llvm dev - Aug 2004 - [LLVMdev] How to get LoopInfo within Pass subclass?

Hi, I have a hopefully quick question. I'm writing a Pass that needs to 
see a whole module at a time and keep some state, so I subclassed Pass. 
However, I want to be able to see the Loops in each Function. Roughly, 
here's what I want:

virtual bool run(Module &M){
     // do stuff...
     for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I){
	    if(! I->isExternal()) visitFunction(*I);
     }
	// ...
       return false;
}

virtual void visitFunction(Function &F){
     LoopInfo *LI = &getAnalysis<LoopInfo>();
     std::vector<Loop*> SubLoops(LI->begin(), LI->end());
     for(unsigned i = 0, e = SubLoops.size(); i != e; ++i){
	   // visit SubLoops[i];
     }
}

So, I add LoopInfo as a required analysis:

virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesAll();
       AU.addRequired<LoopInfo>();
};

However, when running I'm informed that:

PassManagerT.h:421: failed assertion `getAnalysisOrNullUp(P) && 
dynamic_cast<ImmutablePass*>(getAnalysisOrNullUp(P)) && "Pass
available
but not found! " "Perhaps this is a module pass requiring a function 
pass?"'

I couldn't find a clear explanation of which passes can require which 
other ones, and I'm a little confused as to whether what I want is even 
possible. How can I say that I want LoopInfo for each Function in my 
Module?

PS, if I'm not just missing it, and this is a gap in the docs, I'd be 
glad to try writing something to fill the gap. It'd probably be a good 
way to learn more about LLVM.

Thanks,
-mike
--
	Michael O. McCracken
	UCSD CSE Ph.D. Student
	mike at cs.ucsd.edu

On Thu, 5 Aug 2004, Michael McCracken wrote:
> Hi, I have a hopefully quick question. I'm writing a Pass that needs to
> see a whole module at a time and keep some state, so I subclassed Pass.
> However, I want to be able to see the Loops in each Function. Roughly,
ok.
> However, when running I'm informed that:
>
> PassManagerT.h:421: failed assertion `getAnalysisOrNullUp(P) &&
> dynamic_cast<ImmutablePass*>(getAnalysisOrNullUp(P)) &&
"Pass available
> but not found! " "Perhaps this is a module pass requiring a
function
> pass?"'
>
> I couldn't find a clear explanation of which passes can require which
> other ones, and I'm a little confused as to whether what I want is even
> possible. How can I say that I want LoopInfo for each Function in my
> Module?
Unfortunately this is a big missing feature in the pass management system.
In particular, as the assertion says, a "Pass" cannot yet require a
"FunctionPass".  The reason this is tricky is that each (e.g.)
LoopInfo
instance contains loop information for one function.  When we allow Pass
objects to require FunctionPass objects, the PassManager will have to
instantiate one FunctionPass for each function in the module.

This is clearly something that we want to do, but unfortunately we can't
do it yet.  This is actually mentioned here, though I'm not suprised you
missed it: :)
http://llvm.org/docs/WritingAnLLVMPass.html#PassFunctionPass

As a work-around you can make your "Pass" object work as a
FunctionPass.
I don't know what this will do to the logic in the pass, but if this is an
option, it would be the best way to go.

You're right that FunctionPass's are not supposed to have state
(something
that many people overlook :) ), however, for now, nothing will break if it
does have state, and this is really the only way around this.  However, if
you want to work on the PassManager, please let us know! :)

-Chris

-- 
http://llvm.cs.uiuc.edu/
http://nondot.org/sabre/

On Aug 5, 2004, at 5:30 PM, Chris Lattner wrote:
> On Thu, 5 Aug 2004, Michael McCracken wrote:
>
>> Hi, I have a hopefully quick question. I'm writing a Pass that
needs
>> to
>> see a whole module at a time and keep some state, so I subclassed 
>> Pass.
>> However, I want to be able to see the Loops in each Function. Roughly,
>
> ok.
>
>> However, when running I'm informed that:
>>
>> PassManagerT.h:421: failed assertion `getAnalysisOrNullUp(P) &&
>> dynamic_cast<ImmutablePass*>(getAnalysisOrNullUp(P)) &&
"Pass
>> available
>> but not found! " "Perhaps this is a module pass requiring a
function
>> pass?"'
>>
>> I couldn't find a clear explanation of which passes can require
which
>> other ones, and I'm a little confused as to whether what I want is 
>> even
>> possible. How can I say that I want LoopInfo for each Function in my
>> Module?
>
> Unfortunately this is a big missing feature in the pass management 
> system.
> In particular, as the assertion says, a "Pass" cannot yet require
a
> "FunctionPass".  The reason this is tricky is that each (e.g.)
LoopInfo
> instance contains loop information for one function.  When we allow 
> Pass
> objects to require FunctionPass objects, the PassManager will have to
> instantiate one FunctionPass for each function in the module.
>
> This is clearly something that we want to do, but unfortunately we 
> can't
> do it yet.  This is actually mentioned here, though I'm not suprised 
> you
> missed it: :)
> http://llvm.org/docs/WritingAnLLVMPass.html#PassFunctionPass
OK. Yeah, I stopped reading after I saw the first 'future extension'. 
Woops.
> As a work-around you can make your "Pass" object work as a 
> FunctionPass.
> I don't know what this will do to the logic in the pass, but if this 
> is an
> option, it would be the best way to go.
> You're right that FunctionPass's are not supposed to have state 
> (something
> that many people overlook :) ), however, for now, nothing will break 
> if it
> does have state, and this is really the only way around this.
I'm not sure if I can do this. The pass I'm writing is writing info 
about the module and its parts to a file, so I need to see the module, 
if only to print its moduleIdentifier.

I'm not sure if there's a way to get the Module that a Function belongs 
to from within a FunctionPass. If I could do that, and cheat by keeping 
some state, I could do what I want as a FunctionPass. Am I missing 
something, or is that not possible?

Also, out of curiosity, why the stateless restriction - is it because 
passes may someday be run in parallel?
> However, if you want to work on the PassManager, please let us know! :)
This depends. If it's really in my way, I may have little choice. I 
would like to make a contribution, though, and it didn't actually sound 
too bad. Any guess what amount of work that'd be?
Assume I have a passing familiarity with the LLVM architecture, 
slightly rusty average C++ skills, and little shame in asking 
questions. :)

-mike

--
	Michael O. McCracken
	UCSD CSE Ph.D. Student
	mike at cs.ucsd.edu