InstVisitor Example

I just read the LLVM Programmer's Manual, which mentions (but
specifically does not include any details of) the InstVisitor
template. Could someone please provide an example of how to use this
template to find (as an example) all CallSites for the function
strcpy?

Thanks,
Brice Lin

I just read the LLVM Programmer's Manual, which mentions (but
specifically does not include any details of) the InstVisitor
template. Could someone please provide an example of how to use this
template to find (as an example) all CallSites for the function
strcpy?

If this is really what you want to do, then the easiest method is to just get the declaration of the strcpy function, and iterate over its uses.

   Function* const strcpy = m.getFunction("strcpy");
   for (Value::use_iterator i = strcpy->use_begin(), e = strcpy- >use_end(); i != e; ++i)
     if (Instruction* const use = dyn_cast<Instruction>(i)) {
       // Do what you need here
       CallSite call(use);
       ...
     }

If you want to know how to use InstVisitor for other purposes, you just derive a class from the InstVisitor template, and overload the visit routines that you want.

   class StrcpyVisitor : public InstVisitor<StrcpyVisitor> {
     void handleStrcpy(CallSite strcpy);
     string strcpyName;
    public:
     void visitCallInst(CallInst& call) {
       if (call.getName() == strcpyName)
         handleStrcpy(&call);
     }
     void visitInvokeInst(InvokeInst& invoke) {
       if (call.getName() == strcpyName)
         handleStrcpy(&invoke);
     }
   };

Then, you probably want to inherit from ModulePass, or FunctionPass, or BasicBlockPass, or whatever makes sense, and in the relevant runOnX(X& x) callback you say:

   bool runOnX(X& x) {
     visit(x);
     return true/false/something_computed;
   }

Hope this helps,
Luke

I just read the LLVM Programmer's Manual, which mentions (but
specifically does not include any details of) the InstVisitor
template. Could someone please provide an example of how to use this
template to find (as an example) all CallSites for the function
strcpy?

If this is really what you want to do, then the easiest method is to
just get the declaration of the strcpy function, and iterate over its
uses.

I guess I should note that the "use iteration" will only work for direct calls. If you are concerned with indirect calls you need an alias analysis or call graph or some such thing. There are lots of ways to deal with this.

Also, you might need to be smarter about the Instruction check if you expect ConstantExpr casts or any other such use of the declaration.

Luke

Thanks for providing me with those examples. My rather inefficient
pass, which inherits from ModulePass, currently iterates through the
Module, Functions, and BasicBlocks multiple times (once to find
strcpy, another to find strcat, and so on for various other
functions). If I only care about the direct calls, would I benefit
more from switching to multiple use_iterators or the InstVisitor
template?

- Brice

Brice,

As Luke said, you can find all *uses* of the function name 'strcpy' by iterating over all the uses. If you just need to replace one function with another one without changing the arguments being passed, that is enough even for indirect calls (within the code being compiled, not external code) because you will find all the places where the function address is taken.

If you want to modify the arguments being passed, you need to find the actual call sites, which requires the call graph (include/Analysis/CallGraph.h).

--Vikram
Associate Professor, Computer Science
University of Illinois at Urbana-Champaign
http://llvm.org/~vadve

Brice Lin wrote:

Thanks for providing me with those examples. My rather inefficient
pass, which inherits from ModulePass, currently iterates through the
Module, Functions, and BasicBlocks multiple times (once to find
strcpy, another to find strcat, and so on for various other
functions). If I only care about the direct calls, would I benefit
more from switching to multiple use_iterators or the InstVisitor
template?

I think use_iterators are probably the best for you if that's all you want. It is the most efficient because you only touch the actual users of each function, and it leads to really clear code. On the other hand, it works best in a ModulePass setting. If you want something for a smaller granularity the InstVisitor is probably as good a choice, as your runOnX function just ends up calling visit(x).

Luke