[LLVMdev] AnalysisUsage: addRequired vs. addRequiredTransitive

[Owen Anderson]

On Mar 31, 2010, at 2:53 PM, Trevor Harmon wrote:
> 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:
> 

Consider passes like AliasAnalysis or MemoryDependenceAnalysis.  These passes has two phases of existence: when their runOnFunction is called, and when later passes (which requested them via their own AnalysisUsage's) call methods on them to query that information.

Some analyses, like Andersen's AA, do all their computation in their runOnFunction().  Therefore, anything they depended on can be destroyed after the runOnFunction() returns.  Others, like MemoryDependenceAnalysis, are "lazy."  MDA specifically does NOT compute results in its runOnFunction(), instead computing results on-demand when a user queries it.  Because MDA depends on AA, we must ensure that, as long as MDA is alive and responding to queries, AA is alive as well.

That is what addRequiredTransitive does.

--Owen