[llvm-dev] Finding all pointers to functions

Vedant Kumar via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 23 09:17:45 PST 2015

Hi Russell,

> I need to track down all pointers anywhere in a module that could be pointing to functions (because some of the optimizations I want to do, require either identifying every use of a function, or conservatively identifying when such cannot be done).
> A starting point is to look at all the global variables:
>   for (auto &G : M.globals())
>     for (auto &V : G.operands())
>       if (auto F = dyn_cast<Function>(V))
> Of course, instructions can also refer to functions, both as direct calls and otherwise:
>   for (auto &F : M) {
>     for (auto &I : inst_range(F)) {
>       for (auto &V : I.operands())
>         if (auto F = dyn_cast<Function>(V))

Your code is reminiscent of the traversal logic in tools/verify-uselistorder.

There's a chance you might be able to reuse the ValueMapping class from that project.

> But there are other things as well, for example it seems there is something called a personality function that can be a pointer to another function, so need to add that
>     if (F.hasPersonalityFn())
> It seems there are other things called prefix data and prologue data, which are pointers to constants, which could contain pointers to functions, so would need to include those as well.
> Am I correct in thinking that prefix data and prologue data will not be included in the global variables list, so do need special handling?

Prefix data, prologue data, and function personalities do not need to be globals.

This will require special handling. The ValueMapping class has good skeleton code for this.

> Could they be recursive? That is, could those constants contain pointers to other constants... which end up containing pointers to functions... such that none of the intermediate constant objects are in the global variable list?

I don't see how this is possible. If it is, we'd need to file a PR against verify-uselistorder.


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