Parallel Lint

Parallel Lint
Author: Andrey Karpov

Date: 08.06.2009

Abstract
The article describes a new direction in development of static code analyzers - verification of parallel
programs. The article reviews several static analyzers which can claim to be called "Parallel Lint".

Introduction
The article is intended for developers of Windows-applications using C/C++ languages but may be also
useful for many people interested in the issues of static code analysis.

Static code analysis is a process of software verification without actual execution of programs being
examined. Usually it is the source code of the program that is directly analyzed but sometimes one type
of object code is analyzed [1].

Static analysis is a kind of software testing with the help of code review. But while in case of code review
the code being tested must be looked through by a person, in case of static analysis specialized software
called a static analyzer takes part of the work. A static analyzer detects potentially unsafe places in the
program code and suggests the user perform further analysis and correction of these places. It helps
reduce the amount of work of code review.

One of the first static analyzers and the most popular one was Lint utility which appeared in 1979 as part
of Unix 7 distribution kit as the main tool of controlling the quality of software written in C [2, 3]. This
tool is so popular that the word "lint" has nearly become a synonym of "static analyzer". And very often
you can see not "a static analysis tool" but "a lint-like tool".

Some time later new powerful means of static analysis, both general-purpose and specialized, appeared:
Coverity Prevent, PC-Lint, KlocWork K7, PolySpace, Viva64, FXCop, C++Test, JLint, UNO etc.

These static analyzers detect a lot of errors varieties of which are even impossible to list. They are
constantly modified and they are learning to detect more and more types of errors. One of such new
directions is verification of parallel programs.

As multi-core microprocessors appeared parallel programming exceeded the limits of highly tailored
solutions and spread quickly in the sphere of various applications. Parallel programming requires
specialized tools for creating, testing and debugging programs. And while in the sphere of technology of
creating parallel applications everything is rather good, in the sphere of verification and testing of these
applications there are a lot gaps. It is this that urges developers of static analyzers to develop this
territory which is rather new for them - the territory of verification of parallel applications. Now it's the
time of new solutions. Now it's the time of "Parallel Lint".

Parallel Lint is not a name of a concrete tool. But it is a good phrase which describes simply and briefly
the new class of tools for analyzing parallel code. In this article the reader will get acquainted with some

tools in which diagnosing of the new class of parallel errors is implemented and which can be justly
called "Parallel Lint".

PC-Lint tool
In the 9th version of Gimpel Software PC-lint diagnosing of errors in parallel programs built on POSIX
threads and similar technologies is implemented. In other words PC-Lint tool is not focused on some
particular parallel technologies and can be adapted by a user for the programs built on various libraries
implementing parallelism. This flexibility is achived due to arrangement of auxiliary comments for PC-
Lint in the code and its settings. As usual, the price of this flexibility is a higher complexity.

Still, complex setting is needed only if you use a parallel technology unknown to PC-Lint. PC-Lint
supports POSIX threads and can detect errors relating to locks by itself if such functions as
pthread_mutex_lock() and pthread_mutex_unlock() are used.

But anyway PC-Lint requires some hints from the programmer's side and is helpless without them. The
point is that the static analyzer does not know if some code section is executed in parallel mode and can
only make some suggestions. It is not so easy to understand whether this is a parallel call or not. The call
can depend in a complex way on the logic of the algorithm and input data which the static analyzer
lacks.

Let's consider an example of the function (given in the manual to PC-Lint 9.0):

void f()

{

static int n = 0;

/* ... */

}

The problem is that if f() function is called from parallel threads an error of initializing 'n' variable may
occur. To diagnose this case we need to explicitly define to PC-Lint analyzer that f() function can be
called in parallel mode. To do so we need to use a consturcion like:

//lint -sem(f, thread)

Then, when diagnosing the code, you will see the warning: Warning 457: "Thread 'f(void)' has an
unprotected write access to variable 'n' which is used by thread 'f(void) .

If the paralleliling mechanism is built not on POSIX threads you will have to use special additional
directives to hint PC-Lint what functions cause locks and unlocks:

-sem(function-name, thread_lock)

-sem(function-name, thread_unlock)

In this case you can detect errors in the code like the following one:

//lint -sem( lock, thread_lock )

//lint -sem( unlock, thread_unlock )

extern int g();

void lock(void), unlock(void);

void f()

{

//-------------

lock();

if( g() )

return; // Warning 454

unlock();

//-------------

if( g() )

{

lock();

unlock();

unlock(); // Warning 455

return;

}

//-------------

if( g() )

lock();

{ // Warning 456

// do something interesting

}

}

PC-Lint analyzer can efficiently find errors relating to parallelism if you give the necessary 'hints' to it
beforehand. Otherwise, it can perform analysis of a parallel code as of a sequential one and therefore
fail to detect some errors.

Despite the necessity of additional work of arranging the hints, PC-Lint tool is a very convenient and
powerful tool able to detect many parallel errors. It can be especcialy useful when used together with
Visual Lint shell providing a more convenient user interface to this analyzer.

To learn more about diagnostic abilities of PC-Lint 9.0 see the manual to PC-lint/FlexeLint 9.0 Manual
Excerpts [4].

Among disadvantages of this tool we can mention its inability to diagnose parallel code built on OpenMP
technology. It does not process OpenMP directives and hints will not help here. But there are other
static analyzers for diagnosing OpenMP programs and we will speak about them further [5].

VivaMP tool
VivaMP is a powerful specialized tool intended for verification of the code of applications built on
OpenMP technology. This tool has been initially developed with the purpose of testing OpenMP parallel
code and that's why it can claim to be called "Parallel Lint" more than all the others.

The analyzer integrates into Visual Studio 2005/2008 environment and allows you to start work
immediately without complicated settings or arrangement of comments in the program code. It is a
great advantage of the tool for it allows you to try and master the product easily. Like any other static
analyzer VivaMP will reuqire additional setting while operating it to reduce the number of false
responses. But this is an inevitable sacrifice all the static analyzers demand.

The possibility to avoid tiring setting of the analyzer is explained by the peculiarities of OpenMP
technology. Description of OpenMP directives is in itself an explaining comment for the analyzer
providing information about the program's structure: which part is executed in parallel mode, which
resources are local and which are general etc. All this allows the analyzer to perform rather a thorough
analysis without demanding assistance of the programmer.

Let's show the principles of VivaMP operation on several simple examples.

Example N1.

#pragma omp parallel for

for (size_t i = 0; i != n; ++i)

{

float *array =

new float[10000]; // V1302

delete [] array;

}

In this code, VivaMP analyzer diagnoses the following error: "V1302. The 'new' operator cannot be used
outside of a try..catch block in a parallel section.". The error relates to throwing of an exception from a
parallel block. According to OpenMP specification, if you use exceptions inside a parallel block all these
exceptions must be processed inside this block. If you use 'new' operator inside parallel code, you must
provide catching of the exception which will be generated according to C++ standard when an error of
memory allocation occurs.

This example leads to incorrect program behavior and most likely to program crash if an error of
memory allocation occurs.

Correction of the code lies in processing of exceptions inside the parallel block and transferring of
information about the error through other mechanisms or refusing to use 'new' operator.

The following corrected code is safe from the viewpoint of VivaMP analyzer:


for (size_t i = 0; i != n; ++i)

{

try {

float *array =

new float[10000]; // OK

delete [] array;

}

catch (std::bad_alloc &) {

// process exception

}

}

Example N2.

int a = 0;

#pragma omp parallel for num_threads(4)

for (int i = 0; i < 100000; i++)

{

a++; // V1205

}

This is an example of a typical error of race condition. This is a programming error of a multi-task system
where operation of the system depends on the order in which the code parts are executed. Race
condition appears when several threads of a multi-thread application try to get access to data
simultaneously and one thread is performing writing. Race conditions can lead to unexptected results
and are very often difficult to detect. Sometimes the consequences of the race condition can occur only
in a very large time period and in quite a different place of the application. To avoid race conditions
synchronization means are used which allow you to arrange operations executed by different threads in
a right way.

In this code VivaMP analyzer diagnoses the following error: "V1205. Data race risk. Unprotected
concurrent operation with the "a" variable". As all the threads are writing into the same memory space
and reading from it simultaneously, the value of the variable after this cycle cannot be predicted. To
make this operation safe you must put it into a critical section or (for in this example the operation is
elementary) use "#pragma omp atomic" directive:

int a = 0;

#pragma omp parallel for num_threads(4)

for (int i = 0; i < 100000; i++)

{

#pragma omp atomic

a++; // OK

}

To learn more about type errors occurring when developing OpenMP applications see the article "32
OpenMP traps for C++ developers" [6]. Diagnosing of most of the errors described in the article is
already implemented in VivaMP analyzer or will appear in new versions.

Static analysis implemented in Intel C++ compiler
Diagnosing of parallel OpenMP errors is implemented in Intel C++ 11.0 compiler. Guess how this new
mechanism is called in the compiler. Right, the new function of the compiler is called "Parallel Lint" [7].

The static analyzer embedded into Intel C++ compiler allows you to diagnose various synchronization
errors, race conditions etc. For this you must define the key /Qdiag-enable:sc-parallel{1|2|3} to the
compiler, where numbers define the level of analysis. An additional parameter which is important when
analyzing parallel programs is the compiler's key /Qdiag-enable:sc-include which points to the compiler
to search errors in header files as well.

In the mode of testing a parallel program the compiler does not generate the code being executed. At
the stage of compilation special pseudo-object files (*.obj) are created which contain data structures
including information necessary for analysis instead of object code. Then these files are sent to the input
of the static analyzer which performs analysis of the parallel code.

As it was said above, the executed code is not generated when setting the compiler for static analysis of
parallel OpenMP programs. That's why it would be a convenient solution to keep separate
configurations for building of a project and for its static analysis.

As a result, the developer can, for example, detect an error in the following code:

#include <stdio.h>

#include "omp.h"

int main(void)

{

int i;

int factorial[10];

factorial[0]=1;


for (i=1; i < 10; i++) {

factorial[i] = i * factorial[i-1]; // warning #12246

}

return 0;

}

In static analysis mode Intel C++ compiler will show the following warning: "omp.c(13): warning #12246:
flow data dependence from (file:omp.c line:13) to (file:omp.c line:13), due to "factorial" may lead to
incorrect program execution in parallel mode".

To learn more about the technology of Intel C++ static analysis visit Dmitriy Petunin's webinar "Static
Analysis and Intel® C/C++ Compiler ("Parallel Lint" overview)" which can be found in the archives on
Intel site [8].

As you see, functionality of Intel C++ static analyzer is similar to VivaMP's. I cannot say which one is
better and can be useful for your tasks. On the one hand the analyzer in Intel C++ is a tool you get
together with the compiler. On the other hand there are a lot of various functions in Intel C++ compiler
and "Parallel Lint" is only one of them while VivaMP analyzer is a highly tailored product which is rapidly
developing.

Most likely, VivaMP analyzer would be useful for those developers who use Visual C++ as it allows
performing analysis without any changes in a project's configurations and source code. To use Intel C++
abilities the developers will have to adapt their project for building it by this complier at first. That's why
"Parallel Lint" in Intel C++ would be useful first of all for those developers who have been already using
Intel C++ compiler to build their applications.

Conclusion
Of course, there are other tools which can claim to be called "Parallel Lint". But these tools are mostly
dynamic analyzers or combine static and dynamic analysis. So it would be incorrect to describe them in
this article. The readers can get acquainted with some of them in the article "Tools And Techniques to
Identify Concurrency Issues" [9].

And we wish all our readers safe parallel code!

References
1. Wikipedia. Static code analysis. http://www.viva64.com/go.php?url=12
2. Ian F. Darwin. Checking C Programs with lint, O'Reilly, 1988, ISBN 0-937175-30-7.
3. Wikipedia. Lint. http://www.viva64.com/go.php?url=225
4. PC-lint/FlexeLint 9.0 Manual Excerpts. http://www.viva64.com/go.php?url=226
5. Peaceful coexistence of PC-Lint and VivaMP. http://www.viva64.com/blog/en/2009/02/23/31/
6. Alexey Kolosov, Andrey Karpov, Evgeniy Ryzhkov. 32 OpenMP traps for C++ developers.
http://www.viva64.com/art-3-2-1023467288.html
7. David Worthington. Intel adds parallel programming features to compilers.
http://www.viva64.com/go.php?url=227
8. Intel Software Network. Forums. Thread "Parallel Lint". http://www.viva64.com/go.php?url=228

9. Rahul V. Patil and Boby George. Tools And Techniques to Identify Concurrency Issues.
http://www.viva64.com/go.php?url=132

Parallel Lint

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