[llvm-dev] Function call replacement

[zxhuan via llvm-dev]

Thanks for the prompt reply! There is this
function: _ZN7threads11WorkerGroup3RunEv, demangled
as threads::WorkerGroup::Run(), in one of the benchmarks from PARSEC. I
assume this is similar to the run method of a thread class in Java, that is
designed to be invoked by some library function during runtime.

I do notice for the above mentioned function, use_empty() returns false.
However, creating a call site and then trying to locate the call
instruction is futile. That being said, I am not sure if we can use the
use_iterator returned by user_begin() to do something.

Can you give some hint about how to locate and change that array of
function pointers(namely, the vtable)?

Thanks a lot,
Jason

On Wed, Mar 3, 2021 at 9:10 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Wed, Mar 3, 2021 at 4:50 PM zxhuan <huanzhixuan at gmail.com> wrote:
>
>> One example can be a constructor of a class, say class Foo, in C++. When
>> you have a vector of Foo instances, the constructor will be called by the
>> library function of vector.
>>
>
> Even in this example, you'd find the call to Foo's constructor in the
> module somewhere. The "library function of vector" would be instantiated in
> this same translation unit and emitted into the LLVM IR module.
>
> If the call is really coming from outside the module there's not much you
> can do to effect it.
>
>
>> There are many such functions, that you can't know they are actually
>> executed until the runtime.
>>
>> Beside that, I have done some research today. This has to be done by
>> modifying the class's vtable.
>>
>
> If the function is virtual, and if you have access to the vtable (perhaps
> it's in another module/translation unit too).
>
>
>> I have manually modified the IR file and it works. But I just can't find
>> an API function of LLVM to modify the vtable of the class.
>>
>
> LLVM IR doesn't know much about vtables - it's "just" another array of
> function pointers as far as LLVM is concerned. But you could modify that
> array if you like.
>
>
>>
>> On Wed, Mar 3, 2021 at 3:03 PM David Blaikie <dblaikie at gmail.com> wrote:
>>
>>>
>>>
>>> On Wed, Mar 3, 2021 at 11:55 AM zxhuan via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I am trying to replace calls to a function, say foo() with calls to
>>>> my_foo(). In the IR file, there is no call to foo(). However, foo() will be
>>>> called during execution.
>>>>
>>>
>>> Can't say I'm following here ^ there is no call to foo() but it is
>>> called during execution? Is it called indirectly (eg: void (*x)() = foo;
>>> x(); )? is the call in another module/not this IR file?
>>>
>>>
>>>> I can't use the conventional way to do it (i.e. locate the call
>>>> instruction, create a call site and then create a call to my_foo(), then
>>>> replace all uses with the return value of my_foo()), because you can't
>>>> locate the call instruction. So I tried to swap the name of foo() and
>>>> my_foo(). However, this only results in the original function with the new
>>>> name being called. I know I must have missed some steps, resulting in the
>>>> reference to the original function still existing after the renaming. The
>>>> question is, what is the missing step? Thank you so much for reading my
>>>> description. I hope I make my question clear.
>>>>
>>>> Best,
>>>> Jason
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
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