[llvm-dev] How to efficiently extract the calledFunction from a complex CallInst?

[Shen Liu via llvm-dev]

Thanks for your helpful answer David! But one thing still makes me confused
is when i use isa<CallInst> to check the original instruction the answser
is yes.
, which implies it is a callinst. Since it is not a BitcastInst, why does
llvm add an opcode "call" before it? It looks a little weird here.

On Thu, Nov 12, 2015 at 10:48 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Thu, Nov 12, 2015 at 7:27 PM, Shen Liu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all,
>>
>> Usually if we want to get the called Function we can directly use
>> CallInst->getCalledFunction(), however, today i encounter an unusual
>> CallInst as follows:
>>
>>  %call11 = call double (...)* bitcast (double ()* @quantum_frand to
>> double (...)*)()
>>
>> the original C source involve type cast:
>>
>> float u,v;
>> extern double quantum_frand();
>>    u = 2 * quantum_frand() - 1;
>>    v = 2 * quantum_frand() - 1;
>>
>> In this case, CallInst->getCalledFunction() returns a nullptr unusually,
>> I printed out the getOperand(0) and found the operand is the whole thing of
>>  "double (...)* bitcast (double ()* @quantum_frand to double (...)*)()".
>> Any member function calling on that fails so i don't know whether there is
>> an efficient way to exactly get the called function @quantum_frand(...)
>> here?
>>
>
> What you have is a BitCastInst, so you'd have to downcast the Value* to
> that, then look through the bitcast.
>
> But realize you may never actually find a function in the end:
>
> void func(void (*f)(void)) {
>   f();
> }
>
> the f call is a call to a function pointer, so there is no statically
> knowable called function here. Your code will probably need to handle the
> case where this arises.
>
> - David
>
>
>> Thanks!
>>
>>
>> Best regards,
>>
>> Shen
>>
>>
>>
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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>>
>>
>
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