[Cocci] Using or for structural #ifdefs

Julia Lawall julia.lawall at lip6.fr
Tue Jun 9 07:25:11 CEST 2015

> Doesn't the original developer have the responsibility over the order?
> So long as we proceed on the code in order and perform the operations
> asked atomically its not clear to me how this would be an issue.

In the case where you add new things, you control the order, and I think
there is actually no problem.  You just have to check that both things you
considered adding were actually added.  For example if you have one rule
that might add an initializer A at the end of a structure, and then another
rule that might add an initialier B in this position, then you now A is
before B.  If you now that both rules matched then you can just add an
#ifdef before A and an #endif after B.  Coccinelle doesn't even care if
they are added in an unbalanced way, ie one rule could add the #ifdef and
another could add the #endif.

Similarly for function definitions.

There problem comes if you eg just a replace an existing field, rather than
explicitly adding a new one at the end of the structure.  Or put the new
variant next to an existing one.  Or, in terms of functions, if you add a
definition after one function and before another one.  In that case,
Coccinelle doesn't make available any information about the relative
position of the new things.  You could assume that certain things were
always defined together in the original code, but it seems risky.

> > The problem for functions is that Coccinelle only sees one at a time.  It 
> > has no idea whether one function is before or after another, and whether 
> > they are contiguous or there is something between them.
> Understood, similarly -- it would seem to me fair for Coccinelle to trust
> the order already in place and just address disjunctions atomically as
> they are found top - down.
> I may have misunderstood the issue though.
> > Maybe this would be better done with some tool that is separate from 
> > Coccinelle.  There could be a danger of modifying #ifdefs that were in the 
> > original code.  If that would be a bad thing, then the tool could take the 
> > old and new code as inputs, and only eliminate ifdefs that are unique to 
> > the new code.
> A tool to optimize patches seems like a super useful tool. Grammatically
> however I also think it'd be useful to be able to use disjunctions on functions
> / structur declarations. The last item (c) on grouping metavariables would also
> be extremey useful as it would shorten the length of the rules and I think also
> enable more readible SmPL patches.

It would certainly be useful to have disjnctions on more kinds of things.
This is just a development time/priority issue.

I'm not sure to understand the grouping metavariable idea.  People who 
like regular expressions would just write a regular expression for what 
you propose.  Since I don't particularly like regular expressions, and 
since I know that Coccinelle doesn't use them for indexing, I would rather 

\(get_dma \| map_dma \| unmap_dma \| attach_dma \| detach_dma\)

That may not work at the moment in the context in which it is wanted.  But 
that is just another or problem, not a metavariable problem.


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