[llvm-dev] Get function implementation for indirect CallInst.

[Chao Chen via llvm-dev]

Hi All, 
I am using LLVM to analysis some C++ codes. I need to find the related function implementation for a given indirect CallInst. 
To make my question a little bit clear, I construct a simple example here. For “ call void %3(%class.Base* %1)”, I need to figure out 
Which implementation is called ( it can be manually figured out that _ZN1A5helloEv (A::hello() is called,how to figure it out in a pass ?)

My initial thinking is trace back from "call void %3(%class.Base* %1)” until find %obj1 = alloca %class.A, align 8, but how can I associate %class.A with @_ZTV1A, so I can get the function from @_ZTV1A ?

Thanks!

;virtual tables
@_ZTV1A = linkonce_odr unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1A to i8*), i8* bitcast (void (%class.A*)* @_ZN1A5helloEv to i8*)] }, align 8
@_ZTV1B = linkonce_odr unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast ({ i8*, i8*, i8* }* @_ZTI1B to i8*), i8* bitcast (void (%class.B*)* @_ZN1B5helloEv to i8*)] }, align 8

;A::hello()
define linkonce_odr void @_ZN1A5helloEv(%class.A* %this) unnamed_addr #2 align 2 {
entry:
  %this.addr = alloca %class.A*, align 8
  store %class.A* %this, %class.A** %this.addr, align 8
  %this1 = load %class.A*, %class.A** %this.addr, align 8
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([7 x i8], [7 x i8]* @.str, i32 0, i32 0))
  ret void
}

;B::hello()
define linkonce_odr void @_ZN1B5helloEv(%class.B* %this) unnamed_addr #2 align 2 {
entry:
  %this.addr = alloca %class.B*, align 8
  store %class.B* %this, %class.B** %this.addr, align 8
  %this1 = load %class.B*, %class.B** %this.addr, align 8
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([7 x i8], [7 x i8]* @.str.1, i32 0, i32 0))
  ret void
}

define i32 @main() #0 {
entry:
  %obj1 = alloca %class.A, align 8
  %obj2 = alloca %class.B, align 8
  %C = alloca %class.Base*, align 8
  call void @_ZN1AC1Ev(%class.A* %obj1) #4
  call void @_ZN1BC1Ev(%class.B* %obj2) #4
  %0 = bitcast %class.A* %obj1 to %class.Base*
  store %class.Base* %0, %class.Base** %C, align 8
  %1 = load %class.Base*, %class.Base** %C, align 8
  %2 = bitcast %class.Base* %1 to void (%class.Base*)***
  %vtable = load void (%class.Base*)**, void (%class.Base*)*** %2, align 8
  %vfn = getelementptr inbounds void (%class.Base*)*, void (%class.Base*)** %vtable, i64 0
  %3 = load void (%class.Base*)*, void (%class.Base*)** %vfn, align 8
  call void %3(%class.Base* %1)
  ret i32 0
}
 

Related C++ code

class Base {
    public:
        virtual void hello();
};

class A: public Base {
public:
    void hello() {
        printf("hello\n");
    };
};

class B: public Base {
public:
    void hello() {
        printf("world\n");
    };
};


int main() {
    A obj1;
    B obj2;
    Base * C = &obj1;
    C->hello();
}
 
Best Regards,
Chao
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