The Revenge Of The Often Misunderstood GEP Instruction
Introduction

GEP was mysterious and wily at first, but it turned out that the basic workings were fairly comprehensible. However the dragon was merely subdued; now it's back, and it has more fundamental complexity to confront. This document seeks to uncover misunderstandings of the GEP operator that tend to persist past initial confusion about the funky "extra 0" thing. Here we show that the GEP instruction is really not quite as simple as it seems, even after the initial confusion is overcome.

How is GEP different from ptrtoint, arithmetic, and inttoptr?

It's very similar; there are only subtle differences.

With ptrtoint, you have to pick an integer type. One approach is to pick i64; this is safe on everything LLVM supports (LLVM internally assumes pointers are never wider than 64 bits in many places), and the optimizer will actually narrow the i64 arithmetic down to the actual pointer size on targets which don't support 64-bit arithmetic in most cases. However, there are some cases where it doesn't do this. With GEP you can avoid this problem.

Also, GEP carries additional pointer aliasing rules. It's invalid to take a GEP from one object, address into a different separately allocated object, and dereference it. IR producers (front-ends) must follow this rule, and consumers (optimizers, specifically alias analysis) benefit from being able to rely on it.

And, GEP is more concise in common cases.

However, for the underlying integer computation implied, there is no difference.

I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?

You don't. The integer computation implied by a GEP is target-independent. Typically what you'll need to do is make your backend pattern-match expressions trees involving ADD, MUL, etc., which are what GEP is lowered into. This has the advantage of letting your code work correctly in more cases.

GEP does use target-dependent parameters for the size and layout of data types, which targets can customize.

If you require support for addressing units which are not 8 bits, you'll need to fix a lot of code in the backend, with GEP lowering being only a small piece of the overall picture.

Why do struct member indices always use i32?

The specific type i32 is probably just a historical artifact, however it's wide enough for all practical purposes, so there's been no need to change it. It doesn't necessarily imply i32 address arithmetic; it's just an identifier which identifies a field in a struct. Requiring that all struct indices be the same reduces the range of possibilities for cases where two GEPs are effectively the same but have distinct operand types.

How does VLA addressing work with GEPs?

GEPs don't natively support VLAs. LLVM's type system is entirely static, and GEP address computations are guided by an LLVM type.

VLA indices can be implemented as linearized indices. For example, an expression like X[a][b][c], must be effectively lowered into a form like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional array reference.

This means if you want to write an analysis which understands array indices and you want to support VLAs, your code will have to be prepared to reverse-engineer the linearization. One way to solve this problem is to use the ScalarEvolution library, which always presents VLA and non-VLA indexing in the same manner.

What happens if an array index is out of bounds?

There are two senses in which an array index can be out of bounds.

First, there's the array type which comes from the (static) type of the first operand to the GEP. Indices greater than the number of elements in the corresponding static array type are valid. There is no problem with out of bounds indices in this sense. Indexing into an array only depends on the size of the array element, not the number of elements.

A common example of how this is used is arrays where the size is not known. It's common to use array types with zero length to represent these. The fact that the static type says there are zero elements is irrelevant; it's perfectly valid to compute arbitrary element indices, as the computation only depends on the size of the array element, not the number of elements. Note that zero-sized arrays are not a special case here.

This sense is unconnected with `inbounds` keyword. The `inbounds` keyword is designed to describe low-level pointer arithmetic overflow conditions, rather than high-level array indexing rules.

Analysis passes which wish to understand array indexing should not assume that the static array type bounds are respected.

The second sense of being out of bounds is computing an address that's beyond the actual underlying allocated object.

With the `inbounds` keyword, the result value of the GEP is undefined if the address is outside the actual underlying allocated object and not the address one-past-the-end.

Without the `inbounds` keyword, there are no restrictions on computing out-of-bounds addresses. Obviously, performing a load or a store requires an address of allocated and sufficiently aligned memory. But the GEP itself is only concerned with computing addresses.

Can array indices be negative?

Yes. This is basically a special case of array indices being out of bounds.

Can I compare two values computed with GEPs?

Yes. If both addresses are within the same allocated object, or one-past-the-end, you'll get the comparison result you expect. If either is outside of it, integer arithmetic wrapping may occur, so the comparison may not be meaningful.

Can I do GEP with a different pointer type than the type of the underlying object?

Yes. There are no restrictions on bitcasting a pointer value to an arbitrary pointer type. The types in a GEP serve only to define the parameters for the underlying integer computation. They need not correspond with the actual type of the underlying object.

Furthermore, loads and stores don't have to use the same types as the type of the underlying object. Types in this context serve only to specify memory size and alignment. Beyond that there are merely a hint to the optimizer indicating how the value will likely be used.

Can I cast an object's address to integer and add it to null?

You can compute an address that way, but if you use GEP to do the add, you can't use that pointer to actually access the object, unless the object is managed outside of LLVM.

The underlying integer computation is sufficiently defined; null has a defined value -- zero -- and you can add whatever value you want to it.

However, it's invalid to access (load from or store to) an LLVM-aware object with such a pointer. This includes GlobalVariables, Allocas, and objects pointed to by noalias pointers.

If you really need this functionality, you can do the arithmetic with explicit integer instructions, and use inttoptr to convert the result to an address. Most of GEP's special aliasing rules do not apply to pointers computed from ptrtoint, arithmetic, and inttoptr sequences.

Can I compute the distance between two objects, and add that value to one address to compute the other address?

As with arithmetic on null, You can use GEP to compute an address that way, but you can't use that pointer to actually access the object if you do, unless the object is managed outside of LLVM.

Also as above, ptrtoint and inttoptr provide an alternative way to do this which do not have this restriction.

Can I do type-based alias analysis on LLVM IR?

You can't do type-based alias analysis using LLVM's built-in type system, because LLVM has no restrictions on mixing types in addressing, loads or stores.

It would be possible to add special annotations to the IR, probably using metadata, to describe a different type system (such as the C type system), and do type-based aliasing on top of that. This is a much bigger undertaking though.

What's an uglygep?

Some LLVM optimizers operate on GEPs by internally lowering them into more primitive integer expressions, which allows them to be combined with other integer expressions and/or split into multiple separate integer expressions. If they've made non-trivial changes, translating back into LLVM IR can involve reverse-engineering the structure of the addressing in order to fit it into the static type of the original first operand. It isn't always possibly to fully reconstruct this structure; sometimes the underlying addressing doesn't correspond with the static type at all. In such cases the optimizer instead will emit a GEP with the base pointer casted to a simple address-unit pointer, using the name "uglygep". This isn't pretty, but it's just as valid, and it's sufficient to preserve the pointer aliasing guarantees that GEP provides.

Can GEP index into vector elements?

Sort of. This hasn't always been forcefully disallowed, though it's not recommended. It leads to awkward special cases in the optimizers. In the future, it may be outright disallowed.

Instead, you should cast your pointer types and use arrays instead of vectors for addressing. Arrays have the same in-memory representation as vectors, so the addressing is interchangeable.

Can GEP index into unions?

Unknown.

What happens if a GEP computation overflows?

If the GEP has the `inbounds` keyword, the result value is undefined.

Otherwise, the result value is the result from evaluating the implied two's complement integer computation. However, since there's no guarantee of where an object will be allocated in the address space, such values have limited meaning.

What effect do address spaces have on GEPs?

None, except that the address space qualifier on the first operand pointer type always matches the address space qualifier on the result type.

Why is GEP designed this way?

The design of GEP has the following goals, in rough unofficial order of priority:

- Support C, C-like languages, and languages which can be conceptually lowered into C (this covers a lot).
- Support optimizations such as those that are common in C compilers.
- Provide a consistent method for computing addresses so that address computations don't need to be a part of load and store instructions in the IR.
- Support non-C-like languages, to the extent that it doesn't interfere with other goals.
- Minimize target-specific information in the IR.