[cmake-developers] Using CMake to bootstrap clang builtins

[Chris Bieneman]

> On Nov 2, 2015, at 11:52 AM, Brad King <brad.king at kitware.com> wrote:
> 
> On 10/30/2015 03:05 PM, Chris Bieneman wrote:
>> equivalent of try_compile that will test building a static archive
>> instead of an executable?
> 
> For the "source file" signature of try_compile there is currently
> no such option.  The try_compile command also has a mode that builds
> an entire source tree and of course then one could specify any type
> of target because one writes the entire CMakeLists.txt file explicitly.
> That doesn't help for the builtin checks and such though.
> 
> For the basic try_compile signature we hard-code add_executable here:
> 
> https://cmake.org/gitweb?p=cmake.git;a=blob;f=Source/cmCoreTryCompile.cxx;hb=v3.3.2#l477
> 
> We could consider an option to try_compile to ask it to use
> add_library(... STATIC ...) instead of add_executable.  However,
> for your use case one needs this to work for the existing check
> modules that CMake has, not just for your own try_compile calls.
> 
> IIUC in your case you have an already-built cross-compiler and are
> running CMake on the project that provides the builtins.  This
> project uses some of CMake's compiler/platform check macros.  We
> need to make these checks work without running a linker because the
> cross-compiler cannot link until the builtins are built.

Yes, this is exactly what is going on.

> 
> Are you setting CMAKE_TOOLCHAIN_FILE for the build of the builtins?

Yes.

> 
> What we need is for try_compile to learn to check for some global
> option to be set by a toolchain or platform information module that
> tells it there is no linker.  Then try_compile could handle this
> case by using add_library(... STATIC …).

That would be exactly what we need.

-Chris

> 
> -Brad
> 
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