Re: Precompiled headers and templates

[Ian Lance Taylor <iant at google dot com>]

Yang Zhang <yanghatespam@gmail.com> writes:

> Have people found precompiled headers to make significant difference
> with template-heavy headers, esp. things like Boostified projects?
> I've been trying to look up details on what the pre-compilation
> entails, but I still don't have a good handle on this.  I'm mainly
> wondering whether it speeds up anything for templates that are
> instantiated outside of the header (i.e. at the use site).  I'm
> guessing not, since template instantiation can induce arbitrary
> computation.  If I'm right, then do most folks try to include explicit
> instantiations in their precompiled headers?

Producing a pre-compiled header is actually quite simple.

http://gcc.gnu.org/onlinedocs/gcc-4.3.3/gcc/Precompiled-Headers.html#Precompiled-Headers

However, including explicit instantiations in the precompiled header
won't make any significant difference to compilation time.  The
precompiled header saves on parsing and name lookup time, it doesn't
save on code generation time.

I don't personally find that precompiled headers help with compilation
time all that much.  However, for some projects, ones with millions of
lines of header files included in every compilation, I expect that they
would help.

Ian