John Carter <john.carter@xxxxxxxxxx> writes: > Precompiled headers sound really tasty..., I've read the docs three > times but I'm not sure I've quite digested it enough to do a large > implementation. So I'm looking for some practical / standard practice > guidance here. Precompiled headers are a good idea on paper but note that in some actual applications they turn out not to be faster, based on doing actual timings. > I'm always trying to trim a few seconds off the 2 hour compile time to > it takes to compile & link all variants... The most effective technique I know is to get a bunch of machines and turn them into a distcc cluster. http://distcc.samba.org/. This gives a near linear speedup in compilation time, though it doesn't help with link time (PCH doesn't help with link time either, of course). > The docs say... "If the precompiled header file can't > be used, it is ignored." > > Hmm. What does that mean? Does it mean... It means that if the precompiled header was generated by a different version of the compiler, it won't be used. Precompiled header files are compiler version specific. > This rule also takes some digesting... > * Only one precompiled header can be used in a particular > compilation. > > * A precompiled header can't be used once the first C token is seen. > You can have preprocessor directives before a precompiled header; > you can even include a precompiled header from inside another > header, so long as there are no C tokens before the `#include'. > > Do you go around making sure that the (recursively) fattest .h is at the start? > Or > just hope that some speed up is better than nothing? You create a single .h file which includes all your other .h files. You precompile that file. You use the -include option to automatically include it first in any compilation. Ian
