How to improve Visual C++ compilation times?
I am compiling 2 C++ projects in a buildbot, on each commit. Both are around 1000 files, one is 100 kloc, the other 170 kloc. Compilation times are very different from gcc (4.4) to Visual C++ (2008).
Visual C++ compilations for one project take in the 20 minutes. They cannot take advantage of the multiple cores because a project depend on the other. In the end, a full compilation of both projects in Debug and Release, in 32 and 64 bits takes more than 2 1/2 hours.
gcc compilations for one project take in the 4 minutes. It can be parallelized on the 4 cores and takes around 1 min 10 secs. All 8 builds for 4 versions (Debug/Release, 32/64 bits) of the 2 projects are compiled in less than 10 minutes.
What is happening with Visual C++ compilation times? They are basically 5 times slower.
What is the average time that can be expected to compile a C++ kloc? Mine are 7 s/kloc with vc++ and 1.4 s/kloc with gcc.
Can anything be done to speed-up compilation times on Visual C++?
One thing that slows down the VC++ compiler is if you have a header file that initializes concrete instances of non-trival const value types. You may see this happen with constants of type std::string or GUIDs. It affects both compilation and link time.
For a single dll, this caused a 10x slowdown. It helps if you put them in a precompiled header file, or, just declare them in a header and initialize them in a cpp file.
Do take a look into the virus scanner, and be sure to experiment with precompiled headers, without it you won't see VC++ at its best.
Oh yeah, and make sure the %TMP% folder is on the same partition as where your build is written to, as VC++ makes temp files and moves them later.
The projects depending on each other doesn't imply that no parallelization is possible. The build systems are smart enough to figure out and avoid critical depenedancies, Otherwise gcc wouldn't be able to use 4 cores.
So (in addition to other steps), why not just try enabling multiprocessing in Visual Studio using /MP (See http://msdn.microsoft.com/en-us/library/bb385193.aspx).
It's not the direct answer for the question but at my company we are using IncrediBuild for distributed compilation. It really speeds up the compilation process. http://incredibuild.com/visual_studio.htm
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