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Languages and Compilers(SProg og Oversættere)

Code Generation

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Code Generation

– Describe the purpose of the code generator

– Code templates

– Back patching

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The “Phases” of a Compiler

Syntax Analysis

Contextual Analysis

Code Generation

Source Program

Abstract Syntax Tree

Decorated Abstract Syntax Tree

Object Code

Error Reports

Error Reports

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Multi Pass Compiler

Compiler Driver

Syntactic Analyzer

callscalls

Contextual Analyzer Code Generator

calls

Dependency diagram of a typical 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.

input

Source Text

output

AST

input output

Decorated AST

input output

Object Code

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Issues in Code Generation

• Code Selection:Deciding which sequence of target machine instructions will be used to implement each phrase in the source language.

• Storage AllocationDeciding the storage address for each variable in the source program. (static allocation, stack allocation etc.)

• Register Allocation (for register-based machines)How to use registers efficiently to store intermediate results.

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Code Generation

Source Program

let var n: integer; var c: charin begin c := ‘&’; n := n+1end

let var n: integer; var c: charin begin c := ‘&’; n := n+1end

PUSH 2LOADL 38STORE 1[SB]LOAD 0LOADL 1CALL addSTORE 0[SB]POP 2HALT

PUSH 2LOADL 38STORE 1[SB]LOAD 0LOADL 1CALL addSTORE 0[SB]POP 2HALT

Target program

~~

Source and target program must be“semantically equivalent”

Semantic specification of the source language is structured in terms of phrases in the SL: expressions, commands, etc.=> Code generation follows the same “inductive” structure.

Q: Can you see the connection with formal semantics?

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Specifying Code Generation with Code Templates

Example: Code templates specification for Mini TriangleRECAP: The mini triangle AST

Program ::= Command ProgramCommand ::= V-name := Expression AssignCmd | let Declaration in Command LetCmd ...Expression ::= Integer-Literal IntegerExp | V-name VnameExp | Operator Expression UnaryExp | Expression Op Expression BinaryExpDeclaration ::= ...V-name::= IdentifierSimpleVName

Program ::= Command ProgramCommand ::= V-name := Expression AssignCmd | let Declaration in Command LetCmd ...Expression ::= Integer-Literal IntegerExp | V-name VnameExp | Operator Expression UnaryExp | Expression Op Expression BinaryExpDeclaration ::= ...V-name::= IdentifierSimpleVName

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Specifying Code Generation with Code Templates

The code generation functions for Mini Triangle

Phrase Class Function Effect of the generated code

Program

Command

Expres-sionV-name

V-nameDecla-ration

run P

execute C

evaluate E

fetch V

assign Velaborate D

Run program P then halt. Starting and finishing with empty stackExecute Command C. May update variables but does not shrink or grow the stack!Evaluate E, net result is pushing the value of E on the stack.Push value of constant or variable on the stack.Pop value from stack and store in variable VElaborate declaration, make space on the stack for constants and variables in the decl.

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Code Generation with Code Templates

run [C] =execute [C]HALT

The code generation functions for Mini Triangle

Programs:

Commands:

execute [V := E] =evaluate [E]assign [V]

execute [I ( E )] =evaluate [E]CALL p where p is address of the routine named I

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Code Generation with Code Templates

Commands:

execute [C1 ; C2] =execute [C1]execute [C2]

execute [if E then C1 else C2] =evaluate [E]JUMPIF(0) gexecute [C1]JUMP h

g: execute [C2]h:

C1

C2

E

g:

h:

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Code Generation with Code Templates

execute [while E do C] =

JUMP hg: execute [C]h: evaluate[E]

JUMPIF(1) g

C

E

While command

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Developing a Code Generator “Visitor”

public Object visitSequentialCommand(SequentialCommand com,Object arg) {

com.C1.visit(this,arg);com.C2.visit(this,arg);return null;

}

public Object visitSequentialCommand(SequentialCommand com,Object arg) {

com.C1.visit(this,arg);com.C2.visit(this,arg);return null;

}

execute [C1 ; C2] =execute[C1]execute[C2]

LetCommand, IfCommand, WhileCommand => later. - LetCommand is more complex: memory allocation and addresses - IfCommand and WhileCommand: complications with jumps

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Backpatching Example

public Object WhileCommand (WhileCommand com,Object arg) {

short j = nextInstrAddr;emit(Instruction.JUMPop, 0,

Instruction.CBr,0);short g = nextInstrAddr;com.C.visit(this,arg);short h = nextInstrAddr;code[j].d = h; com.E.visit(this,arg);emit(Instruction.JUMPIFop, 1,

Instruction.CBr,g);return null;

}

public Object WhileCommand (WhileCommand com,Object arg) {

short j = nextInstrAddr;emit(Instruction.JUMPop, 0,

Instruction.CBr,0);short g = nextInstrAddr;com.C.visit(this,arg);short h = nextInstrAddr;code[j].d = h; com.E.visit(this,arg);emit(Instruction.JUMPIFop, 1,

Instruction.CBr,g);return null;

}

execute [while E do C] =JUMP h

g: execute [C]h: evaluate[E]

JUMPIF(1) g

dummy address

backpatch

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Constants and Variables

We have not yet discussed generation of LetCommand. This is the place in MiniTriangle where declarations are.

execute [let D in C] =elaborate[D]execute [C]POP(0) s if s>0

where s = amount of storage allocated by D

How to know these?

fetch [V] =LOAD d[SB] where d = address of V relative to SB

assign [V] =STORE d[SB] where d = address of V relative to SB

Calculated during generation forelaborate[D]

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Code Template: Global Procedure

elaborate [proc I () ~ C] =JUMP g

e: execute [C]RETURN(0) 0

g:

C

execute [I ()] =CALL(SB) e

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Code Template: Global Procedure

Example: let var n: Integer; proc double() ~ n := n*2in begin n := 9; double() end

let var n: Integer; proc double() ~ n := n*2in begin n := 9; double() end

0: PUSH 11: JUMP 72: LOAD 0[SB]3: LOADL 24: CALL mult5: STORE 0[SB]6: RETURN(0) 07: LOADL 98: STORE 0[SB]9: CALL(SB) 210:POP(0) 111:HALT

n := n*2

var n: Integer

proc double() ~ n := n*2

n := 9double()