The Often Misunderstood GEP Instruction

1. Introduction
2. The Questions
   1. Why is the extra 0 index required?
   2. What is dereferenced by GEP?
   3. Why can you index through the first pointer but not subsequent ones?
   4. Why don't GEP x,0,0,1 and GEP x,1 alias?
   5. Why do GEP x,1,0,0 and GEP x,1 alias?
3. Summary

Written by: Reid Spencer.

Introduction

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.

The Questions

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.

1. Why is the extra 0 index required?
2. What is dereferenced by GEP?
3. Why can you index through the first pointer but not subsequent ones?
4. Why don't GEP x,0,0,1 and GEP x,1 alias?
5. Why do GEP x,1,0,0 and GEP x,1 alias?

Why is the extra 0 index required?

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:

1. The type of `%MyStruct` is not `{ float*, int }` but rather `{ float*, int }*`. That is, `%MyStruct` is a pointer to a structure containing a pointer to a `float` and an `int`.
2. Point #1 is evidenced by noticing the type of the first operand of the GEP instruction (`%MyStruct`) which is `{ float*, int }*`.
3. The first index, `long 0` is required to dereference the pointer associated with `%MyStruct`.
4. The second index, `ubyte 1` selects the second field of the structure (the `int`).

What is dereferenced by GEP?

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.

Why can you index through the first pointer?

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):

- idx1 = &MyVar + 0
- idx2 = &MyVar + 4
- idx3 = &MyVar + 8

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.

Why don't GEP x,0,0,1 and GEP x,1 alias?

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.

Why do GEP x,1,0,0 and GEP x,1 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] }*`.

Summary

In summary, here's some things to always remember about the GetElementPtr instruction:

1. The GEP instruction never accesses memory, it only provides pointer computations.
2. The first operand to the GEP instruction is always a pointer and it must be indexed.
3. There are no superfluous indices for the GEP instruction.
4. Trailing zero indices are superfluous for pointer aliasing, but not for the types of the pointers.
5. Leading zero indices are not superfluous for pointer aliasing nor the types of the pointers.

Appendix: Discussion

The following is a real discussion from the #llvm IRC channel about the GEP instruction. You may find this instructive as it was the basis for this document.

User | Comment
--- | ---
Yorion | If x & y must alias, are [ getelementptr x,0,0,1,2 ] and [ getelementptr x,1,2 ] aliased? (they obviously have different types, but they should alias...)
Yorion | oops, for the second one I meant [ getelementptr y,1,2 ]
Reid | I don't see how that could be, Yorion but I'm not the authority on this
Yorion | hmm..
Reid | the two geps, by definition, are going to produce different pointers which are not aliased
Yorion | would [ GEP x,1,0 ] and [ GEP y,1 ] be aliased?
Reid | if the second gep was [gep y,0,0,1,2] then they should be aliased as well
Reid | no, I wouldn't expect that to work either :)
Reid | you can't just arbitrarily drop leading or trailing indices :)
Reid | (.. leading or trailing 0 indices, I mean)
Reid | this instruction walks through a data structure and generates a pointer to the resulting thing
Reid | if the number of indices are different, you're ending up at a different place and by definition they'll have different addresses
Yorion | oh, I see, because of different types, [ GEP x,0,1 ] & [ GEP x,1 ] actually might refer to different fields, but might also refer to the same ones...
Reid | or, at least, that's my crude understanding of it :)
Reid | no, they'll definitely refer to different fields
nicholas | GEP x,0,1 ==> &((*(x+0))+1)? vs. GEP x,1 ==> &(*(x+1))?
Reid | lemme grok that for a sec
Reid | that might be true in some limited definition of x, but it wouldn't be generally
nicholas | oh. fields of different sizes in a structure.
Reid | yup
Yorion | is perhaps the type unification the reason why [ GEP x,0,1 ] and [ GEP x,1 ] cannot alias?
Reid | no
Reid | they may or may not have the same type, but they are definitely different pointers
Reid | lets use a concrete example for "x"
Reid | suppose x is "struct {int a, float b} *"
Reid | GEP X,0,1 is going to return the address of b
Reid | GEP X,1 is going to return the address of the *second* "a" (after the first b)
Yorion | ah, I see...
Yorion | trailing zeros are still a bit confusing...
Reid | same thing .. you're just selecting the 0th member of an array or structure
Yorion | you don't move away from the pointer, only the type is changed
Reid | no, you still move away from the pointer .. the type might change, or not
Reid | the pointer definitely changes
Reid | lets look at an example for trailing zero
Reid | suppose x is "int x[10][10][10][10]" (in C)
Reid | GEP X,0,0 will yield you a 3 dimensional array
Reid | GEP X,0,0,0,0,0 will yield you an "int"
Reid | make sense?
Yorion | yes
Reid | so, I think there's a law here: if the number of indices in two GEP instructions are not equivalent, there is no way the resulting pointers can alias
Reid | (assuming the x and y alias)
Yorion | I was confused with some code in BasicAliasAnalysis that says that two pointers are equal if they differ only in trailing zeros
Yorion | BasicAliasAnalysis.cpp:504-518
Reid | lemme look
nicholas | if y1 = GEP X, 0, 0 and y2 = GEP X, 0, 0, 0, 0, 0 (from Reid's example)
nicholas | then doesn't *y1 and *y2 both refer to the same "int"?
Reid | they shouldn't
Reid | hmm .. actually, maybe you're right :)
Reid | they definitely have different *types*
Yorion | true
nicholas | different types just doesn't cut it. :)
Reid | .. thinking on this :)
nicholas | similarly, i could create a yucky with a struct that has a char *, then have you GEP right through the pointer into the pointed-to data. That could mean that the resulting point might alias anything.
Yorion | my theory (after reading BAA) is that all zeros can be omitted, and that the pointers alias if they have the same sequence of indices
Yorion | however, this screws the typing, so that's why zeros are for
Yorion | nicholas, does that match your hunch?
nicholas | I have to admit, I've had much grief with GEPIs already. I wish the semantics were plainly documented as part of their own language, instead of just relying on C aliasing rules and C semantics...
nicholas | Yorion: leading zeroes can't be omitted.
Reid | okay, if you have two GEPs and their leading indices are an exact match, if the one with more indices only has trailing 0s then they should alias
nicholas | must alias, i think.
Reid | yes, must alias, sorry
Yorion | okay
Yorion | I'm glad we cleared this up
Reid | so, if y1 = GEP X, 1,2,0 and if y2 = GEP X, 1,2,0,0,0 then y1 "must alias" y2 :)
Reid | but that doesn't work for leading 0s :)
Yorion | yes, true... I was wrong
Reid | I too have been having fun with GEP recently :)
Yorion | but, there're cases like [a = GEP x,1,0; b = GEP a,1,0; c = GEP b,1,0], and that should be equivalent to GEP x,1,0,1,0,1
Reid | not quite
nicholas | I'm sure another rule can be written for GEPIs, but they would only apply to type-safe code.
nicholas | another *tautology
Yorion | Reid: why not, only the type should be different...
Reid | its should be equivalent to GEP x,1,0,1,0,1,0
Yorion | and that must alias GEP x,1,0,1,0,1
Reid | hmm, by the previous rule, I guess you're right :)
Yorion | I'm a bit scared that even you're a bit confused about GEP
Reid | I'm glad I'm not the only one that gets a little confused wrapping my head around this stuff :)
Reid | GEP has always confused me .. partly because I think its wrong :)
Reid | well, actually, not so much that GEP is wrong, but that gvars being pointers without storage
Reid | i.e. when you say "%x = global int" in LLVM, the type of X is int*
Reid | yet, there is no storage for that pointer
Reid | its magically deduced
nicholas | well, it makes no sense to have globals be SSA...
Reid | heh
Reid | yeah, well .. practicalities :)
Yorion | true
Yorion | sabre gave me a reference on how globals are handled in SSA
Reid | anyway, gotta run
Yorion | the paper was crappy, but I do understand now why is it implemented that way in LLVM
Yorion | thx for the interesting discussion Reid
Reid | heh .. its one that Chris and I keep having .. he just tells me that C has rotted my brain :)
nicholas | lol
Yorion | hehehe
Reid | he might be right :)
Yorion | sabre: have you seen the discussion on GEP?
sabre | no
sabre | I'll read the backlog, j/s
sabre | ok, there's a lot
sabre | what's the executive summary?
sabre | do you have a q?
Yorion | is it possible that GEP x,0,0,1 and GEP x,1 alias?
sabre | no
Yorion | and b) GEP x,1,0,0 and GEP x,1 should alias, right?
sabre | I assume you mean for size = 1 ?
sabre | b) yes
Yorion | although they have different types
sabre | right
Yorion | okay
Yorion | I'm still not 100% convinced that: a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0 cannot alias Z=GEP x,1,1,1
Yorion | (that c and z cannot alias)
sabre | that's becuse they do alias
sabre | mustalias in fact
Yorion | but then: GEP x,1,0,1,0,1,0 = GEP x,1,1,1
sabre | Yorion: no
sabre | c != GEP x,1,0,1,0,1,0
sabre | the first index doesn't work like that
Yorion | how does then the first index work? c and z must alias, but GEP x,1,0,1,0 != GEP x,1,1 ??
sabre | *sigh*
Reid | :)
Reid | we need an FAQ on this
sabre | Yorion: how did you get
sabre | "GEP x,1,0,1,0"?
Yorion | look
sabre | you can't just concatenate subscripts
Yorion | why?
sabre | because... it doesn't work that way?
sabre | consider C
Yorion | how does it work?
sabre | if I have blah* P
sabre | P[0][1][2][3][4]
sabre | this is *not* the same as:
sabre | t = &P[0][1][2] ... t[3][4]
sabre | Yorion: Consider: struct *P
sabre | P->X == P[0].X
sabre | You're losing the 0.
sabre | P->X->Y == P[0].X[0].Y
sabre | Not P.X.Y
sabre | actually that's a bad analogy
sabre | because C dereferences in this case
sabre | Try: (&(P->X))->Y
Yorion | so, a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0, can you construct the definition of c in terms of x?
sabre | yes, but you're going out of bounds :)
sabre | consider this:
sabre | { float, { double , { int } } } *P
sabre | int *X = gep P, 0, 1, 1, 0
sabre | do you understand the leading zero?
sabre | alternatively:
sabre | t = gep P, 0, 1
sabre | t2 = gep t, 0, 1
sabre | X = gep t, 0, 0
Yorion | what's t2 for?
sabre | oh
sabre | sorry :)
sabre | X = gep t2, 0, 0
Yorion | give me a minute please
sabre | ok
Yorion | sabre: shouldn't the type be: { float, { double, { int }* } }* P ?
sabre | nope
sabre | why the extra * ?
sabre | if it helps, the type of t is { double, {int}}* and t2 is {int}* and X is int*
Yorion | wait... 0 represents dereference, natural number i represents &A[i] ?
sabre | gep does no dereferences, ever
sabre | gep P, 0, 1 is equivalent to &P[0].X
sabre | aka &P->X
sabre | gep P, 1 == &P[1] aka P+1
sabre | so gep P, 0, 1 can't alias gep P, 1 just like &P->Y can't alias P+1
sabre | assuming P is a pointer to struct {X, Y }
Yorion | sabre: is it possible to come up with a general rule for "arithmetic of GEP indices"?
sabre | Yorion: of course, it's very simple
sabre | just not what you're expecting :)
sabre | See langref.html
Yorion | for example, a=GEP x,0,0,1 b=GEP a,0,0,1, c=GEP b,0,0,1, that should be equal to GEP x,0,1,1,0, right?
Yorion | I did
Yorion | oops, equal to GEP x,0,1,1,1,0
sabre | that would be:
sabre | GEP X, 0, 0, 1, 0, 1, 0, 1
Yorion | you're killing me
sabre | The basic rule when turning: A = GEP B, C D = GEP A, 0, E
sabre | is that you drop the 0, turning it into
sabre | GEP B, C, E
Yorion | okay, that's good. any other rules?
nicholas | what if it isn't a 0?
sabre | more generally: A = GEP Ptr, B, C, ... D = GEP A, 0, E, F, ...
sabre | D = GEP Ptr, B, C, ... E, F, ...
sabre | if it's not zero, you generally cannot concatenate them
sabre | unless the first gep has one subscript
sabre | in which case you drop the zero
sabre | if you look in InstCombiner::visitGetElementPtrInst, it should have this logic
Yorion | what if there is no zero? how can I compute the offset from the base pointer in that case?
Yorion | like A=GEP B,C and D=GEP A,E,F
sabre | you don't have to combine them to compute an offset
sabre | are you *just* trying to get a byte offset from the pointer?
Yorion | I'm trying to get offset of D from B
sabre | why didn't you say so? :)
sabre | with all constant subscripts, it's trivial
sabre | look at SelectionDAGLowering::visitGetElementPtr
sabre | in CodeGen/SelectionDAG/SelectionDAGISel.cpp
sabre | basically the rule is that you multiply the index by the size of the thing indexed
sabre | there is also a Support/GetElementPtrIterator or something
sabre | that makes it trivial to see what type is indexed by which subscript
sabre | for each subscript it gives you a type
sabre | For an array subscript you multiply the index by the indexed type
sabre | for a struct subscript, you add the field offset
sabre | s/array/sequentialtype/ if you're in a pedantic mood
Yorion | let's focus on structs: in that case, the above given example would be: D = GEP B,C,E,F?
sabre | no
sabre | you drop the E if it's zero
sabre | if it's not you can't concat
sabre | are you trying to trick me into saying "yes, just append the indices"? :)
Yorion | okay, let's assume E is not zero, how do I compute offset from B for D for a struct?
sabre | Why are you framing this in terms of concatenation?
Yorion | no, I'm trying to understand it
sabre | computing an offset and concatenating are entirely different
sabre | Lets consider a specific example
Yorion | because I want to express certain properties in the terms of base pointers either globals or parameters
Yorion | I want to eliminate locals from my analysis
sabre | you realize that parmeters can point into the middle of structs?
Yorion | yes
sabre | you realize invalid access paths can be constructed with geps/
sabre | ?
Yorion | what do you mean by invalid access paths?
Yorion | like offseting out of the struct which is passed to the function?
sabre | The case where the subscript isn't zero is invalid code
sabre | from a type-safety perspective
DannyB | he means untypesafe things that seem valid :)
DannyB | IE they point somewhere in the struct, but not to any particular field
DannyB | (or whatever)
sabre | right
Yorion | okay
sabre | or they might point in some other struct :)
sabre | It's the equivalent to saying:
sabre | struct Foo { int A, int B; }
sabre | Foo* P =
sabre | T = &P->B;
sabre | S = T+1
sabre | that is:
sabre | T = gep 0, 1
sabre | S = gep T, 1
sabre | you can't concat those and get a type-safe access path
sabre | remember T = &P->B === T = &P[0].B
sabre | understand?
Yorion | let me think for a minute
sabre | Consider what the C case does, it will be most clear if you're used to C
sabre | :)
sabre | Consider the byte offset and why it doesn't point into P-> anything
sabre | it points into P[1] not P[0]
Yorion | it's perfectly fine if GEP offsets out of the type. I'd still need to express GEP in the terms of base pointers. Take the example above: T=GEP P,0,1; S=GEP T,1
Yorion | type safety is not crucial in my case
sabre | That specific example is GEP P, 1, 0
sabre | however, you can form geps that are NOT equivalent to anything else
sabre | for example, consider:
sabre | struct X { int, char}
Yorion | that is fine. they're equivalent to something in the calling context
sabre | the same sequence points into padding
sabre | and there is no gep that can do that
Yorion | the bottom line is: if the program is valid, interprocedural analysis will match that offset with something in one of the functions on the call stack
Yorion | and that's all I care about
Yorion | can you give me a formula for structs for computing offsets that takes into account the case GEP T,&lt:non_zeros> and the size of the structs/fields?
sabre | yes, I did above
sabre | Two things:
sabre | GEP Ptr, A, X, Y, Z
sabre | The result is Ptr + A * sizeof(struct) + fieldoffs(X) + fieldoffs(Y) + fieldoffs(Z)
sabre | simple enough?
sabre | you see why "A" is special?
Yorion | give me a min, I'm constructing an example
Reid | sabre: I think I finally understand
Reid | your comment that GEP *never* dereferences makes a lot of sense
Reid | it is only doing address calculation, so the first one is taking the address of the var
sabre | If C didn't conflate lvalues and rvalues, GEP would be much easier to understand for people
Reid | yeah
Yorion | so, for example: M=GEP A,B,C; N=GEP M,D,E; N = [ A + B*sizeof(struct) + fieldoffs(C) ]:(of type T) + D*sizeof(T) + fieldoffs(E)
Reid | I just remember learning a hard lesson about the difference between char* A and char A[] .. long time ago when I was learning C
sabre | of type T*
sabre | otherwise fine
Yorion | okay, I think I finally understand it
sabre | without the T* your D sizeof will be wrong
Yorion | a suggestion: the formula you gave above explains it all
Yorion | I'd suggest explaining it that way in documentation
sabre | That's not right though
sabre | it doesn't include arrays or packed types
sabre | so it is, at best, a half truth
Yorion | tell me, how to compute the fieldoffs for an index?
sabre | arrays can be in structs :)
Yorion | in bytes
sabre | idx * sizeof(arrayelt)
sabre | just like for pointers (the first index)
sabre | There are two cases: structs and sequentials
sabre | for sequentials you use idx*sizeof(sequenced type)
sabre | for structs you add their offset
sabre | it's really very simple :)
sabre | the first index of a gep is always over the pointer
Yorion | no I meant in LLVM, how do I convert the field offset into bytes?
sabre | which is why it's strange
sabre | if you only think about structs
sabre | TargetData::getFieldOffset
sabre | or something
sabre | look in SelectionDAGISel.cpp (visitGEP) as I suggested.
Yorion | do you still have the energy to go over sequential types? :-)
Yorion | what is the offset formula for sequential types?
Reid | we just went over that: idx * sizeof(elementType)
Yorion | so, if there's an array hidden somewhere in the struct, essentially I need to compute idx*sizeof() instead of fieldoffs() and that's it?
sabre | yes
Reid | yes
Yorion | excellent.
sabre | There are two cases: structs and sequentials
sabre | [9:17pm] sabre: for sequentials you use idx*sizeof(sequenced type)
sabre | [9:17pm] sabre: for structs you add their offset
sabre | [9:17pm] sabre: it's really very simple :)
Yorion | now when I understand it, it is simple...
Yorion | thx