[llvm-dev] CallInst::getCalledFunction returns null?

[John Criswell via llvm-dev]

On 9/8/15 11:16 AM, Christian Convey via llvm-dev wrote:
> I was wondering if someone could explain why 
> CallInst::getCalledFunc behaves the way it does.
>
> For simple, direct call instructions in my IR, that method behaves 
> just as one would expect.
>
> However, for instructions like this:
>
>     %25 = call i32 (%struct._IO_FILE*, ...)* bitcast (i32 (...)*
>     @close to i32 (%struct._IO_FILE*, ...)*)(%struct._IO_FILE* %24),
>     !dbg !695
>
>
> getCalledFunc returns null.
>
> I know getCalledFunc is expected to return null 
> <http://llvm.org/docs/doxygen/html/classllvm_1_1CallInst.html#a0bcd4131e1a1d92215f5385b4e16cd2e> on 
> indirect calls, but I would have thought naming the callee via a 
> constant expression like the one here would be considered a direct call.

The getCalledFunc() method is not very sophisticated.  It turns NULL if 
the called value is not trivially a function constant.

I'm not sure for the reasoning behind this.  My best guess is that a 
constant expression could do all sorts of interesting things.  For 
example, a constant expression can select one of several values or pull 
a value out of an array using a constant expression GEP.

You'll need to do what SAFECode does; use 
getCalledValue()->stripPointerCasts() to get the called value and remove 
any casts that stand between you and the function value. It's 
inconvenient, but it works.

Alternatively, you could see if the CallSite() class provides a smarter 
method of getting the called function, but I'm not sure if it has such a 
method.

Regards,

John Criswell

>
> Thanks,
> Christian
>
>
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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