Re: [Boost-build] Another funny Boost Build behaviour.

From: Jurko Gospodnetić (jurko.gospodnetic_at_[hidden])
Date: 2008-09-03 15:46:36

   Hi Volodya.

> I know what's going on. For performance reasons, Boost.Build caches the result
> of attempt of converting targets of type X to targets of type Y.
> 
> So, when my-lib is built, we compute the list of all source types that eventually
> can become OBJ. Later one, when other-lib is built we get this:
> 
>              *** construct OBJ
>                  from { source.extension.MY_TYPE }
>                  properties: <asynch-exceptions>off <debug-symbols>on <doxygen.doxproc.index>no <doxygen.processor>xsltproc <exception-handling>on <extern-c-nothrow>off <format>html <
>              ***  4  viable generators
>                ** generator 'rc.compile.resource' pruned
>                ** generator 'gcc.compile.c++' pruned
>                ** generator 'gcc.compile.c' pruned
>                ** generator 'gcc.compile.asm' pruned
> 
> This 'pruned' means 'no, there's no way this generator can do anything with this type'. The bug is that
> first we compute this cached knowledge, and when a new generator is added, via import in other/Jamfile,
> we don't update our cache.
> 
> In code, generators.variable-source-types-for-generator returns the set of source type can
> a generator can consume, after some chain of transformation. As you see, the set is computed when
> we first encounter a generator, and is never recomputed. The most easiest way to fix
> this would be to clean entire cache of viable source type whenever a new generator is defined.
> Smarter approaches are possible, but they are harder.

   Could we refactor this so that the generator selection is done only 
after all the user Jamfiles have been processed?

   I have not looked at the code yet so I do not have a feeling of how 
difficult this would be but I guess that would be the most natural 
implementation as the whole Boost Build syntax is quite declarative. 
I.e. first we define what we want to happen, and only then does the 
actual build process commence, not requiring any further interaction.

   Best regards,
     Jurko Gospodnetić