[CMake] EXEC_PROGRAM and inheritance

[Brad King]

Brian Hone wrote:
> Hi folks,
> 
> I see a lot of posts which seem to indicate that there is popular 
> support for allowing inheritance of the EXEC_PROGRAM command to 
> subdirectories.
> 
> Looking at the code, commands which allow inheritance simply add the 
> following to the .h file:
> 
> virtual bool IsInherited <cid:part1.07070002.03030705 at baesystems.com>() 
> {return true;}
> 
> 
> Am I missing something?  Would this cause the command to be re-executed 
> from sub-directories, or the value to be cached and saved?  It's a real 
> devil to save the value from an external command and pass it around, 
> which sort of defeats the purpose of an external command.

Yes, it is easy to make the command inherited but there are also cases 
where it should not be inherited.  When a command is inherited it gets 
executed for its own directory and again for every subdirectory.  This 
is desirable for commands like SET so that the state of variables is 
preserved in subdirectories, but it is not necessarily desirable for 
expensive commands like CONFIGURE_FILE and EXEC_PROGRAM.

This all comes from a previous methodology left from ancient CMake 
versions in which CMakeLists.txt code was not supposed to be a language. 
  We are planning to eventually change the evaluation order to allow 
things like processing subdirectories before their parents.  We have 
chosen to delay this EXEC_PROGRAM issue until then, though no schedule 
has been set.

-Brad