LLVM getelementptr

The pointer arithmetic instruction. Used to calculate an offset pointer given a value and a struct.

In essence, given a object and a field number, GEP returns the pointer to that object’s field at field number.

Example

define ptr @get_inner_in_array(ptr %p, i64 %idx) {
  %q = getelementptr %MyStruct, ptr %p, i64 %idx, i32 1, i32 1
  ret ptr %q
}

• Takes an untyped pointer ptr into a struct of %MyStruct with ptr %p
• Take a field number i64 %idx
• Firstindex means it starts searching in the 1st sub-element of %MyStruct (assuming that the 1st sub element is an iterable like a list or vector)
• Secondindex means it starts searching at the 1st sub-element of %MyStruct.1
• Return the pointer to the field at field number

2019 EuroLLVM Talk Explanation

https://www.youtube.com/watch?v=m8G_S5LwlTo&t=1753s

First Index Offset

the firstindex only offsets the basetype with Pointer Arithmetic. This means if the pointer’s base-type is int, it offsets pointer by i32. If the pointer’s base-type is a 4-integer list, then it offsets by i128 (as 32 * 4 is 128).

n-th index offset

All subsequent offsets will then be offsets of the inner-element of the first-index’s data-type.