Question

Short story:

I can’t make precompiled headers work properly with gcc -c option.

Long story:

Folks, I’m using gcc-4.4.1 on Linux and before trying precompiled headers in a really large project I decided to test them on simple program. They “kinda work” but I’m not happy with results and I’m sure there is something wrong about my setup.

First of all, I wrote a simple program(main.cpp) to test if they work at all:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

int main()
{
  return 0;
}

Then I created the precompiled headers file pre.h(in the same directory) as follows:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

…and compiled it:

$ g++ -I. pre.h

(pre.h.gch was created)

After that I measured compile time with and without precompiled headers:

with pch

$ time g++ -I. -include pre.h main.cpp

real    0m0.128s
user    0m0.088s
sys  0m0.048s

without pch

$ time g++ -I. main.cpp 

real    0m0.838s
user    0m0.784s
sys  0m0.056s

So far so good! Almost 7 times faster, that’s impressive! Now let’s try something more realistic. All my sources are built with -c option and for some reason I can’t make pch play nicely with it. You can reproduce this with the following steps below…

I created the test module foo.cpp as follows:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

int whatever()
{
  return 0;
}

Here are the timings of my attempts to build the module foo.cpp with and without pch:

with pch

$ time g++ -I. -include pre.h -c foo.cpp 

real    0m0.357s
user    0m0.348s
sys 0m0.012s

without pch

$ time g++ -I. -c foo.cpp 

real    0m0.330s
user    0m0.292s
sys 0m0.044s

That’s quite strange, looks like there is no speed up at all!(I ran timings for several times). It turned out precompiled headers were not used at all in this case, I checked it with -H option(output of “g++ -I. -include pre.h -c foo.cpp -H” didn’t list pre.h.gch at all).

What am I doing wrong?

Answer 1

Ok, I think I’ve found the solution: -fpch-preprocess should be used alongside with -c option. It works like a charm!

Here’s the timings:

with pch

$ time g++ -I. -include pre.h -c foo.cpp -fpch-preprocess

real    0m0.028s
user    0m0.016s
sys 0m0.016s

without pch

$ time g++ -I. -c foo.cpp 

real    0m0.330s
user    0m0.292s
sys 0m0.044s

Update: I asked the same question on the gcc help mailing list and Ian Lance Taylor explained this strange behavior by my usage of distcc/ccache. These tools first preprocess the source that’s why this options is required.