[LLVMdev] llvm passes

[Chris Lattner]

On Fri, 22 Sep 2006, Ryan M. Lefever wrote:
> In the instructions for writing LLVM passes, it says "[c]urrently 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."

Right.

> I'm a little confused by that.  It was my understanding that if pass X
> required pass Y, then pass Y would have to fully run and build up its
> internal data structures before pass X ran.  Then, when pass X was run,
> X would call getAnalysis<Y> and get a reference to the pass Y object,
> which it could query for information stored during pass Y's execution.
> So, I'm wondering why the types of the passes matter.  The statement
> that there would be "nowhere to store information for all of the
> functions in the program," confuses me.

Imagine you have a functionpass X, for example DominatorSet.  DominatorSet 
runs on a function at a time and captures information about the current 
function's dominator information in its instance variables.

Now you have ModulePass Y that runs and requires X to be run.  Because it 
is a functionpass, X will be run on every function in the module before Y 
starts.  Further, because X only has one set of instance variables, Y 
could see, at most, information for one function.

-Chris

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