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Code Generation

Frédéric Haziza <[email protected]>

Department of Computer Systems

Uppsala University

Spring 2008

Operating SystemsProcess Management

Memory Management

Storage Management

CompilersCompiling process &Lexical analysis

Parsing

Semantic &Code generation

Lexicalanalysis

Syntaxanalysis

Semanticsanalysis

Analysis

Machineindepen-dant codegeneration

Optimizationof machine

inde-pendant

code

StorageAllocation

Machinecode

generation

Optimizationof machine

code

Synthesis

Intermediate code Machine code Optimizations

Outline

1 Intermediate code

2 Machine code

3 Optimizations

4 OSKomp’08 | Code Generation


Good IR

easy to translate from AST

easy to translate to assembly/machine code

easy to optimize

easy to retarget



Well-known examples

1 Three-address code

2 P-code (for Pascal)

3 Bytecode (for Java)



P-Code

Stack-based intermediate code, for Pascal.

Instruction format: F P Q

F is a function

P, Q may be absent

P used to specify a static block level

Q: offset within a frame or immediate operand (ie constant)

Compile-time address are (static level, offset)



Instructions

Instructions with no parameter operate on the stack’s top• AND, DIF, NGI, FLT, FLO, INN

One or two-address instruction to load/store a value on thetop of the stack

• LDCI, LODI, LDA, STRI

Jump instructions:• UJP L7: unconditional jump• FJP L8: jump to L8 if top of the stack is false

Labels


if (expression) statement1 else statement2

Code to put the value of expression on top of the stack

FJP L1

Code to implement statement1UJP L2

L1

Code to implement statement2L2

while (expression) statement

L1

Code to put the value of expression on top of the stack

FJP L2

Code to implement statement

UJP L1

L2


Java Run-Time system

Execution engine (executes bytecode instructions)

Memory manager (manages heap in which all objects and arrays are stored)

Error and Exception manager (used to catch runtime failures in a planned and

systematic manner)

Threads interface (handles concurrency)

Class Loader (loads, links and initializes classes)

Security manager deals with attempts to run ’hostile’ programs



Principal types of bytecode instructions

Stack manipulation

Performing arithmetic

Handling objects and arrays

Control flow

Method invocation

Handling exceptions and concurrency



Manipulating the stack

Instructions Meanings

iconst_4 load the integer constant 4 on to the stack

iload_4 load the value of local variable number 4 on to the stack

pop discard top value of the stack

dup duplicate top item on the stack

swap interchange top two values of the stack

istore_4 store the value on top of the stack in the local variable number 4



Arithmetic and Array


iadd add the two integers on the top of the stack

fadd add the two floats on the top of the stack

fmul multiply the two floats on the top of the stack


iaload puts the value of an array element on top of the stack,

assuming the array reference and the index of the array are already on the stack



Conditional and unconditional Branches


ifeq L1 jump to L1 if the integer value on top of the stack is zero

if_icmpne L1 jump to L1 if two integer values on top of the stack are not equal

goto L1 jump to L1


if (expression) statement1 else statement2

Bytecode to put the value of expression on top of the stack

ifeq L1

Bytecode to implement statement1goto L2

L1

Bytecode to implement statement2L2

while (expression) statement

L1

Bytecode to put the value of expression on top of the stack

ifeq L2

Bytecode to implement statement

goto L1

L2


Considerations

CISC complex instruction set computer

* Wide range of addressing mode* Small number of registers (<16)* Many special purpose registers* 2-address instructions A + B → A* Variable length instructions* Instructions with side-effect

* Different exec time for instr

RISC reduced instruction set computer

* Simple addressing mode (withregisters)* Many registers (>32)* All registers are ’general purpose’* 3-address instructions r3 = r1 + r2

* Fixed length instructions (32 bits)* No side-effect, one result per instr

* Similar exec time for instr

Instruction selectionRegister allocation



Example

t1 = a + bt2 = c + dt3 = t1 ∗ t2

Must keep t1 and t2 until t3 is evaluated



Example – 2

a ∗ b + c ∗ d + e ∗ f

t1 = a ∗ bt2 = c ∗ dt3 = t1 + t2t4 = e ∗ ft5 = t3 + t4

Temporaries Register

t1 1t2 2t3 3t4 1t5 2

Do we really need distinct registers?



1) n = 02) sum2 = 03) sum3 = 04) t1 = n < 10 : L1

5) t2 = not t16) if t2 goto L2

7) n = n + 18) m = n ∗ n9) sum2 = sum2 + m10) t3 = m ∗ n11) sum3 = sum3 + t312) goto L1

13) L2

n = 0;sum2 = 0;sum3 = 0;while (n<10){n = n+1;m = n*n;sum2 = sum2 + m;sum3 = sum3 + m*n;

}

Variable Live Registern 1..12 1sum2 2..12 2sum3 3..12 3t1 4..5 4m 8..10 4t2 5..6 4t3 10..11 4



Liveness analysis

Equations of the form:

1 inn = usen ∪ (outn\defn)

2 outn =⋃

s∈succ(n) ins

where

usen: set of all variables whose values are used in statement noutn: set of all variables that are live on leaving statement ndefn: set of all variables that are defined in statement n

inn: set of all variables that are live on reaching statement n



Typical local optimizations

Constant folding

Strength reduction

Elimination of unnecessary instructions



Typical global optimizations

Analysis of control and data flow:

Dead code elimination

Common subexpression elimination

Loop optimizations


code generation - it.uu.se filefrédéric haziza department of computer systems uppsala university...

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