[CMake] How to have generated source compiled multiple ways?

[Eric Noulard]

2013/8/12 Paul Smith <paul at mad-scientist.net>:
> Hi all.
>
> I've got a situation where I'm creating generated source files (from
> bison/flex, actually, but I'm writing my own rules for it since I need
> specialized support for C++ output, etc.)
>
> However I need to compile the generated output into two different libraries,
> building two different ways (with different #defines/compiler flags).
>
> So I have tried using add_custom_command() with OUTPUT, but the
> documentation says:
>
> "Do not list the output in more than one independent target that may build
> in parallel or the two instances of the rule may conflict (instead use
> add_custom_target to drive the command and make the other targets depend on
> that one)."
>
>
> Sure enough, it seems that if I have my different libraries depend on these
> outputs I get problems during parallel builds.  So then I tried to use
> add_custom_target(), but that says:
>
> "The target has no output file and is ALWAYS CONSIDERED OUT OF DATE"
>
>
> and, sure enough, if I do it this way my source files (and everything that
> depends on them) rebuild every time I run the build, even if nothing has
> changed.  This is a big bummer.

I did never tried befoire but think you should "simply" keep your
current add_custom_command
and create a new target with add_custom_target whose only purpose is
to "serialize"
the dependency for the 2 (or more) independent libraries which use the
output of your custom command
concurrently.

Something like:
cmake_minimum_required(VERSION 2.8.10)
project(ParallelAddCustomCommand C)

add_custom_command(OUTPUT  ${CMAKE_CURRENT_BINARY_DIR}/generated.c
  COMMAND ${CMAKE_COMMAND} -E copy
${CMAKE_CURRENT_SOURCE_DIR}/template.c
${CMAKE_CURRENT_BINARY_DIR}/generated.c)

add_custom_target(SerializeTarget
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/generated.c)

add_library(MyLib1 ${CMAKE_CURRENT_BINARY_DIR}/generated.c lib1.c)
set_target_properties(MyLib1 PROPERTIES COMPILE_FLAGS -DDEFGEN=3)
add_dependencies(MyLib1 SerializeTarget)

add_library(MyLib2 ${CMAKE_CURRENT_BINARY_DIR}/generated.c lib2.c)
set_target_properties(MyLib2 PROPERTIES COMPILE_FLAGS -DDEFGEN=4)
add_dependencies(MyLib2 SerializeTarget)

> How can I have generated source files compiled different ways into different
> libraries, but not have them (and anything that depends on them) rebuilt
> every time?
>
>
> The only way I've thought of so far is to have two different
> add_custom_command() options with the same input but different output (in
> different directories maybe), one for each target.  This means I need to
> generate the files twice even though the output source files will have
> identical content.  But as long as I don't have to REBUILD them every time
> maybe that's not so bad.  I haven't tried this yet though.  Are there other
> options?

I tried to craft some working example which is attached to this mail.

-- 
Erk
L'élection n'est pas la démocratie -- http://www.le-message.org
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