[LLVMdev] ModulePasses requiring FunctionPasses

Patrick Meredith pmeredit at uiuc.edu
Mon May 1 12:48:24 PDT 2006

On May 1, 2006, at 1:00 PM, Balpreet Pankaj wrote:

> I am trying to write a ModulePass which requires PostDominator sets  
> for every function in the module. Now finding post dominators is  a  
> function pass. The link on the llvm.org website says that :
> "Currently it is illegal for a ModulePass to require a  
> FunctionPass. This is because there is only one instance of the  
> FunctionPass object ever created, thus nowhere to store information  
> for all of the functions in the program at the same time. Although  
> this has come up a couple of times before, this has always been  
> worked around by factoring one big complicated pass into a global  
> and an interprocedural part, both of which are distinct. In the  
> future, it would be nice to have this though."
> This says that we can work around by factoring the code into a  
> global and interprocedural part. If so, is our global pass a Module  
> pass ? And then, how do the global and inter-procedural passes  
> interact ? Is there a specific example where this has been done  
> before ?

Ahhh, you must remember the terribly confusing terminology.  Global  
means function level.  This is sort of like doing an iterative data  
flow where
you have a part that computes local basic block analyses, and then  
the global part merges those analyses at exits and entries to  
blocks.   Now
you are going to do something on a function level, store it to data  
structures of some sort and use a ModulePass to merge at calls.  The  
problem I see is this is still an issue of a Module Pass Requiring a  
FunctionPass, so you are going to have to declare some sort of structure
like a std::vector of whatever analysis structure you need, and "push  
a new one on" in the beginning of run on Function.  A hash_map
from Function* to analysis information might be better however.   
Actually, if you want to get this working really fast it might be better
to just edit PostDominator set to have a map of PostDominatorSets,  
and make it a ModulePass.

> Thanks,
> -Balpreet
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