[LLVMdev] getAnalysisIfAvailable<>(...)

[Garrison Venn]

Is it consistent to have a Pass instance's run method's implementation use getAnalysisIfAvailable<AnalysisType>() (vs just using getAnalysis< AnalysisType >) when that same instance's getAnalysisUsage(AnalysisUsage &au) implementation invokes au.addRequired<AnalysisType>()?

For example in the implementation of SelectionDAGISel::runOnMachineFunction(...) (called from MachineFunctionPass::runOnFunction(...)): 

DwarfWriter *DW = getAnalysisIfAvailable<DwarfWriter>();

is used vs:

DwarfWriter &DW = getAnalysis<DwarfWriter>();

even though SelectionDAGISel::getAnalysisUsage(AnalysisUsage &AU) sets:

AU.addRequired<DwarfWriter>();

Since this occurs in 2.6 and trunk, and this pattern is repeated with other analysis passes, I'm wondering if I'm not understanding something. 

Thanks in advance 

Garrison