The order of elements in sets cannot be relied upon, because the hash
values are randomized by Python. Whenever sets are converted to lists
we need to keep their order stable, or random changes in the command line
cause ninja to rebuild a lot of files unnecessarily. To stabilize them,
use either sort or OrderedSet. Sorting is not always applicable, but it
can be faster because it's done in C and it can produce slightly nicer
output.
'if_true' sources should be built with their dependencies, as
illustrated by test case change.
Ideally, I think we would want only the files with the dependencies to
be built with the flags, but that would probably change the way
sourceset are used.
In QEMU a single set of source files is built against many different
configurations in order to generate many executable. Each executable
includes a different but overlapping subset of the source files; some
of the files are compiled separately for each output, others are
compiled just once.
Using Makefiles, this is achieved with a complicated mechanism involving
a combination of non-recursive and recursive make; Meson can do better,
but because there are hundreds of such conditional rules, it's important
to keep meson.build files brief and easy to follow. Therefore, this
commit adds a new module to satisfy this use case while preserving
Meson's declarative nature.
Configurations are mapped to a configuration_data object, and a new
"source set" object is used to store all the rules, and then retrieve
the desired set of sources together with their dependencies.
The test case shows how extract_objects can be used to satisfy both
cases, i.e. when the object files are shared across targets and when
they have to be separate. In the real-world case, a project would use
two source set objects for the two cases and then do
"executable(..., sources: ... , objects: ...)". The next commit
adds such an example.
Paolo Bonzini