在llvm ir中增加ptr

Question

I am trying to understand the getelementptr instruction in llvm IR, but not fully understanding it.

I have a struct like below -

struct Foo {
    int32_t* p;
}

I want to do this -

foo.p++;

What would be the right code for this?

%0 = getelementptr %Foo* %fooPtr, i32 0, i32 0
%1 = getelementptr i32* %0, i8 1
store i32* %1, i32* %0

I am wondering if value in %0 needs to be first loaded using "load" before executing 2nd line.

Thanks!

Answer 1

You can see the GEP instruction as an operation that performs arithmetic operations on pointers. In LLVM IR the GEP instruction is your instruction of choice to perform operations on pointers easyly. You don't have to do cumbersome calculate the size of your types and offsets to manually perform such operations.

In your case:

%0 = getelementptr %Foo* %fooPtr, i32 0, i32 0

selects the member inside the structure. It uses the pointer operatand %fooPtr to calculate %0 = ((fooPtr + 0) + 0) . GEP does not know about fooPtr just pointing to one element of Foo, this is why two indices are used to select the member.

%1 = getelementptr i32* %0, i8 1

As mentioned above the GEP performs pointer arithmetic and in your case get %1 = (p + 1);

Since you are operating on pointers using GEP you don't need to load the value of p. GEP will do this implicitly for you.

Now you can store the new index back to the position of the p member inside the Foo struct pointed to by fooPtr .

For further reading: The Often Misunderstood GEP Instruction