[cmake-developers] Idea for Multi-Toolchain Support

[frodak17]

On Sat, Dec 22, 2018 at 10:58 AM Torsten Robitzki <Torsten at robitzki.de>
wrote:

> Hi,
>
> > Am 17.12.2018 um 21:18 schrieb Kyle Edwards via cmake-developers <
> cmake-developers at cmake.org>:
> >
> > Then, executables and libraries could have a toolchain specified:
> >
> > add_executable(BuildUtility TOOLCHAIN DEFAULT ...)
> > add_library(MyLibrary TOOLCHAIN CrossToolchain …)
>
> or even toolchains:
>
>     add_library(feature1 TOOLCHAINS CrossToolChain HostToolChain)
>
>     add_executable(Firmware TOOLCHAIN CrossToolChain)
>     target_link_libraries(Firmware feature1)
>
>     add_executable(Test_Feature1 TOOLCHAIN HostToolChain)
>     target_link_libraries(Test_Feature1 feature1)
>
> A nice feature would be to let toolchain propagate along the dependency
> graph defined by `target_link_libraries`. So in the example above, the
> TOOLCHAINS option to `add_library` would be optional, because the target
> Firmware requires feature1 to be build with CrossToolChain and
> Test_Feature1 to be build with HostToolChain. Which requires the build to
> have different binary directories for every toolchain. (Which for me is an
> indication that having multidimensional build types would be the more
> natural way to go)
>
>
How would different binary directories for every toolchain work?
CMAKE_CURRENT_BINARY_DIR wouldn't even be accurate let alone all of the
code that tracks these things.  Using target_compile_options() and file
level compile options would have to use generator expressions to pass the
correct flags to the correct toolchain.  If TOOLCHAINS is specified and the
CMakeLists.txt file only contains the library target then nothing is built
because there is nothing linking against it?

Offhand it seems to me that adding multiple binary directories based on
which toolchain was used is a larger change to the infrastructure then
adapting a generator to use multiple toolchains in its output.


> What I’m still missing is support to define dependencies between different
> targets (build by different toolchains), so that I can describe that I
> require a tool that is build with the HostToolChain, to generate a file
> that is required by the part of the build, that is using the
> CrossToolChain. And, of cause, if the source of that tool changes, that
> tool have to be rebuild and the generated file from the CrossToolChain part
> of the build have to be rebuild.
>
>
All the commands already support this.  add_executable() rebuilds the host
tool if the source file change for the tool is updated.
add_custom_command() runs whenever any target specified in DEPENDS is
rebuilt.  So in this case you specify the custom command to run the host
tool and generate the output and set DEPENDS to host tool target.  If the
host tool is updated due to a source file change then the tool is rebuilt
and the file is regenerated.  The cross toolchain target that uses the
generated file rebuilds because the timestamp on generated file was
updated.
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