Compiler Construction Compiler Construction Semantic Analysis I.
Compiler Construction€¦ · start brushing up on Java ... Compiler Construction Introduction 2....
Transcript of Compiler Construction€¦ · start brushing up on Java ... Compiler Construction Introduction 2....
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Compiler Construction
Important facts:Name: Prof. Dr. Peter ThiemannEmail: [email protected]
Office: 079-00-015
Exercises:Name: Dipl.-Inform. Manuel GeffkenEmail: [email protected]
Office: 079-00-014
Basis for grades:50% homeworks50% final exam
Copyright c©2012 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work forpersonal or classroom use is granted without fee provided that copies are not made or distributed for profit orcommercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, torepublish, to post on servers, or to redistribute to lists, requires prior specific permission and/or fee. Requestpermission to publish from [email protected].
Compiler Construction Introduction 1
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Things to do
• read Appel chapter 1
• make sure you have a working account
• start brushing up on Java
• review Java development tools
• find the official course webpagehttp://proglang.informatik.uni-freiburg.de/teaching/compilerbau/2012ws/
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Compilers
What is a compiler?
• a program that reads an executable program in one language andtranslates it into an executable program in another language
• we expect the program produced by the compiler to be better, in someway, than the original
What is an interpreter?
• a program that reads an executable program and produces the results ofrunning that program
• usually, this involves executing the source program in some fashion
This course deals mainly with compilers
Many of the same issues arise in interpreters
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Motivation
Why study compiler construction?
Why build compilers?
Why attend class?
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Interest
Compiler construction is a microcosm of computer science
artificial intelligencegreedy algorithms, learning algorithms
algorithmsgraph algorithms, union-find, dynamic programming
theoryDFAs for scanning, parser generators, lattice theory
systemsallocation and naming, locality, synchronization
architecturepipeline management, hierarchy management, instruction set use
Inside a compiler, all these things come together
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Isn’t it a solved problem?
Machines are constantly changing
Changes in architecture⇒ changes in compilers
• new features pose new problems
• changing costs lead to different concerns
• old solutions need re-engineering
Changes in compilers should prompt changes in architecture
• New languages and features
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Intrinsic Merit
Compiler construction is challenging and fun
• interesting problems
• primary responsibility for performance (blame)
• new architectures⇒ new challenges
• real results
• extremely complex interactions
Compilers have an impact on how computers are used
Some of the most interesting problems in computing
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Experience
You have used several compilers
What qualities are important in a compiler?
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Experience
You have used several compilers
What qualities are important in a compiler?
1. Correct code
2. Output runs fast
3. Compiler runs fast
4. Compile time proportional to program size
5. Support for separate compilation
6. Good diagnostics for syntax errors
7. Works well with the debugger
8. Good diagnostics for flow anomalies
9. Cross language calls
10. Consistent, predictable optimization
Each of these shapes your expectations about this course
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Abstract view
errors
compilercode codesource machine
Implications:
• recognize legal (and illegal) programs
• generate correct code
• manage storage of all variables and code
• agreement on format for object (or assembly) code
Big step up from assembler — higher level notations
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Traditional two pass compiler
codesource
codemachinefront
endbackend
IR
errors
Implications:
• intermediate representation (IR)
• front end maps legal code into IR
• back end maps IR onto target machine
• simplify retargeting
• allows multiple front ends
• multiple passes⇒ better code
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A fallacy
backend
frontend
FORTRANcode
frontend
frontend
frontend
backend
backend
code
code
code
C++
CLU
Smalltalk
target1
target2
target3
Can we build n×m compilers with n+m components?
• must encode all the knowledge in each front end
• must represent all the features in one IR
• must handle all the features in each back end
Limited success with low-level IRs — but LLVMCompiler Construction Introduction 12
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Front end
codesource tokens
errors
scanner parser IR
Responsibilities:
• recognize legal procedure
• report errors
• produce IR
• preliminary storage map
• shape the code for the back end
Much of front end construction can be automatedCompiler Construction Introduction 13
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Front end
codesource tokens
errors
scanner parser IR
Scanner:
• maps characters into tokens – the basic unit of syntaxx = x + y;
becomes<id, x> = <id, x> + <id, y> ;
• character string value for a token is a lexeme
• typical tokens: number, id, +, -, *, /, do, end
• eliminates white space (tabs, blanks, comments)
• a key issue is speed⇒ use specialized recognizer (as opposed to lex)
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Front end
codesource tokens
errors
scanner parser IR
Parser:
• recognize context-free syntax
• guide context-sensitive analysis
• construct IR(s)
• produce meaningful error messages
• attempt error correction
Parser generators mechanize much of the work
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Front end
Context-free syntax is specified with a grammar
<sheep noise> ::= baa
| baa <sheep noise>
The noises sheep make under normal circumstances
This format is called Backus-Naur form (BNF)
Formally, a grammar G = (S,N,T,P) where
S is the start symbolN is a set of non-terminal symbolsT is a set of terminal symbolsP is a set of productions or rewrite rules
(P : N→ N∪T )
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Front end
Context free syntax can be put to better use
1 <goal> ::= <expr>2 <expr> ::= <expr> <op> <term>3 | <term>4 <term> ::= number
5 | id
6 <op> ::= +
7 | -
Simple expressions with addition and subtraction over tokens id and number
S = <goal>T = number, id, +, -N = <goal>, <expr>, <term>, <op>P = 1, 2, 3, 4, 5, 6, 7
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Front end
Given a grammar, valid sentences can be derived by repeated substitution.
Prod’n. Result<goal>
1 <expr>2 <expr> <op> <term>5 <expr> <op> y
7 <expr> - y
2 <expr> <op> <term> - y
4 <expr> <op> 2 - y
6 <expr> + 2 - y
3 <term> + 2 - y
5 x + 2 - y
To recognize a valid sentence in some CFG, we reverse this process and buildup a parseCompiler Construction Introduction 18
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Front end
A parse can be represented by a parse, or syntax, tree
2><num:
<id:x>
<id: >y
goal
op
termopexpr
expr term
expr
term
-
+
A parse tree contains a lot of unnecessary information
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Front end
So, compilers often use a more concise abstract syntax tree
<id:x> 2><num:
<id: >y+
-
Abstract syntax trees (ASTs) are often used as an IR between front end andback end
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Back end
errors
IR allocationregister
selectioninstruction machine
code
Responsibilities
• translate IR into target machine code
• choose instructions for each IR operation
• decide what to keep in registers at each point
• ensure conformance with system interfaces
Automation has been less successful here
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Back end
errors
IR allocationregister machine
codeinstructionselection
Instruction selection:
• produce compact, fast code
• use available addressing modes
• pattern matching problem
– ad hoc techniques– tree pattern matching– string pattern matching– dynamic programming
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Back end
errors
IRmachine
codeinstructionselection
registerallocation
Register Allocation:
• have value in a register when used• limited resources• changes instruction choices• can move loads and stores• optimal allocation is difficult
Modern allocators often rely on graph coloringCompiler Construction Introduction 23
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Traditional three pass compiler
IR
errors
IRmiddlefront back
end end endsourcecode code
machine
Code Improvement
• analyzes and changes IR
• goal is to reduce runtime
• must preserve values
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Optimizer (middle end)
opt nopt1 ... IR
errors
IR IRIR
Modern optimizers are usually built as a set of passes
Typical passes
• constant propagation and folding
• code motion
• reduction of operator strength
• common subexpression elimination
• redundant store elimination
• dead code elimination
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The MiniJava compiler
Parse TranslateLexCanon-Semantic
Analysis calize
Instruction
Selection
Frame
Layout
Parsing
Actions
Sourc
e P
rogra
m
Tokens
Pass 10
Reductions
Abstr
act S
ynta
x
Tra
nsla
te
IR T
rees
IR T
rees
Frame
Tables
Environ-
ments
Assem
Control
Flow
Analysis
Data
Flow
Analysis
Register
Allocation
Code
Emission Assembler
Machin
e L
anguage
Assem
Flo
w G
raph
Inte
rfere
nce G
raph
Regis
ter
Assig
nm
ent
Assem
bly
Language
Relo
cata
ble
Obje
ct C
ode
Pass 1 Pass 4
Pass 5 Pass 8 Pass 9
Linker
Pass 2
Pass 3
Pass 6 Pass 7
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The MiniJava compiler phases
Lex Break source file into individual words, or tokensParse Analyse the phrase structure of programParsingActions
Build a piece of abstract syntax tree for each phrase
SemanticAnalysis
Determine what each phrase means, relate uses of variables to their defini-tions, check types of expressions, request translation of each phrase
Frame Layout Place variables, function parameters, etc., into activation records (stackframes) in a machine-dependent way
Translate Produce intermediate representation trees (IR trees), a notation that is nottied to any particular source language or target machine
Canonicalize Hoist side effects out of expressions, and clean up conditional branches, forconvenience of later phases
InstructionSelection
Group IR-tree nodes into clumps that correspond to actions of target-machine instructions
Control FlowAnalysis
Analyse sequence of instructions into control flow graph showing all possi-ble flows of control program might follow when it runs
Data FlowAnalysis
Gather information about flow of data through variables of program; e.g.,liveness analysis calculates places where each variable holds a still-needed(live) value
RegisterAllocation
Choose registers for variables and temporary values; variables not simulta-neously live can share same register
CodeEmission
Replace temporary names in each machine instruction with registers
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A straight-line programming language
Stm → Stm ; Stm CompoundStmStm → id := Exp AssignStmStm → print ( ExpList ) PrintStmExp → id IdExpExp → num NumExpExp → Exp Binop Exp OpExpExp → ( Stm , Exp ) EseqExpExpList → Exp , ExpList PairExpListExpList → Exp LastExpListBinop → + PlusBinop → − MinusBinop → × TimesBinop → / Div
An example straight-line program:
a := 5+3; b := (print(a,a−1),10×a); print(b)
prints:
8 7
80
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Tree representation
a := 5+3; b := (print(a,a−1),10×a); print(b)
AssignStm
CompoundStm
a OpExp
PlusNumExp
5
NumExp
3
AssignStm
b EseqExp
PrintStm
PairExpList
IdExp
a
LastExpList
OpExp
MinusIdExp NumExp
a 1
OpExp
NumExp Times IdExp
a10
PrintStm
LastExpList
IdExp
b
CompoundStm
This convenient internal representation is convenient for a compiler to use.
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Java classes for trees
abstract class Stm {}class CompoundStm extends Stm {
Stm stm1, stm2;CompoundStm(Stm s1, Stm s2){ stm1=s1; stm2=s2; }
}class AssignStm extends Stm {
String id; Exp exp;AssignStm(String i, Exp e){ id=i; exp=e; }
}class PrintStm extends Stm {
ExpList exps;PrintStm(ExpList e){ exps=e; }
}
abstract class Exp {}class IdExp extends Exp {
String id;IdExp(String i) {id=i;}
}
class NumExp extends Exp {int num;NumExp(int n) {num=n;}
}class OpExp extends Exp {
Exp left, right; int oper;final static int
Plus=1,Minus=2,Times=3,Div=4;OpExp(Exp l, int o, Exp r){ left=l; oper=o; right=r; }
}class EseqExp extends Exp {
Stm stm; Exp exp;EseqExp(Stm s, Exp e){ stm=s; exp=e; }
}abstract class ExpList {}class PairExpList extends ExpList {
Exp head; ExpList tail;public PairExpList(Exp h, ExpList t){ head=h; tail=t; }
}class LastExpList extends ExpList {
Exp head;public LastExpList(Exp h) {head=h;}
}
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