TM
Idea: We can use cross compilation from HM to TM to bootstrap
the TM compiler.
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Half Bootstrap
Idea: We can use cross compilation from HM to M to bootstrap the
M compiler.
Step 1: Implement Ada->TM compiler in Ada
Ada->TM
Ada
Step 2: Compile on HM
Ada->TM
Ada->TM
Ada Ada->HM HM
HM
HM
Cross compiler:
running on HM but
emits TM code
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Half Bootstrap
Step 3: Cross compile our TM compiler.
Ada->TM
Ada
Ada->TM
Ada->TM
HM
HM
DONE!
TM
From now on we can develop subsequent versions of the compiler
completely on TM
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Bootstrapping to Improve Efficiency
The efficiency of programs and compilers:
Efficiency of programs:
- memory usage
- runtime
Efficiency of compilers:
- Efficiency of the compiler itself
- Efficiency of the emitted code
Idea: We start from a simple compiler (generating inefficient code)
and develop more sophisticated version of it. We can then use
bootstrapping to improve performance of the compiler.
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Bootstrapping to Improve Efficiency
We have:
Step 1
Ada->Mslow
Ada
Ada-> Mslow
Mslow
We implement:
Ada->Mfast
Ada
Ada->Mfast
Ada->Mfast
Ada Ada-> Mslow Mslow
Mslow
M
Step 2
Ada->Mfast
Ada->Mfast
Ada Ada-> Mfast Mfast
Mslow
Fast compiler that
emits fast code!
M
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Conclusion
•
•
•
To write a good compiler you may be writing several
simpler ones first
You have to think about the source language, the target
language and the implementation language.
Strategies for implementing a compiler
1. Write it in machine code
2. Write it in a lower level language and compile it using an
existing compiler
3. Write it in the same language that it compiles and bootstrap
•
The work of a compiler writer is never finished, there
is always version 1.x and version 2.0 and …
40
Compilation
So far we have treated language processors (including
compilers) as “black boxes”
Now we take a first look "inside the box": how are
compilers built.
And we take a look at the different “phases” and their
relationships
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The “Phases” of a Compiler
Source Program
Syntax Analysis
Error Reports
Abstract Syntax Tree
Contextual Analysis
Error Reports
Decorated Abstract Syntax Tree
Code Generation
Object Code
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Different Phases of a Compiler
The different phases can be seen as different
transformation steps to transform source code into
object code.
The different phases correspond roughly to the different
parts of the language specification:
• Syntax analysis <-> Syntax
• Contextual analysis <-> Contextual constraints
• Code generation <-> Semantics
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Example Program
We now look at each of the three different phases in a little
more detail. We look at each of the steps in transforming
an example Triangle program into TAM code.
! This program is useless except for
! illustration
let var n: integer;
var c: char
in begin
c := ‘&’;
n := n+1
end
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1) Syntax Analysis
Source Program
Syntax Analysis
Error Reports
Abstract Syntax Tree Note: Not all compilers construct an
explicit representation of an AST. (e.g. on
a “single pass compiler” generally no need
to construct an AST)
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1) Syntax Analysis -> AST
Program
LetCommand
SequentialCommand
SequentialDeclaration
AssignCommand
AssignCommand
VarDecl
VarDecl
SimpleT
Ident
n
Ident
Integer
Char.Expr
BinaryExpr
VNameExp Int.Expr
SimpleT SimpleV
Ident
Ident
c
Char
SimpleV
Ident Char.Lit Ident
c
‘&’
n
Ident Op Int.Lit
n
+
1
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2) Contextual Analysis -> Decorated AST
Abstract Syntax Tree
Contextual Analysis
Error Reports
Decorated Abstract Syntax Tree
Contextual analysis:
• Scope checking: verify that all applied occurrences of
identifiers are declared
• Type checking: verify that all operations in the program are
used according to their type rules.
Annotate AST:
• Applied identifier occurrences => declaration
• Expressions => Type
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2) Contextual Analysis -> Decorated AST
Program
LetCommand
SequentialCommand
SequentialDeclaration
VarDecl
Ident
n Integer
AssignCommand
BinaryExpr :int
Char.Expr
VNameExp Int.Expr
VarDecl
SimpleT
Ident
AssignCommand
:char
:int
SimpleT SimpleV
SimpleV
:char
Ident
Ident
c Char
:int
Ident Char.Lit Ident
c
‘&’
:int
n
Ident Op Int.Lit
n
+ 1
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Contextual Analysis
Finds scope and type errors.
Example 1:
AssignCommand ***TYPE ERROR (incompatible types in
:int
assigncommand)
:char
Example 2:
foo not found
SimpleV ***SCOPE ERROR: undeclared variable foo
Ident
foo
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3) Code Generation
Decorated Abstract Syntax Tree
Code Generation
Object Code
• Assumes that program has been thoroughly
checked and is well formed (scope & type rules)
• Takes into account semantics of the source
language as well as the target language.
• Transforms source program into target code.
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3) Code Generation
let var n: integer;
var c: char
in begin
c := ‘&’;
n := n+1
end
VarDecl address = 0[SB]
PUSH 2
LOADL 38
STORE 1[SB]
LOAD 0
LOADL 1
CALL add
STORE 0[SB]
POP 2
HALT
SimpleT
Ident
Ident
n Integer
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Compiler Passes
• A pass is a complete traversal of the source program, or
a complete traversal of some internal representation of
the source program.
• A pass can correspond to a “phase” but it does not have
to!
• Sometimes a single “pass” corresponds to several phases
that are interleaved in time.
• What and how many passes a compiler does over the
source program is an important design decision.
52
Single Pass Compiler
A single pass compiler makes a single pass over the source text,
parsing, analyzing and generating code all at once.
Dependency diagram of a typical Single Pass Compiler:
Compiler Driver
calls
Syntactic Analyzer
calls
Contextual Analyzer
calls
Code Generator
53
Multi Pass Compiler
A multi pass compiler makes several passes over the program. The
output of a preceding phase is stored in a data structure and used by
subsequent phases.
Dependency diagram of a typical Multi Pass Compiler:
Compiler Driver
calls
calls
calls
Syntactic Analyzer
Contextual Analyzer
Code Generator
input
output input
output input
output
Source Text
AST
Decorated AST
Object Code
54
Example: The Triangle Compiler Driver
public class Compiler {
public static void compileProgram(...) {
Parser parser = new Parser(...);
Checker checker = new Checker(...);
Encoder generator = new Encoder(...);
Program theAST = parser.parse();
checker.check(theAST);
generator.encode(theAST);
}
}
public void main(String[] args) {
... compileProgram(...) ...
}
55
Compiler Design Issues
Single Pass
Multi Pass
Speed
better
worse
Memory
Modularity
better for
large programs
worse
(potentially) better
for small programs
better
Flexibility
worse
better
“Global” optimization
impossible
possible
Source Language
single pass compilers are not possible
for many programming languages
56
Language Issues
Example Pascal:
Pascal was explicitly designed to be easy to implement
with a single pass compiler:
– Every identifier must be declared before it is first use.
?
var n:integer;
procedure inc;
begin
n:=n+1
end
procedure inc;
begin
n:=n+1
end; Undeclared Variable!
var n:integer;
57
Language Issues
Example Pascal:
– Every identifier must be declared before it is used.
– How to handle mutual recursion then?
procedure ping(x:integer)
begin
... pong(x-1); ...
end;
procedure pong(x:integer)
begin
... ping(x); ...
end;
58
Language Issues
Example Pascal:
– Every identifier must be declared before it is used.
– How to handle mutual recursion then?
forward procedure pong(x:integer)
procedure ping(x:integer)
begin
... pong(x-1); ...
end;
OK!
procedure pong(x:integer)
begin
... ping(x); ...
end;
59
Language Issues
Example Java:
– identifiers can be declared before they are used.
– thus a Java compiler need at least two passes
Class Example {
void inc() { n = n + 1; }
int n;
void use() { n = 0 ; inc(); }
}
60
Keep in mind
There are many issues influencing the design of a new
programming language:
– Choice of paradigm
– Syntactic preferences
– Even the compiler implementation
• e.g no of passes
• available tools
There are many issues influencing the design of new
compiler:
– No of passes
– The source, target and implementation language
– Available tools
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