llvm dev - Jul 2010 - [LLVMdev] Controlling the order of a FunctionPass

Hi,

I would like my FunctionPasses to be invoked in reverse call graph  
order (callees before callers). However, "Writing an LLVM Pass" notes
that "FunctionPass's do not require that they are executed in a  
particular order." Is there any way at all to specify an ordering?

If this is not possible, I'm thinking of a workaround in which my  
runOnFunction method does not actually process the function but  
instead copies the Function object to a "work set". When runOnFunction
has been invoked for all functions, I would then perform the analysis  
on each saved function in the order that I desire. Does this sound  
like the right approach? (Note that my pass is analysis-only -- it  
does not modify the functions.)

Thanks,

Trevor

Trevor Harmon wrote:
> Hi,
>
> I would like my FunctionPasses to be invoked in reverse call graph  
> order (callees before callers). However, "Writing an LLVM Pass"
notes
> that "FunctionPass's do not require that they are executed in a  
> particular order." Is there any way at all to specify an ordering?
>
> If this is not possible, I'm thinking of a workaround in which my  
> runOnFunction method does not actually process the function but  
> instead copies the Function object to a "work set". When
runOnFunction
> has been invoked for all functions, I would then perform the analysis  
> on each saved function in the order that I desire. Does this sound  
> like the right approach? (Note that my pass is analysis-only -- it  
> does not modify the functions.)
>   
If you write your pass as a ModulePass, then you can iterate over the 
functions in any order that you want.

Unless writing your pass as a CallGraphSCCPass gets you the right 
iteration order, writing your pass as a ModulePass looks like the proper 
way to do this.  FunctionPass'es are supposed to do only local analysis 
and are required to be completely independent; a FunctionPass should not 
record information between invocations of its runOnFunction() method.

-- John T.
> Thanks,
>
> Trevor
>
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On Jul 22, 2010, at 2:05 PM, John Criswell wrote:
> If you write your pass as a ModulePass, then you can iterate over the
> functions in any order that you want.
I had considered that, but my FunctionPass depends on other passes  
processing the functions first:

void MyPass::getAnalysisUsage(AnalysisUsage &AU) const {
     AU.addRequired<UnifyFunctionExitNodes>();
     AU.addRequired<LoopInfo>();
     AU.addPreserved<LoopInfo>();
}

bool MyPass::runOnFunction(Function &function) {
     LoopInfo &loopInfo = getAnalysis<LoopInfo>();
     ...
}

I don't know how to convert the above code into its equivalent form  
for a ModulePass.

Trevor