[llvm-dev] Finding the entry point function in a LLVM IR

[mats petersson via llvm-dev]

If you want to know which functions are (or may be) called from where, in
the entire program, then you will need to do some sort of "LLVM-IR Linking"
(there are tools that will do that for you, such as "llvm-link").

Of course, even then, there's possible cases where it's impossible to know
whether a function is ACTUALLY called until at runtime - function pointers,
including those in vtables, may or may not actually get called, depending
on the exact dynamic behaviour of the code.

Then there will be functions implemented outside of the LLVM-IR for the
program anyway. atexit is a good example of a function that takes a
function pointer. Your code will not know what (if anything) atexit does
with that function pointer, or if/when that function gets called. Of
course, we, as humans, know how atexit works and when the function gets
called, but some code will not know that unless you write code to
understand it's behaviour - and there are many types of functions that take
a pointer to a function, some of which are much more complex than atexit -
for example signal handlers or call-backs that gets called on errors.
Imagine a function being called only when a memory allocation fails...

As David says, it's of course highly dependent on what you are trying to
achieve, exactly what approach you should take (or if there is an approach
that is meaningful at all).

On 23 October 2017 at 09:03, David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 21 Oct 2017, at 12:51, mohie pokhriyal via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > I want to be able to find out that main is the entry point function of
> the program.
> > main and boo both do not have any predecessors or successors , such that
> I can make a cfg to figure out who’s calling whom ?
> >
> > Is there a way I can achieve this ?
>
> The fact that main is the entry point is not known to LLVM (except in a
> couple of places that special-case main, such as the internalise pass),
> because it is an artefact of C/C++, not a generic property.  On most *NIX
> platforms, the real entry point for a program is something like __start or
> _start, which then call main.  In most compilation units, there is no
> single entry point, because they do not contain the program entry point and
> so can be entered by any externally visible function.
>
> It might help if you explained why you need this.
>
> David
>
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