[LLVMdev] AnalysisUsage: addRequired vs. addRequiredTransitive

[Trevor Harmon]

Hi,

I'm a bit confused about the distinction between addRequired and  
addRequiredTransitive in AnalysisUsage. From PassAnalysisSupport.h:

"REQUIRES (must be available when the pass runs), REQUIRES TRANSITIVE  
(must be available throughout the lifetime of the pass)."

The part that's confusing me is "must be available when the pass  
runs". I don't see how that's any different from "must be available  
throughout the lifetime of the pass". For example, if a pass is  
"available" when my pass runs, then I should be able to use it when my  
pass is running, correct? For instance:

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

bool MyPass::runOnFunction(Function &function) {
    UnifyFunctionExitNodes &pass =  
getAnalysis<UnifyFunctionExitNodes>();
    // ... use the pass ...
}

But if the above is okay, then why would addRequiredTransitive be  
necessary?

I suspect that the REQUIRES definition is simply not worded well.  
Perhaps it should say, "(must have run before the pass runs)". Any  
thoughts?

Trevor