64-bit Loki

Loki Team and OOO "Program Verification Systems".
Andrei Alexandrescu, Andrey Karpov, Evgeniy Ryzhkov, Peter Kümmel, Rich Sposato.

September 2009

Abstract

The article is a report about testing of portability of Loki library with 64-bit systems with the help of Viva64 code analyzer performed by OOO "Program Verification Systems" Company. It contains recommendations for users of the library. The article will be also useful for the users of other libraries built on templates for it describes the peculiarities of analysis of such libraries.

Introduction

Loki library has been developed by Andrei Alexandrescu as part of the highly acclaimed book "Modern C++ Design: Generic Programming and Design Patterns Applied". An abstract of the book declares: "This book introduces the concept of generic components-reusable design templates that produce boilerplate code for compiler consumption-all within C++. Generic components enable an easier and more seamless transition from design to application code, generate code that better expresses the original design intention, and support the reuse of design structures with minimal recoding."

OOO "Program Verification Systems" Company creating Viva64 code analyzer for developing 64-bit applications participates actively in communication with the authors of various software projects. Once one of Loki project administrators, Rich Sposato, asked us to check the code of the library for portability to 64-bit systems with the help of our code analyzer Viva64. The Loki library is known for using the most powerful and up-to-date abilities of C++ language; so if Viva64 managed Loki then it can easily manage simpler projects.

We should notice that this article is based on the Loki version of May 2009 (it is more modern than the official version of that time Loki 0.1.7), so the stated problems will be removed in further Loki versions.

So, we downloaded the latest version of Loki from the SVN repository on SourceForge and set to work.

Building of the 64-bit version of Loki in Microsoft Visual Studio 2005

Viva64 code analyzer integrates into Microsoft Visual Studio development environment, that's why it would be sensible to build a version of Loki for this particular environment. Loki package contains ready files of solutions for Visual Studio 2005 and Visual Studio 2008. But now, in May 2009, these solutions contain only 32-bit configurations. That's why we needed to create configurations for x64 platform in Visual Studio. After adding the necessary configurations we could launch compilation of the 64-bit version.

Loki library is intended for operating on many various platforms and is built with the help of many most popular compilers. It explains why the 64-bit Loki version has been compiled nearly at once. The Loki library is actually only one of 20 projects in the most recent version of Loki. The other 19 projects are test programs which exercise various parts of Loki to demonstrate the code compiles and runs correctly. By including so many test programs with Loki, the programmers can provide guarantees for much of Loki's functionality despite the complexity of the Loki source code. Out of 20 projects in the solution only SafeFormat failed to build:

========== Build: 19 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

------ Build started: Project: SafeFormat, Configuration: Debug x64 ------
Compiling...
main.cpp
.\main.cpp(255) : error C3066: there are multiple ways 
that an object of this type can be called with these arguments
        ..\..\include\loki/SafeFormat.h(109): could be 'Loki::PrintfState<Device,Char> 
&Loki::PrintfState<Device,Char>::operator ()(bool)'
...
        while trying to match the argument list '(UInt)'

The text of the error is abridged a bit for it would take the whole page in a full form. Let's consider the code causing the error:

void test_dword()
{
    typedef signed int Int;
    typedef unsigned int UInt;
    typedef signed long Long;
    typedef unsigned long ULong;
    Int i(0);
    UInt ui(0);
    Long l(0);
    ULong ul(0);
    Printf("%d")(i);
    Printf("%d")(ui); // the problem is in this line
    Printf("%d")(l);
    Printf("%d")(ul);
}

The PrintfState struct uses the LOKI_PRINTF_STATE_FORWARD macro to cast various parameter types to unsigned long. We found one problem here where a 64 bit parameter may get sliced down to 32 bits by this code snippet inside the file SafeFormat.h:

#if (defined(_WIN32) || defined(_WIN64))
        LOKI_PRINTF_STATE_FORWARD(unsigned long)
#else

We recommend using this snippet to avoid slicing 64 bit types:

#if (defined(_WIN32) || defined(_WIN64))
#if (defined(_WIN64))
        LOKI_PRINTF_STATE_FORWARD(unsigned int)
#endif
        LOKI_PRINTF_STATE_FORWARD(unsigned long)
#else

After this single correction the compilation error disappears and all the 20 projects of the library compiled with only some diagnostic warnings. One such warning tells us about a potentially unsafe conversion of size_t type inside typicalUse() function in the file CachedFactoryTest.cpp:

// Registering objects
for(size_t i=0;i<objectKind;i++)
  CC.Register(i, createProductNull);

The control variable (objectKind) of the loop is an unsigned type. After changing the type of the loop counter to unsigned so it matches the type of the control variable, the warning disappears:

 // Registering objects
for(unsigned i=0;i<objectKind;i++)
  CC.Register(i, createProductNull);

After these small corrections the 64-bit library compiles successfully and does not show any diagnostic warnings on 64-bit mode. But is the library's code actually correct? By running the many test projects and seeing no incorrect results or assertion failures, we can confidently say the Loki library is correct. We also used our code analyzer, Viva64, to perform static analysis of Loki.

Testing of the 64-bit Loki version with the help of Viva64

To make sure that Loki is compatible with 64-bit systems, let's perform analysis of the code using Viva64. Viva64 code analyzer is intended for developing new 64-bit applications and porting of existing 32-bit ones on a 64-bit platform.

During the analysis of Loki, Viva64 detected 89 potentially unsafe syntactic constructions. It does not mean Loki projects contain 89 errors related to 64-bit code, but that the developers should review these 89 places should be to understand if these are errors. Of course, we have studied these locations in the code to ascertain if there are actual errors. Many of the 89 locations are inside test projects rather than inside Loki itself.

1 Incorrectly used constant LONG_MIN

Let's begin with an error relating to the incorrectly used constant LONG_MIN in the following function:

char* RenderWithoutSign(LOKI_SAFEFORMAT_SIGNED_LONG n, 
    char* bufLast, unsigned int base, bool uppercase)

It is located in the file SafeFormat.h. The problem is in this line:

if (n != LONG_MIN) {

LOKI_SAFEFORMAT_SIGNED_LONG type is defined as a type capable of storing 64-bit values in a 64-bit system. In Unix-systems (with LP64 data model) long type is used for this purpose. But in 64-bit Windows-systems (LLP64 data model) long type remained 32-bit. That's why LOKI_SAFEFORMAT_SIGNED_LONG type is defined in Loki in this way:

#if defined(_WIN32) || defined(_WIN64)
  #define LOKI_SAFEFORMAT_SIGNED_LONG intptr_t
  #define LOKI_SAFEFORMAT_UNSIGNED_LONG uintptr_t
#else
  #define LOKI_SAFEFORMAT_SIGNED_LONG signed long
  #define LOKI_SAFEFORMAT_UNSIGNED_LONG unsigned long
#endif

As long type remained 32-bit in 64-bit Windows-systems, LONG_MIN constant defines the minimum value of the 32-bit variable. This means that using it is incorrect when operating 64-bit types (in this case intptr_t). A strongly recommended solution is to use your own constant so you can guarantee its value remains the same for all platforms, or so you can control its value per platform rather than relying on the compiler. One possible correction is as follows:

#if defined(_WIN32) || defined(_WIN64)
#  define LOKI_SAFEFORMAT_SIGNED_LONG intptr_t
#if defined(_WIN64)
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MIN_VALUE LLONG_MIN
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MAX_VALUE LLONG_MAX
#else
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MIN_VALUE LONG_MIN
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MAX_VALUE LONG_MAX
#endif
...
#else
#  define LOKI_SAFEFORMAT_SIGNED_LONG signed long
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MIN_VALUE LONG_MIN
#  define LOKI_SAFEFORMAT_SIGNED_LONG_MAX_VALUE LONG_MAX
...
#endif

Consequently, the line

if (n != LONG_MIN) {

must be replaced with

if (n != LOKI_SAFEFORMAT_SIGNED_LONG_MIN_VALUE) {

Fortunately, the creators of Loki library must be praised because this is the only line needing this correction. All the other comments described further may be interesting but they are not important.

2 Some types of magic numbers - errors or not?

Many of the problems detected by the Viva64 analyzer in Loki are because the lines seem to contain magic numbers. From the viewpoint of migration of code from a 32-bit platform on a 64-bit one, some numbers are the most dangerous. Perhaps, a programmer wants to have a definite size of a data type in some code section and this may cause a problem. Those who look through the warnings of the code analyzer often complain that they are irrelevant. Really, why should the code analyzer about number 4 in lines like these?

::Loki::ScopeGuard guard4 = ::Loki::MakeGuard( &HasFour, 1, 2, 3, 4 );
::Loki::ScopeGuard guard5 = ::Loki::MakeGuard( &HasFive, 1, 2, 3, 4, 5 );

Sometimes there appear such constructions that must be studied very thoroughly. For example, in the file SafeFormat\main.cpp we see the code:

case 'X':
    // TestCase(formatSpec, RandomInt(-10000, 10000));
    // don't test negative values on 64bit systems, because 
    // snprintf does not support 64 Bit values
    TestCase(formatSpec, 
        RandomInt( -10000 * (sizeof(size_t)>4 ? 0 : 1) , 10000));   
    break;
case 'e':

Of course, this particular use of a magic number is not an actual problem, but detecting and diagnosing magic numbers helps make the code more portable from one operating system to another and from 32 bits to 64 bits.

3 Addition of int to a pointer as a potential error

The file flex\simplestringstorage.h contains the function:

void resize(size_type newSize, E fill)
{
  const int delta = int(newSize - size());
  if (delta == 0) return;
  if (delta > 0)
  {
    if (newSize > capacity())
    {
      reserve(newSize);
    }
    E* e = &*end();
    flex_string_details::pod_fill(e, e + delta, fill);
  }
  pData_->pEnd_ = pData_->buffer_ + newSize;
}

Viva64 analyzer warns about a potential problem here:

flex_string_details::pod_fill(e, e + delta, fill);

The defect occurs in adding, delta, a variable of int type, to e, a pointer. This is a potential problem since the pod_fill function will not be able to process a data size of more than 2 GB (INT_MAX characters). This particular instance is not a no problem for there are hardly any lines that occupy more than 2 GB. Even so, it is better to change delta from an int type to a ptrdiff_t type:

const ptrdiff_t delta = ptrdiff_t(newSize - size());

4 Using int for indexing arrays is incorrect

To access very large data arrays - more than INT_MAX elements - we recommend using types ptrdiff_t or size_t instead of unsigned int or unsigned long. The file SmallObj\SmallObjBench.cpp contains a large macro LOKI_SMALLOBJ_BENCH_ARRAY in which an int is used to index an array since compilers often use 32 bit variables for ints, but must use an appropriately sized data type for size_t.

5 Right arguments of functions

The file CachedFactory\CachedFactoryTest.cpp contains the following function:

template< class Cache >
milliSec typicalUse(Cache &CC, unsigned objectKind, 
                    unsigned maxObjectCount, unsigned maxIteration)

We recommend using the size_t type for the objectKind parameter, but as this code exists only in a test project, the code does not affect the Loki library itself.

Loki library is compatible with 64-bit systems - does it mean that we can say the same about a program using it?

All the few problems of Loki library described above are easy to correct. Does that mean that if Loki does not contain any 64-bit problems (and this is true) than any application using this library is also safe from the viewpoint of 64-bit code? Unfortunately, it is not!

The point is that Loki library uses templates actively, so when the code analyzer examines a template code sometimes it cannot detect a problem. To be absolutely sure the analyzer needs to perform instancing of template classes and functions.

Let's give an example that does not relate to Loki library. Among other problems Viva64 analyzer can detect non-optimal data structures in code:

template <class T>
struct TClass
{
  int m_a;
  T m_b;
  int m_c;
};

If T has int type here the structure is optimal. But if T has size_t type the structure will occupy 24 bytes instead of possible 16 bytes. When there are many such objects, it is better to rewrite the code in this way:

template <class T>
struct TClass
{
  T m_b;
  int m_a;
  int m_c;
};

But the analyzer can check it only by instancing the template. That is, you cannot detect a problem when there is only definition of a class in the header.

Another example, again not relating to Loki, concerns type conversion:

template<typename T1, typename T2>
class TemplateClass
{
public:
	void test1()
	{
		m_a.m_value = m_b.m_value; // is there an error here?
	}
private:
	T1 m_a;
	T2 m_b;
};

In this code the error of type conversion can or cannot occur depending on the parameters with which instancing of TemplateClass template is performed. Without performing instancing, by simply analyzing the function's code, the analyzer cannot detect an error.

The two examples of template classes described above do not relate to Loki library but they are important for understanding the principles of code analyzers' operation. The peculiarity of template libraries like Loki is that even if a library is completely compatible with 64-bit systems it does not mean that the code using it is correct. This point changes completely the approach to verification of applications. Unlike common (non-template) libraries when complete compatibility with 64-bit systems is enough to make sure that the whole application is correct, in case of template libraries you cannot be sure about it.

All this means that although Loki library does not contain problems relating to 64-bit code, a user application which uses it must be tested by the code analyzer additionally to see if there are such problems, for errors depend on the parameters with which instancing of templates is performed.

Conclusion

The results of testing Loki library on compatibility with 64-bit systems performed by the workers of OOO "Program Verification Systems" Company allowed us to draw the following conclusions:

The library is fully compatible with 64-bit systems and does not contain potential errors. The errors stated in this article are likely to be corrected very easily.

Viva64 code analyzer intended for developing 64-bit applications and porting existing 32-bit ones turned out to be very efficient when testing a complicated template code of the library. It proves a very good quality of the code analyzer.

Although Loki library does not contain 64-bit problems they can occur in user applications using Loki. As the end code depends on the parameters with which templates have been instanced, it is necessary to test user applications with the help of the code analyzer. Only then you can be sure that the user application is compatible with 64-bit systems.

Acknowledgements

We would like to thank the following people who assisted us with our analysis of Loki library, or reviewed our work on Loki:

  • We thank Program Verification Systems technical team who reviewed Loki library and carried out verification of its 64-bit compliance: Andrey Karpov and Evgeniy Ryzhkov.
  • We thank Loki team: Andrei Alexandrescu, Peter Kümmel, and Rich Sposato for cooperation and for checking our article, editing it and providing valuable advice.
  • We are glad that Rich Sposato expressed the will of working together in future and we would like to cooperate in future with the team of Loki developers.

References

  1. Loki Library. http://sourceforge.net/projects/loki-lib/.
  2. Viva64 Tool. http://www.viva64.com/viva64-tool.


About the Author

Andrey Karpov

Andrey Karpov is technical manager of the OOO "Program Verification Systems" (Co Ltd) company developing the PVS-Studio tool which is a package of static code analyzers integrating into the Visual Studio development environment.

PVS-Studio is a static analyzer that detects errors in source code of C/C++ applications. There are 3 sets of rules included into PVS-Studio:

  1. Diagnosis of 64-bit errors (Viva64)
  2. Diagnosis of parallel errors (VivaMP)
  3. General-purpose diagnosis

Andrey Karpov is also the author of many articles on the topic of 64-bit and parallel software development. To learn more about the PVS-Studio tool and sources concerning 64-bit and parallel software development, please visit the www.viva64.com site.

Best Articles:

My page on LinkedIn site: http://www.linkedin.com/pub/4/585/6a3

E-mail: karpov@viva64(dot)com

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