Archive for the ‘C++ Compilers’ Category

Visual Studio 2012 First Impressions

Tuesday, October 9th, 2012

A few weeks ago we released ODB 2.1.0. Besides a large number of new features, this version also added support for Visual Studio 2012. Specifically, all the runtime libraries, examples, and tests now come with project/solution files for and were successfully tested with Visual Studio 2012 in addition to 2010 and 2008. This blog post is a collection of notes on differences between Visual Studio 2010 and 2012 that we encountered while working on adding this support. The notes primarily focus on C++ and you may find them useful if you plan to migrate to 2012 or if you want to add support for this version of Visual Studio in your project.

The first thing that you will notice when installing Visual Studio 2012 (VS2012 from now on) is that you can no longer install just the C++ development environment. Now all the languages are always installed. So make sure you have enough free space available. VS2012 co-exists fine with VS2010 and VS2008 provided you install SP1 for VS2010. Failed that you will get a weird error from the linker saying that the COFF file is corrupt. To test ODB we have all three versions of VS installed on the same virtual machine and everything works fine.

After installing VS2012 I was preparing myself mentally for the mind-numbing task of setting up all the include/library search paths in VC++ Directories. In my case those are needed for all the 5 databases that ODB supports, all the ODB runtime libraries, plus Boost and Qt. Multiply that by two for 32 and 64-bit configurations, and you end up with a lot of directories. BTW, getting to the VC++ Directories dialog in VS2012 is exactly the same as in VS2010. That is, open the Property Manager tab, then open Microsoft.Cpp.Win32.User or Microsoft.Cpp.x64.User sheet.

Imagine my surprise when I opened this dialog for the first time and saw all the directories pre-populated! For a moment I thought, finally, Microsoft actually did something sensible for a change. However, my joy was short lived. At first I thought that VS2012 simply copied the directories from VS2010 and all I needed to do is just tweak them a bit. But then I realized that Microsoft could have also done something else: they could have shared the entries with VS2010! So I quickly modified one entry in VS2012 and sure enough I saw the same modification appearing in VS2010.

Why is this bad news? We all know that mixing libraries built using one version of VS with applications that use another is asking for trouble. In fact, some libraries, such as Qt, go a step further and actively prevent you from doing this by prefixing their symbols with the VS version. The fact that you are now forced to share VC++ Directories between VS2010 and 2012 just shows that Microsoft still doesn’t understand how developers are using their product.

Luckily, there is a fairly easy workaround for this problem. The idea is to include the VS version number into the path and then use the $(VisualStudioVersion) variable in the VC++ Directories entries. Here is an example for the ODB runtime library, libodb. First step is to create two directories with the library source code, say libodb-vc10.0 and libodb-vc11.0. Then build libodb-vc10.0 with VS2010 and libodb-vc11.0 with VS2012. Once this is done, the final step is to open the VC++ Directories dialog (doesn’t matter whether it is VS2010 or 2012) and add the following paths:

Include: ...\libodb-vc$(VisualStudioVersion)
Library: ...\libodb-vc$(VisualStudioVersion)\lib

In ODB, VS project and solution files are automatically generated from templates. So for us the first step in adding support for VS2012 was to come up with suitable library and executable project templates. As it turned out, it was actually easier to convert VS2010 projects to VS2012 rather than create ones from scratch. Comparing the 2010 and 2012 project files (.vcxproj) revealed that the only difference between the two is the addition of the following PlatformToolset tag after UseDebugLibraries in each configuration:


Similar to the project files, the VS2012 solution files (.sln) are exactly the same except for the VS version embedded in them. The project filters files (.vcxproj.filters) haven’t changed. So all that was needed to convert VS2010 project/solution templates to VS2012 was a couple of simple sed scripts. Overall, in case of ODB, it took about half a day to create all the templates and update all the scripts. And another day to build and test all the configurations for all the databases.

I also haven’t encountered any spurious errors or warnings when compiling ODB with VS2012 compared to 2010. Though ODB source code was already fairly clean thanks to being tested with the latest versions of GCC and Clang with all the warnings enabled.

Compilation speed-wise, I haven’t noticed any significant improvements. While I haven’t done any thorough testing in this area, SQLite build times show a marginal improvement (39s for VS2012 vs 43s for 2010).

Finally, when it comes to C++11 support in VS2012, it appears Microsoft concentrated on user-visible features (like range-based for-loop) at the expense of library-level ones. As a result, the list of unsupported C++11 features that could be useful in ODB is exactly the same for VS2012 as for 2010:

#if _MSC_VER >= 1600
#  define ODB_CXX11
#  define ODB_CXX11_NULLPTR

GCC can now be built with a C++ compiler

Tuesday, May 8th, 2012

You probably heard about the decision to allow the use of C++ in GCC itself. But it is one thing to say this and completely different to actually making a large code base like GCC to even compile with a C++ compiler instead of C. Well, GCC 4.7 got one step closer to this and can now be compiled with either a C or C++ compiler. Starting with 4.8, it is planned to build GCC in the C++ mode by default. Here is the C++ Build Status page for GCC 4.8 on various targets.

To enable the C++ mode in GCC 4.7, we use the --enable-build-with-cxx GCC configure option. As one would expect, different distributions made different decisions about how to build GCC 4.7. For example, Debian and Ubuntu use C++ while Arch Linux uses C. These differences are not visible to a typical GCC user which is why neither the GCC 4.7 release notes nor the distributions mention any of this. In fact, I didn’t know about the new C++ build mode until ODB, which is implemented as a GCC plugin, mysteriously failed to load with GCC 4.7. This “war story” is actually quite interesting so I am going to tell it below. At the end I will also discuss some implications of this change for GCC plugin development.

But first a quick recap on the GCC plugin architecture: GCC plugin is a shared object (.so) that is dynamically-loaded using the dlopen()/dlsym() API. As you may already know, with such dynamically-loaded shared objects, symbol exporting can work both ways: the executable can use symbols from the shared object and the shared object can use symbols from the executable, provided this executable was built with the -rdynamic option in order to export its symbols. This back-exporting (from executable to shared object) is quite common in GCC plugins since to do anything useful a plugin will most likely need to call some GCC functions.

Ok, so I built ODB with GCC 4.7 and tried to run it for the first time. The error I got looked like this:

cc1plus: error: cannot load plugin undefined symbol: instantiate_decl

Since the same code worked fine with GCC 4.5 and 4.6, my first thought was that in GCC 4.7 instantiate_decl() was removed, renamed, or made static. So I downloaded GCC source code and looked for instantiate_decl(). Nope, the function was there, the signature was unchanged, and it was still extern.

My next guess was that building GCC itself with the -rdynamic option was somehow botched in 4.7. So I grabbed Debian build logs (this is all happening on a Debian box with Debian-packaged GCC 4.7.0) and examined the configure output. Nope, -rdynamic was passed as before.

This was getting weirder and weirder. Running out of ideas, I decided to examine the list of symbols that are in fact exported by cc1plus (this is the actual C++ compiler; g++ is just a compiler driver). Note that these are not the normal symbols which we see when we run nm (and which can be stripped). These symbols come from the dynamic symbol table and we need to use the -D|--dynamic nm option to see them:

$ nm -D /usr/lib/gcc/x86_64-linux-gnu/4.7.0/cc1plus | 
grep instantiate_decl
0000000000529c50 T _Z16instantiate_declP9tree_nodeib

Wait a second. This looks a lot like a mangled C++ name. Sure enough:

nm -D -C /usr/lib/gcc/x86_64-linux-gnu/4.7.0/cc1plus | 
grep instantiate_decl
0000000000529c50 T instantiate_decl(tree_node*, int, bool)

I then ran nm without grep and saw that all the text symbols are mangled. Then it hit me: GCC is now built with a C++ compiler!

Seeing that the ODB plugin is written in C++, you may be wondering why did it still reference instantiate_decl() as a C function? Prior to 4.7, GCC headers that a plugin had to include weren’t C++-aware. As a result, I had to wrap them in the extern "C" block. Because GCC 4.7 can be built either in C or C++ mode, that extern "C" block is only necessary in the former case. Luckily, the config.h GCC plugin header defines the ENABLE_BUILD_WITH_CXX macro which we can use to decide how we should include the rest of the GCC headers:

#include <config.h>
extern "C"
} // extern "C"

There is also an interesting implication of this switch to the C++ mode for GCC plugin writers. In order to work with GCC 4.7, a plugin will have to be compiled with a C++ compiler even if it is written in C. Once the GCC developers actually start using C++ in the GCC source code, it won’t be possible to write a plugin in C anymore.

Who calls this function?

Wednesday, February 29th, 2012

Let’s say we have a large project and we want to find out from which places in our code a particular function is called. You may be wondering why would you want to know? The most common reason is to eliminate dead code; if the function is not called, then it may not be needed anymore. Or maybe you just want to refresh your memory about this area of your code base. The case that triggered this post involved changing one function call to another. I was adding support for composite object ids in ODB and was gradually changing the code to use a more generalized version of a certain function while still maintaining the old version for the code still to be ported. While I knew about most of the areas that needed changing, in the end I needed to verify that nobody was calling the old function and then remove it.

So how do we find out who calls a particular function? The method that I am sure most of you have used before is to comment the function out, recompile, and use the C++ compiler error messages to pin-point the calls. There are a few problems with this approach, however. First of all, depending on your build system, the compilation may stop before showing you all the call sites (make -k is helpful here but is still not a bulletproof solution). So to make sure that you have seen all the places, you may also have to keep commenting the calls and recompiling until you get no more errors. This is annoying.

This approach will also not work if a call can be resolved to one of the overloaded versions. This was exactly the situation I encountered. I had two functions that looked like this:

class traverser
  void traverse (type&);   // New version.
  void traverse (class_&); // Old version.

Where class_ derives from type so if I commented the old version out, the calls were happily resolved to the new version without giving any errors.

Another similar situation is when you have a function in the outer namespace that will be used if you comment a function in the inner namespace:

void f ();
namespace n
  void f ();
  void g ()
    // Will resolve to outer f() if inner f() is
    // commented out.
    f ();

What’s worse is that in complex cases involving implicit conversions of arguments, some calls may be successfully resolved to an overloaded or outer version while some will trigger an error. As a result, you may not even realize that you didn’t see all the call sites.

Ok, so that approach didn’t work in my case. What else can we try? Another option is to just comment the definition of the function out and see if we get any unresolved symbol errors during linking. There are many problems with this method as well. First of all, if the function in question is virtual, then this method won’t work because the virtual function table will always contain a reference to the function. Plus, all the calls to this function will go through the vtable.

If the function is not virtual, then, at best, a linker will tell you that there is an undefined reference in a specific function in a specific translation unit. For example, here is an output from the GNU Binutils ld:

/tmp/ccXez0jI.o: In function `main':
test.cxx:(.text+0×10): undefined reference to `f()'
test.cxx:(.text+0×15): undefined reference to `f()'

In particular, there will be no line information so if a function calls the function of interest multiple times, we will have no way of knowing which call triggered the undefined symbol.

This approach also won’t work if we are building a shared library (unless we are using the -no-undefined or equivalent option) because the undefined reference won’t be reported until we link the library to an executable or try to load it (e.g., with dlopen()). And when that happens all we will get is just a note that there is an undefined reference in a library: undefined reference to `f()'

In my case, since ODB is implemented as a shared library, all this method did was to confirm that I still had a call to the old version of the function. I, however, had no idea even which file(s) contained these calls.

As it happens, just the day before I was testing ODB with GCC in the C++11 mode. While everything worked fine, I got a few warnings about std::auto_ptr being deprecated. As I saw them scrolling by, I made an idle note to myself that when compiled in the C++11 mode libstdc++ probably marks auto_ptr using the GCC deprecated attribute. A day later this background note went off like a light bulb in my head: I can mark the old version of the function as deprecated and GCC will pin-point with a warning every single place where this function is called:

class traverser
  void traverse (type&);
  void traverse (class_&) __attribute__ ((deprecated));

And the diagnostics is:

model.cxx: In function ‘void object_columns::traverse(data_member&)’:
model.cxx:22:9: warning: ‘void traverser::traverse(class_&)’ is

This method is also very handy to find out which overloaded version was selected by the compiler without resolving to the runtime test:

void f (bool) __attribute__ ((deprecated));
void f (int) __attribute__ ((deprecated));
void f (double) __attribute__ ((deprecated));
void g ()
  f (true);
  f (123);
  f (123.1);

And the output is:

test.cxx:7:10: warning: ‘void f(bool)’ is deprecated
test.cxx:8:9: warning: ‘void f(int)’ is deprecated
test.cxx:9:11: warning: ‘void f(double)’ is deprecated

The obvious drawback of this method is that it relies on a GCC-specific extension, though some other compilers (Clang and probably Intel C++ for Linux) also support it. If you know of a similar functionality in other compilers and/or IDE’s, please mention it in the comments.