[cmake-developers] cmake build does too much work

[Bill Hoffman]

On 12/11/2013 5:13 PM, Matthew Woehlke wrote:
> I've been working on a project lately that isn't *that* massively large,
> but has an unusually high number of library and executable targets. One
> thing that's been bugging me is that any trivial change in a "lower
> level" library causes more than a hundred targets to be relinked, for no
> good reason I know of.
>
> Now, I *do* get that relinking is good if the library ABI changes.
> However, that's not the case here, and I am wondering if it would be
> possible for CMake to generate an additional, intermediary step after
> library linking to somehow export a file representing the ABI of the
> library (with overwrite checks to not modify the file if the ABI has not
> changed), and to use *those*, rather than the actual libraries, as the
> dependencies for targets linking to the libraries. I think this could
> produce a significant speed-up for incremental builds in some cases, as
> it would allow the build to short-circuit the relink of many targets
> when it turns out a library's ABI has not changed.
>
> Does this sound like something CMake could/should do?
>
> (I'm thinking something like running objdump on the resulting library
> with suitable arguments and doing a copy_if_different on the output. I
> guess this would only apply to shared libraries, and probably should be
> an optional feature.)

There is this property:
http://www.cmake.org/cmake/help/git-master/variable/CMAKE_LINK_DEPENDS_NO_SHARED.html

If you wanted to use that for your project I think you can just run 
cmake -DCMAKE_LINK_DEPENDS_NO_SHARED=TRUE.

The other option sounds interesting but hard to implement in a cross 
platform manner.

-Bill