This document seeks to dispel the mystery and confusion surrounding LLVM's GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are probably the most frequently occuring questions once a developer gets down to coding with LLVM. Here we lay out the sources of confusion and show that the GEP instruction is really quite simple.
When people are first confronted with the GEP instruction, they tend to relate it to known concepts from other programming paradigms, most notably C array indexing and field selection. However, GEP is a little different and this leads to the following questions, all of which are answered in the following sections.
Quick answer: there are no superfluous indices.
This question arises most often when the GEP instruction is applied to a global variable which is always a pointer type. For example, consider this:
%MyStruct = uninitialized global { float*, int }
...
%idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1
The GEP above yields an int* by indexing the int typed field of the structure %MyStruct. When people first look at it, they wonder why the long 0 index is needed. However, a closer inspection of how globals and GEPs work reveals the need. Becoming aware of the following facts will dispell the confusion:
Quick answer: nothing.
The GetElementPtr instruction dereferences nothing. That is, it doesn't access memory in any way. That's what the Load instruction is for. GEP is only involved in the computation of addresses. For example, consider this:
%MyVar = uninitialized global { [40 x int ]* }
...
%idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17
In this example, we have a global variable, %MyVar that is a pointer to a structure containing a pointer to an array of 40 ints. The GEP instruction seems to be accessing the 18th integer of of the structure's array of ints. However, this is actually an illegal GEP instruction. It won't compile. The reason is that the pointer in the structure must be dereferenced in order to index into the array of 40 ints. Since the GEP instruction never accesses memory, it is illegal.
In order to access the 18th integer in the array, you would need to do the following:
%idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
%arr = load [40 x int]** %idx
%idx = getelementptr [40 x int]* %arr, long 0, long 17
In this case, we have to load the pointer in the structure with a load instruction before we can index into the array. If the example was changed to:
%MyVar = uninitialized global { [40 x int ] }
...
%idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17
then everything works fine. In this case, the structure does not contain a pointer and the GEP instruction can index through the global variable pointer, into the first field of the structure and access the 18th int in the array there.
Quick answer: Because its already present.
Having understood the previous question, a new question then arises:
Why is it okay to index through the first pointer, but subsequent pointers won't be dereferenced?
The answer is simply because memory does not have to be accessed to perform the computation. The first operand to the GEP instruction must be a value of a pointer type. The value of the pointer is provided directly to the GEP instruction without any need for accessing memory. It must, therefore be indexed like any other operand. Consider this example:
%MyVar = unintialized global int ... %idx1 = getelementptr int* %MyVar, long 0 %idx2 = getelementptr int* %MyVar, long 1 %idx3 = getelementptr int* %MyVar, long 2
These GEP instructions are simply making address computations from the base address of MyVar. They compute, as follows (using C syntax):
Since the type int is known to be four bytes long, the indices 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No memory is accessed to make these computations because the address of %MyVar is passed directly to the GEP instructions.
Note that the cases of %idx2 and %idx3 are a bit silly. They are computing addresses of something of unknown type (and thus potentially breaking type safety) because %MyVar is only one integer long.
Quick Answer: They compute different address locations.
If you look at the first indices in these GEP instructions you find that they are different (0 and 1), therefore the address computation diverges with that index. Consider this example:
%MyVar = global { [10 x int ] }
%idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, byte 0, long 1
%idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
In this example, idx1 computes the address of the second integer in the array that is in the structure in %MyVar, that is MyVar+4. The type of idx1 is int*. However, idx2 computes the address of the next structure after %MyVar. The type of idx2 is { [10 x int] }* and its value is equivalent to MyVar + 40 because it indexes past the ten 4-byte integers in MyVar. Obviously, in such a situation, the pointers don't alias.
Quick Answer: They compute the same address location.
These two GEP instructions will compute the same address because indexing through the 0th element does not change the address. However, it does change the type. Consider this example:
%MyVar = global { [10 x int ] }
%idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, byte 0, long 0
%idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
In this example, the value of %idx1 is %MyVar+40 and its type is int*. The value of %idx2 is also MyVar+40 but its type is { [10 x int] }*.
In summary, here's some things to always remember about the GetElementPtr instruction: