No speedup with precompiled headers on gcc (but large speedup with visual studio)

Question

I'm working on a large project that must builds under multiple environment, chiefly linux/gcc and windows/msvc. To speed up the build, we use precompiled headers.

The Windows implementation is very efficient: on my quad-core hyperthreaded i7 build time goes down from 9 minutes to 1.5 minutes. However using precompiled headers doesn't seem to improve performance: in both cases it builds in 22 minutes under a virtual box on the same computer, or about 40 minutes on a real server.

So I'm thinking the obvious, that I somehow got something wrong and that the precompiled header isn't kicking in. I can't find what however.

Our Makefiles are generated by CMake, so I can copy paste the code used to compile the header and the object files that uses them.

Creating the header:

/usr/bin/c++ -O3 -DNDEBUG --no-warnings "-I/mnt/code/server a/src/game"
"-I/mnt/code/server a/src/game/vmap" "-I/mnt/code/server a/dep/include/g3dlite"
"-I/mnt/code/server a/dep/include" "-I/mnt/code/server a/src/shared"
"-I/mnt/code/server a/src/framework" "-I/mnt/code/server a/buildlinux"
"-I/mnt/code/server a/buildlinux/src/shared" -I/usr/include/mysql
"-I/mnt/code/server a/dep/acelite" -DDO_MYSQL -DHAVE_CONFIG_H
-DVERSION=\"0.6.1\" -DSYSCONFDIR=\"../etc/\" -D_RELEASE -D_NDEBUG -x c++-header
-o "/mnt/code/server a/buildlinux/src/game/pchdef.h.gch" "/mnt/code/server
a/src/game/pchdef.h"

Compiling an object file:

/usr/bin/c++ $(CXX_DEFINES) $(CXX_FLAGS) "-I/mnt/code/server
a/buildlinux/src/game" -include pchdef.h -Winvalid-pch -o
CMakeFiles/game.dir/AccountMgr.cpp.o -c "/mnt/code/server
a/src/game/AccountMgr.cpp"

Insights are appreciated, even if they don't directly derive from the snippets above.

Answer 1

There are a couple of things that you need to pay attention to when using precompiled headers in GCC. First of all, the precompiled header must be created with the same arguments as the cpp files are being compiled. Also I assume you have actually included the precompiled header in AccountMgr.cpp?

Try to compile with the -H flag, this will output which include files are being considered. Check that the pchdef-file is mentioned, and see what other include files are being parsed. To have gcc complain about invalid PCH files, consider using -Winvalid-pch.