[CMake] Secret precompiled header support?

[Bill Hoffman]

On 5/18/2012 12:51 PM, Robert Dailey wrote:

>
> I certainly agree with all of your points but consider this. Even though
> PCH support may not exist on every compiler, every company that wants to
> add PCH support to CMake through scripting logic is essentially doing
> the same work over and over again. Everybody is going to do the same things:
>
>  1. Restrict the functionality to MSVC only via conditional branching
>  2. Call the same CMake commands to add the same compiler flag with the
>     same parameters
>
> Just because every compiler doesn't support it doesn't mean CMake can't
> provide some way to eliminate the boilerplate. If we want to use PCH
> through CMake, there is certain logic that must exist. It's really just
> a question of WHERE that should exist: In CMake C++ code or in CMake as
> a script.
>
> What do we do if PCH support isn't provided by a compiler, but we have
> enabled it in our scripts? There are several things you can do, but what
> I commonly do is just ignore the request. For example, for all platforms
> I may invoke my custom function to enable PCH, but on the platforms that
> don't support it, it's just going to make sure that those files get
> compiled. Even if you don't enable PCH or can't use it, the PCH header
> file is still included in all CPP files that need includes from it, so
> the code will still compile. In this specific case, ignoring the request
> still works, and the user doesn't have to worry about the lack of the
> feature resulting in the build failing.
>
> You know all of this but the main point I wanted to make is that even
> though CMake can't guarantee PCH support on all platforms, it's still
> adding value by eliminating boilerplate. People won't be redundantly
> implementing and re-testing the same complex logic to add the feature in
> a modular way. Maybe a good middle ground would be to have a CMake built
> in module that provides "convenience" functions for operations like
> this. That way, CMake C++ code isn't changing, but we still give the
> users a subset of functionality to work with.
>
> Just a thought!


I agree, I hate to see code duplication. We do have a test to make sure 
it is possible that is run each night:

Tests/PrecompiledHeader


I have never used it, so I am not sure how hard it is.  I am thinking 
maybe the thing to do is create a module for each compiler that we add 
support for PCH.   Something like Modules/MSVCPCH.cmake.  Since it is so 
compiler specific.  Then the implementation would be in cmake code in a 
module and it will be clear that it is for one compiler. Perhaps you 
could generalize what you have already done into a module like this that 
could be included in CMake?

-Bill