[llvm-dev] How to know the CallInst is a virtual call ?

[David Blaikie via llvm-dev]

On Thu, Jun 18, 2020 at 10:52 AM PenYiWang <s89162504 at gmail.com> wrote:
>
> So if I want to know whether a CallInst is a C++ virtual call or not.
>
> I have to get the information at frontend/Clang.
>
> and then pass the information to middle-end/LLVM IR by myself.
>
> Is it right?

Yes... but you maybe shouldn't be? What do you plan to do with that information?

(not that there aren't/haven't been efforts to improve virtual call
performance - I think there's something using ThinLTO to do a whole
program analysis of virtual calls you could look into as examples of
optimizations on virtual function calls)

>
> Thank you
>
>
>
> David Blaikie <dblaikie at gmail.com> 於 2020年6月19日 週五 上午1:26寫道：
>>
>> On Thu, Jun 18, 2020 at 9:53 AM PenYiWang via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi
>> >
>> > I know that a virtual call looks like this :
>> >
>> >   %4 = load %class.base*, %class.base** %1, align 8
>> >   %5 = bitcast %class.base* %4 to void (%class.base*)***
>> >   %6 = load void (%class.base*)**, void (%class.base*)*** %5, align 8
>> >   %7 = getelementptr inbounds void (%class.base*)*, void (%class.base*)** %6, i64 0
>> >   %8 = load void (%class.base*)*, void (%class.base*)** %7, align 8
>> >   call void %8(%class.base* %4)
>> >
>> > There may be some action to get function pointer on vtable .
>> >
>> > But, when I scan a llvm ir file, if I just see a CallInst and it is an indirect call
>> >
>> > Is there any way to know whether the CallInst is a virtual call or not ?
>>
>> Not exactly, no - LLVM has no concept of virtual calls specifically -
>> they are "just" indirect calls through a vtable. LLVM optimizations
>> generally shouldn't be trying to reconstruct more high level semantics
>> than exist in LLVM IR - an optimization to improve virtual function
>> calls (using the existing IR - not accounting for some special cases
>> that might involve adding extra metadata, etc) should be framed in
>> terms of indirect calls in general (perhaps indirect calls from
>> functions in constant arrays - maybe that's the specific subcase you
>> want to target, etc).
>>
>> - Dave