[LLVMbugs] Re: [LLVMdev] Tarjan+function_ptrs == trouble ?

[Chris Lattner]

> I've been through the documentation, and if I visit main, i think
> everything will turn out correctly...  Printing out the Call graph reveals
> that main is calling this external node i think you are making reference
> to.  Could this be the problem?

The tarjan SCC iterator should visit main, so I think it's broken.  I'm
just refering to the fact that the indirect call node is wierd and needs
to be known about by clients of the CallGraph class.

For visualization you can use the 'analyze -print-callgraph' to turn a
call graph into a dot file.  You'll probably want to commit this patch
first though:

http://mail.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20021202/001850.html

-Chris



>  On Fri, 6 Dec 2002, Chris Lattner wrote:
>
> > > if you take the time to print out the function each non-looping node iter
> > > traverses, it never reaches main...
> >
> > Ok, first note that the CallGraph class does really stupid (but correct)
> > things for function pointers, so you're not going to see anything
> > remarkably nice here... Make sure to read the CallGraph.h comments to
> > understand exactly what is going on.
> >
> > Despite that, I can verify that the SCC iterator seems to not be doing the
> > right thing.  On the testcase (below), and with the pass below, I get the
> > following:
> >
> > $ opt -load /localhome/lattner/llvm/lib/Debug/libtest.so -test < test.o > /dev/null
> > New SCC
> >   Node: __main
> > New SCC
> >   Node: func_one
> > New SCC
> >   Node: func_two
> > New SCC
> >   Node: func_three
> >
> > Which doesn't visit main.  Vikram, could you look into this if you get a
> > chance?
> >
> > -Chris
> >
> > ------------------- Pass code ----------------------------
> >
> > #include "llvm/Analysis/CallGraph.h"
> > #include "llvm/Function.h"
> > #include "llvm/Pass.h"
> > #include "Support/TarjanSCCIterator.h"
> >
> > struct Test : public Pass {
> >   virtual bool run(Module &F) {
> >     typedef TarjanSCC_iterator<CallGraph*> MyTarjan;
> >     CallGraph& callGraph = getAnalysis<CallGraph>();
> >     for (MyTarjan I = tarj_begin(&callGraph), E = tarj_end(&callGraph); I != E; ++I) {
> >       SCC<CallGraph*> *S = *I;
> >       assert(S);
> >       std::cerr << "New SCC\n";
> >       for (unsigned i = 0, e = S->size(); i != e; ++i)
> >         if (Function *F = (*S)[i]->getFunction())
> >           std::cerr << "  Node: " << F->getName() << "\n";
> >         else
> >           std::cerr << "  Indirect node\n";
> >     }
> >     return false;
> >   }
> >   virtual void getAnalysisUsage(AnalysisUsage &AU) const {
> >     AU.setPreservesAll();
> >     AU.addRequired<CallGraph>();
> >   };
> > };
> > RegisterOpt<Test> Z("test", "test");
> >
> >
> > -------------------- Testcase ------------------------
> >
> > #include <stdlib.h>
> > #include <stdio.h>
> >
> > //#define WORKS
> >
> > int * a_global;
> >
> > int a_predicate = 1;
> > int another_pred =0;
> >
> > int func_one()
> > {
> >     return 1;
> > }
> >
> > int func_two()
> > {
> >     return 2;
> > }
> >
> > int func_three()
> > {
> >     return 3;
> > }
> >
> > int main()
> > {
> >     int (*func_ptr)();
> >
> >     if(a_predicate)
> >         func_ptr = func_one;
> >     else if(another_pred)
> >         func_ptr = func_two;
> >     else
> >         func_ptr = func_three;
> >
> > #ifdef WORKS
> >     return func_one();
> > #else
> >     return func_ptr();
> > #endif
> > }
> >
> >
>
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-Chris

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