Introduction to CompilerJungsik Choi

chjs@khu.ac.kr

Formal Languages and Compiler (CSE322)

2018. 3. 8

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• High level functions• Recognize legal program, generate correct code (OS & linker can accept)

• Manage the storage of all variables and code

• Two passes• Use an intermediate representation (IR)

• Front end maps legal source code into IR 𝑂(𝑛) or 𝑂(𝑛 log 𝑛)

• Back end maps IR into target machine code typically NP-complete

• Admits multiple front ends & multiple passes (better code)

Traditional Two-pass Compiler

Front End Back EndTarget

ProgramSource

Program

IR

Errors

Compiler

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• Responsibilities• Recognize legal (& illegal) programs

• Report errors in a useful way

• Produce IR & preliminary storage map

• Shape the code for the back end

• Much of front end construction can be automated

Front End

SourceProgram

IR

Errors

Scanner Parsertokens

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• Scanner (Lexical Analysis)• Maps character stream into words (basic units of syntax)

• sum=x+y; becomes <id,sum> <eq,=> <id,x> <add,+> <id,y> <sc,;>

• word ≅ lexeme, part of speech ≅ token type

• Typical tokens include number, identifier, keyword, operator• Scanner eliminates white space and comments

• Produced by automatic scanner generator

Front End - Scanner

SourceProgram

IR

Errors

Scanner Parsertokens

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• Parser• Recognizes context-free syntax

• Guides context-sensitive (“semantic”) analysis

• E.g. type checking

• Builds IR for source program

• Produced by automatic parser generators

Front End - Parser

SourceProgram

IR

Errors

Scanner Parsertokens

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• Context-free syntax can be put to better use

• This grammar defines simple expressions with addition & subtraction over “number” and ‘id”

• This grammar, like many, falls in a class called “context-free grammars”, abbreviated CFG

Front End – Example (1)

1.goal → expr

2.expr → expr op term

3. | term

4.term → number

5. | id

6.op → +

7. | -

S = goal

T = {number, id, +, -}

NT = {goal, expr, term, op}

P = {1, 2, 3, 4, 5, 6, 7}

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• A parse can be represented by a tree (parse tree or syntax tree)

Front End – Example (2)

1.goal → expr

2.expr → expr op term

3. | term

4.term → number

5. | id

6.op → +

7. | -

goal

expr

opexpr term

opexpr term

term

<id,x>

+ <number,2>

− <id,y>

𝑥 + 2 − 𝑦

• This contains a lot of unneeded information

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• Compilers often use an Abstract Syntax Tree (AST)• AST summarizes grammatical structure, without including

detail about the derivation

• This is much more concise

• ASTs are one kind of intermediate representation (IR)

Front End – Example (3)

−

+

<id,x> <number,2>

<id,y>

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• Responsibilities• Translate IR into target machined code

• Choose instructions to implement each IR operation

• Decide which values to keep in registers

• Find optimal order of instruction execution

• Ensure conformance with system interfaces

• Automation has been less successful in the back end

Back End

IR

Errors

InstructionSelection

IR TargetProgram

InstructionScheduling

RegisterAllocation

IR

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• Instruction Selection• Produce fast, compact code

• Take advantage of target features such as addressing modes

• Usually viewed as a pattern matching problem

• ad hoc methods, pattern matching, dynamic programming

Back End – Instruction Selection

IR

Errors

InstructionSelection

TargetProgram

InstructionScheduling

RegisterAllocation

IR IR

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• Instruction Scheduling• Avoid hardware stalls and interlocks

• Use all functional units productively

• Can increase lifetime of variables (changing the allocation)

• Optimal scheduling is NP-Complete in nearly all cases

• Heuristic techniques are well developed

Back End – Instruction Scheduling

IR

Errors

InstructionSelection

TargetProgram

InstructionScheduling

RegisterAllocation

IR IR

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• Register Allocation• Have each value in a register when it is used

• Manage a limited set of resources

• Can change instruction choices & insert LOADs & STOREs

• Optimal allocation is NP-Complete

• Compilers approximate solutions to NP-Complete problems

Back End – Register Allocation

IR

Errors

InstructionSelection

TargetProgram

InstructionScheduling

RegisterAllocation

IR IR

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• Code Optimizations• Analyzes IR and rewrites (or transforms) IR

• Primary goal is to reduce

• Execution time, Space usage, Power consumption, …

• Must preserve “meaning” of the code

Optimizing Compiler

Front End Back EndTarget

ProgramSource

Program

IR

Errors

Compiler

Middle EndIR

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• Simple Treewalk for initial code

• Peephole matching for desired code

Instruction Selection Example

load 4 r5loadAO r0,r5 r6loadI 8 r7loadAO r0,r7 r8mult r6,r8 r9

loadAI r0,4 r5loadAI r0,8 r6mult r5,r6 r7

IDENT<a, ARP, 4>

IDENT<b, ARP, 4>

×

Tree Treewalk Code Desired Code

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• Schedule Instructions considering• Latency

• Dependences

• Generate fast executing code

Instruction Scheduling Example

a: loadAI r0,@w r1b: add r1,r1 r1c: loadAI r0,@x r2d: mult r1,r2 r1

a

b

d

c

The Code The Precedence Graph

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• Instruction selection assume infinite # of registers (virtual registers)

• Mapping virtual registers to physical registers

• Sometimes need register spill/fill code

Register Allocation Example

loadI 4 r1loadAO r0,r1 r2loadI 8 r3loadAO r0,r3 r4mult r2,r4 r5

6 virtual registers

loadI 4 r1loadAO r0,r1 r1loadI 8 r2loadAO r0,r2 r2mult r1,r2 r2

3 physical registers

(only r5 is used later) (r5 is renamed with r2)

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• Front End: Process high-level programming language

• Middle End: Apply optimization for speed, power, space, …

• Back End: Produce machine-level assembly code

Summary

Front End Back EndTarget

ProgramSource

Program

IR

Errors

Compiler

Middle EndIR