[CMake] dependencies of cross compiliations

[Craig Scott]

On Tue, Dec 4, 2018 at 6:56 PM Torsten Robitzki <Torsten at robitzki.de> wrote:

> > Am 27.11.2018 um 19:55 schrieb Eric Noulard <eric.noulard at gmail.com>:
> >
> > However from my point of view and my cross-compiling experience when you
> cross-compile you have:
> >
> > 1) the host compiler which is used to compile "host tools"
> > 2) the target compiler (may be several of them) to "cross-compile"
> >
> > My assumption are:
> >  a) when you cross-compile your build is a "whole" and you shouldn't
> have to setup some superbuild
> >    structure for building host tools ht_exe and another for target1 tool
> t1t_exe and another one for target2 tool t2t_exe.
> >
> >  b) what you want is to build:
> >      ht_exe for the host
> >      possibly use ht_exe during the build to generate some [source] file
> >      t1t_exe for the [cross]target1
> >      t2t_exe for the [cross]target2
> >
> >  c)  you seldomly compile the same source for the host AND the target,
> but it may happen.
>
> In case, you are doing unit tests, it’s normal to have the same code
> running in a test on the host platform and in the final binary on the
> target.
>
> I think, having more than 1 target platform becomes more and more normal
> as it becomes more usual to have multiple microcontrollers in a project.
>
> Previously, I have encoded this in the build type. So instead of just
> having Debug and Release, I had HOST_Debug, HOST_Release NRF51_Debug,
> NRF51_Release, STM8_Debug, STM8_Release and so on. It doesn’t annoy me very
> much, that I have to run CMake 3 times to get all the binaries for a
> release build. The problem that I have, are dependencies between this
> builds. If I write a tool that (for example) generates source files for one
> of the target platforms, the build for the host platform must run before
> the build for that target platform. And when I make changes to that tool, I
> want the build to regenerate the generated source files.
>
> Keeping track of this dependencies to solve this kind of ordering issues
> and to allow minimum rebuilds, is the main purpose of any build system. To
> solve this with CMake, I think we need a way to define the dependencies
> between build types (in the example above, from the generator from the host
> build to the generated source file in one of the target builds) and CMake
> needs to know the build directory for all build types (not only the
> current).
>

Perhaps a superbuild would be the cleanest approach here? The host tools
would be one subproject and the cross-compile builds would depend on the
host tools' build. You could then choose to build everything via the top
level superbuild or just work on one of the subprojects if that's all you
needed once the initial tools build had been done. You could even set up as
many different sub-projects for the different architectures as needed.
Packaging would require a little more work, but it shouldn't be
prohibitively so.

Another alternative is the approach described in this stackoverflow article
<https://stackoverflow.com/questions/36084785/building-a-tool-immediately-so-it-can-be-used-later-in-same-cmake-run>
which performs the host tools build off to the side in a secondary build
during configure. This works well when the host tools don't change much (we
use it extensively at work with very large, complex hierarchical projects).
It wouldn't help though if you need to build more than one cross-compiled
architecture.



> > The wish-season is coming up, so that's sort of what I would like to
> > have. Now it's your turn. No bikeshedding please, only deliveries ;)
>
> How about ``add_dependencies()`` allowing me to define dependencies
> between different build types? :-)
>

A superbuild would already give you the equivalent capability.


-- 
Craig Scott
Melbourne, Australia
https://crascit.com

New book released: Professional CMake: A Practical Guide
<https://crascit.com/professional-cmake/>
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