A Case Study on Partial Evaluation in Embedded Software Design

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A Case Study on Partial Evaluation in Embedded Software

Design

Michael JungRalf Laue

Sorin Alexander Huss

Integrated Circuits and Systems LabDarmstadt University of Technology

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Introduction

• Embedded systems software design: software generators help to achieve both resuability and efficiency

• Generators usually implemented by hand:– time-consuming and error-prone

• Implementing software generators with partial evaluation constitute way out

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Overview

• Problem illustration• Introduction to basics

– OSEK– Partial Evaluation

• Implementation• Results

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Problem Illustration

High Degree ofConfigurability

(Parameters, Indirections, ...)

Hard economical constraints led to the fact that embedded software is usually highly specialized for the application at hand and therefore not reusable

ConflictingGoals

Reusability EfficientImplementation

Hand Optimized andSpecialized Code

High Level ofAbstraction (e.g.OOP)

Low Level Machine-

Intimate Code

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Binding Time of Parameters

• Parameterization happens at different times during product life cycle:

• Parameters bound to concrete values before run time are called static. Run time parameters are called dynamic

• This distinction is usually not exploited in reusable software modules

t

End of Assembly Line Time Run TimeDevelopment Time

e.g. bus bandwidth e.g. amount of fuel to inject

Configuration

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Generative Programming

• Generative Programming takes advantage of static parameters– Domain Specific Language

• specifies static parameters at a high level of abstraction

– Generator• generates optimized code from generic components

Generator ImplementationComponents

TranslatorSpecification in a DSL

[2]

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OSEK OS - Generator

• OSEK is a specification for embedded OS (developed by university and automotive industry)

• Domain Specific Language for OSEK is OIL (OSEK Implementation Language)– OIL specifies features like number of tasks and

resources or level of error management

• Generator applies OIL specification and generates specialized code containing only needed functionality

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OSEK Generation Flow

Application Configuration

File (OIL)

optionalOSEK Builder

SystemGenerator

Files produced by

SG Compiler

Linker

Executable File

Executable File

OSEK OSKernel

Device Driver

User's source code

"C" code

"C" code"C" code

Object libraries

[1]

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Shortcomings of Generators

• Generators are typically implemented by hand (e.g. OSEK)

• Verifying correctness of hand-written generators is hard

• Partial Evaluation promises some means of automation

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• A Partial Evaluator computes a Generator from– a generic parameterizable software module– a division of the parameters in static and dynamic ones

• Given concrete values for the static parameters this Generator computes a specialized software module exhibiting the same behaviour as the original program with the same inputs

Partial Evaluation

SpecializedProgram

GeneratorPartial Evaluator

GenericProgram

Divisonstatic vs.dynamic

parameters

static input

dynamicinput output

[3]

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Partial Evaluation: Exampleint power(int x, unsigned n) { int r=1; while (n) { if (n&1) r=r*x; x=x*x; n=n>>1; } return r;}

x : dynamicn : static

void power_gen(unsigned n) { printf(„void power(int x, unsigned n) {\n“); printf(„ int r=1;\n“); printf(„ assert(n==%d);\n“, n); while (n) { if (n&1) printf(„ r=r*x;\n“); printf(„ x=x*x;\n“); n=n>>1; } printf(„ return r;\n}“);}

n = 3

void power(int x, unsigned n) { int r=1; assert(n==3); r=r*x; x=x*x; r=r*x; x=x*x; return r;}

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Partial Evaluation: Advantages

• Generators are typically implemented by hand:– division of parameters in static and dynamic

ones is build into generator cannot easily be changed later on

– With Partial Evaluation generators for all 2P can automatically be synthesized

• Verifying correctness of hand-written generators is hard:– Assuming the Partial Evaluator is correct: only

the generic program has to be checked

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Implementation – Basic Ideas

• OSEK-Implementation with C-Mix/II (which processes the complete C-syntax)

• C-Mix/II creates an OSEK-generator out of generic OS code

• Given the information specified in the OIL-file, this generator computes specialized OS code

• Instead of writing an own parser for OIL, we mapped OIL to XML and used an XML-parser

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Implementation

• Two function-types have to be specialized:– Initialization-functions: creation(malloc) and

initialization of OS-data-structures– API-functions: include only the necessary code

paths for the chosen functionality

• For this, the functions get an additional parameter containing the XML-tree containing the OIL-information, which is static and will not be present in the specialized code

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Implementation

Code to bespecialized

Generator

Partitionstatic/dynamic

OIL/XML

Compiler/Linker

SpecializedCode

General Code Application Code

Executable

C-Mix/II

M6

8K

i386

User written/designed

Part of OSEK-Implementation

3rd party tools

Text-file

Program

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Example: Initialization

TaskRefType *OSEK_ready_queues_head;TaskRefType *OSEK_ready_queues_back;

void OSEK_init_ready_queue(xmlNodePtr config){ unsigned int *priority_mapping, maxPriority;

priority_mapping = create_priority_mapping(config); maxPriority = priority_mapping[max_priority(config)]+1; OSEK_ready_queues_head = malloc(sizeof(TaskRefType)*maxPriority); OSEK_ready_queues_back = malloc(sizeof(TaskRefType)*maxPriority); for (i=0; i<maxNeededPriority; i++) { OSEK_ready_queues_head[i] = INVALID_TASK; OSEK_ready_queues_back[i] = INVALID_TASK; } // for} // OSEK_init_ready_queue


void OSEK_init_ready_queue(xmlNodePtr config){ unsigned int *priority_mapping, maxPriority;

priority_mapping = create_priority_mapping(config); maxPriority = priority_mapping[max_priority(config)]+1; OSEK_ready_queues_head = malloc(sizeof(TaskRefType)*maxPriority); OSEK_ready_queues_back = malloc(sizeof(TaskRefType)*maxPriority); for (i=0; i<maxNeededPriority; i++) { OSEK_ready_queues_head[i] = INVALID_TASK; OSEK_ready_queues_back[i] = INVALID_TASK; } // for} // OSEK_init_ready_queue

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Example: Intialization

TaskRefType OSEK_ready_queues_head[3] =

{ INVALID_TASK, INVALID_TASK, INVALID_TASK };

TaskRefType OSEK_ready_queues_back[3] =


TaskRefType OSEK_ready_queues_head[3] =


TaskRefType OSEK_ready_queues_back[3] =


• Number of tasks > 3

• Exactly 3 different unique priorities were assigned to the tasks

• Gaps between priorities possible

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Example: API-Function

StatusType InitMessage(

SymbolicName msg,

ApplicationDataRef data,

xmlNodePtr config)

{

read_COM_xml(config);

if (COM_cmix_use_parameter_access)

{

COM_error_last_message = msg;

COM_error_last_dataref = data;

} // if

if (COM_cmix_use_GetServiceId)

COM_error_last_service_called =

COMServiceId_InitMessage;

...

StatusType InitMessage(

SymbolicName msg,

ApplicationDataRef data,

xmlNodePtr config)

{

read_COM_xml(config);

if (COM_cmix_use_parameter_access)

{

COM_error_last_message = msg;

COM_error_last_dataref = data;

} // if

if (COM_cmix_use_GetServiceId)

COM_error_last_service_called =

COMServiceId_InitMessage;

...

if (COM_cmix_COMStatus_is_extended) { if ((msg < 0) || (msg >= COM_cmix_message_amount) || (COM_MessageStore[msg].type != RECEIVE_UNQUEUED)) { if (COM_cmix_user_error_hook && !COM_cmix_ErrorHook_is_active) { COM_cmix_ErrorHook_is_active = 1; COMErrorHook(E_COM_ID); COM_cmix_ErrorHook_is_active = 0; } // if return E_COM_ID; } // if } // if COM_MessageStorage[msg].content.\ receive_unqueued_internal.data=*data; return E_OK;} // InitMessage

if (COM_cmix_COMStatus_is_extended) { if ((msg < 0) || (msg >= COM_cmix_message_amount) || (COM_MessageStore[msg].type != RECEIVE_UNQUEUED)) { if (COM_cmix_user_error_hook && !COM_cmix_ErrorHook_is_active) { COM_cmix_ErrorHook_is_active = 1; COMErrorHook(E_COM_ID); COM_cmix_ErrorHook_is_active = 0; } // if return E_COM_ID; } // if } // if COM_MessageStorage[msg].content.\ receive_unqueued_internal.data=*data; return E_OK;} // InitMessage

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Example: API-Function

StatusType InitMessage(SymbolicName msg,

ApplicationDataRef data)

{

if ((msg < 0) ||

(msg >= COM_cmix_message_amount) ||

(COM_MessageStorage[msg].type !=

RECEIVE_UNQUEUED))

{

return E_COM_ID;

} // if

COM_MessageStorage[msg].content.\

receive_unqueued_internal_data=*data;

return E_OK;

} // InitMessage

StatusType InitMessage(SymbolicName msg,

ApplicationDataRef data)

{

if ((msg < 0) ||

(msg >= COM_cmix_message_amount) ||

(COM_MessageStorage[msg].type !=

RECEIVE_UNQUEUED))

{

return E_COM_ID;

} // if

COM_MessageStorage[msg].content.\

receive_unqueued_internal_data=*data;

return E_OK;

} // InitMessage

Selected:

•No error reporting

•No error hooks

•Sanity checks

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Result: Code Sizes

artifact before specialization

after specialization

standard mode

extended mode

source code 94,849 Byte 66,946 Byte 90,574 Byte

object code (i386)

44,424 Byte 15,244 Byte 22,084 Byte

object code (M68K)

N/A 19,804 Byte 22,676 Byte

artifact OSEK implementation

traditional C-Mix/II-based

flashable image 17,340 Byte 17,336 Byte

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Results: Observed Weaknesses

Although the implementation was feasible, there were some problems, because of limited functionality of the used Partial Evaluator (C-Mix/II):• Reflection:

– Need to access objects (i.e., task functions) via their name– void* GetFuncPtr( const char* name)

• Cross partial evaluation:– Generator is executed on host machine, generating code

for embedded system hardware– Need for identical semantics on both machines

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Results: Observed Weaknesses

• Code bloat:– Unrolling loops, different versions of one function– In our case: only elimination of unused code

• Function signatures:– Partial evaluator changes function signature

not desirable for reusable software modules

• No splitting of structs:– C-Mix/II: if any part of a struct is dynamic, whole

struct is dynamic

• Spaghetti code:– C-Mix/II: control flow implemented with goto

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Conclusion

• Conclusion:– We consider partial evaluation a promising

approach to build efficient reusable software for embedded systems

– A couple of problems still have to be addressed with better tools

• Future Work:– Implementation of a Partial Evaluator considering

the domain of resource constrained embedded systems:

• Cross-Partial Evaluation• Code Bloat

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Thanks for your attention

References[1] The OSEK Group: OSEK/VDX Operating System Version 2.2.2, July 2004[2] K. Czarnecki, U.W. Eisenecker: Generative Programming - Methods, Tools, and Applications,

Addison Wesley, 2000[3] N. D. Jones, C. K. Gomard, and P. Sestoft: Partial Evaluation and Automatic Program Generation,

Prentice Hall, 1993[4] R. Laue: Fallstudie: Implementierung von Software Generatoren mittels partieller Evaluierung,

TU Darmstadt, Diplomarbeit, November 2003[5] M. Jung, and S.A. Huss: Fast Points-to Analysis for Languages with Structured Types, SCOPES

2004, Springer LNCS 3199, 2004

Questions?

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Result: Initialization

unsigned char heap_location_1[12];unsigned char heap_location_2[12];


void OSEK_init_ready_queue(){ OSEK_ready_queues_head = heap_location_1; OSEK_ready_queues_back = heap_location_2; OSEK_ready_queues_head[0] = INVALID_TASK; OSEK_ready_queues_back[0] = INVALID_TASK; OSEK_ready_queues_head[1] = INVALID_TASK; OSEK_ready_queues_back[1] = INVALID_TASK; OSEK_ready_queues_head[2] = INVALID_TASK; OSEK_ready_queues_back[2] = INVALID_TASK;} // OSEK_init_ready_queue

unsigned char heap_location_1[12];unsigned char heap_location_2[12];


void OSEK_init_ready_queue(){ OSEK_ready_queues_head = heap_location_1; OSEK_ready_queues_back = heap_location_2; OSEK_ready_queues_head[0] = INVALID_TASK; OSEK_ready_queues_back[0] = INVALID_TASK; OSEK_ready_queues_head[1] = INVALID_TASK; OSEK_ready_queues_back[1] = INVALID_TASK; OSEK_ready_queues_head[2] = INVALID_TASK; OSEK_ready_queues_back[2] = INVALID_TASK;} // OSEK_init_ready_queue

Documents

A Case Study on Partial Evaluation in Embedded Software Design