The Often Misunderstood GEP Instruction

Introduction

This document seeks to dispel the mystery and confusion surrounding LLVM's - GetElementPtr (GEP) instruction. - Questions about the wily GEP instruction are - probably the most frequently occurring 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. -

Address Computation

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. GEP closely resembles C array indexing - and field selection, however it's is a little different and this leads to - the following questions.

- - -

- What is the first index of the GEP instruction? -

Quick answer: The index stepping through the first operand.

The confusion with the first index usually arises from thinking about - the GetElementPtr instruction as if it was a C index operator. They aren't the - same. For example, when we write, in "C":

- -

-AType *Foo;
-...
-X = &Foo->F;
-

- -

it is natural to think that there is only one index, the selection of the - field F. However, in this example, Foo is a pointer. That - pointer must be indexed explicitly in LLVM. C, on the other hand, indices - through it transparently. To arrive at the same address location as the C - code, you would provide the GEP instruction with two index operands. The - first operand indexes through the pointer; the second operand indexes the - field F of the structure, just as if you wrote:

- -

-X = &Foo[0].F;
-

- -

Sometimes this question gets rephrased as:

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 as an operand without any need for accessing memory. It - must, therefore be indexed and requires an index operand. Consider this - example:

- -

-struct munger_struct {
-  int f1;
-  int f2;
-};
-void munge(struct munger_struct *P) {
-  P[0].f1 = P[1].f1 + P[2].f2;
-}
-...
-munger_struct Array[3];
-...
-munge(Array);
-

- -

In this "C" example, the front end compiler (llvm-gcc) will generate three - GEP instructions for the three indices through "P" in the assignment - statement. The function argument P will be the first operand of each - of these GEP instructions. The second operand indexes through that pointer. - The third operand will be the field offset into the - struct munger_struct type, for either the f1 or - f2 field. So, in LLVM assembly the munge function looks - like:

- -

-void %munge(%struct.munger_struct* %P) {
-entry:
-  %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
-  %tmp = load i32* %tmp
-  %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
-  %tmp7 = load i32* %tmp6
-  %tmp8 = add i32 %tmp7, %tmp
-  %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
-  store i32 %tmp8, i32* %tmp9
-  ret void
-}
-

- -

In each case the first operand is the pointer through which the GEP - instruction starts. The same is true whether the first operand is an - argument, allocated memory, or a global variable.

To make this clear, let's consider a more obtuse example:

- -

-%MyVar = uninitialized global i32
-...
-%idx1 = getelementptr i32* %MyVar, i64 0
-%idx2 = getelementptr i32* %MyVar, i64 1
-%idx3 = getelementptr i32* %MyVar, i64 2
-

- -

These GEP instructions are simply making address computations from the - base address of MyVar. They compute, as follows (using C syntax): -

- -

-idx1 = (char*) &MyVar + 0
-idx2 = (char*) &MyVar + 4
-idx3 = (char*) &MyVar + 8
-

- -

Since the type i32 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.

The obtuse part of this example is in the cases of %idx2 and - %idx3. They result in the computation of addresses that point to - memory past the end of the %MyVar global, which is only one - i32 long, not three i32s long. While this is legal in LLVM, - it is inadvisable because any load or store with the pointer that results - from these GEP instructions would produce undefined results.

- - -

- 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*, i32 }
-...
-%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
-

- -

The GEP above yields an i32* by indexing the i32 typed - field of the structure %MyStruct. When people first look at it, they - wonder why the i64 0 index is needed. However, a closer inspection - of how globals and GEPs work reveals the need. Becoming aware of the following - facts will dispel the confusion:

The type of %MyStruct is not { float*, i32 } - but rather { float*, i32 }*. That is, %MyStruct is a - pointer to a structure containing a pointer to a float and an - i32.
Point #1 is evidenced by noticing the type of the first operand of - the GEP instruction (%MyStruct) which is - { float*, i32 }*.
The first index, i64 0 is required to step over the global - variable %MyStruct. Since the first argument to the GEP - instruction must always be a value of pointer type, the first index - steps through that pointer. A value of 0 means 0 elements offset from that - pointer.
The second index, i32 1 selects the second field of the - structure (the i32).

- - -

- 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 and Store instructions are for. - GEP is only involved in the computation of addresses. For example, consider - this:

- -

-%MyVar = uninitialized global { [40 x i32 ]* }
-...
-%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 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 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 i32]* }* %, i64 0, i32 0
-%arr = load [40 x i32]** %idx
-%idx = getelementptr [40 x i32]* %arr, i64 0, i64 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 i32 ] }
-...
-%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 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, - into the first field of the structure and access the 18th i32 in the - array there.

- - -

- 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 i32 ] }
-%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
-%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 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 i32*. However, - idx2 computes the address of the next structure after - %MyVar. The type of idx2 is { [10 x i32] }* 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 i32 ] }
-%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
-%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
-

- -

In this example, the value of %idx1 is %MyVar+40 and - its type is i32*. The value of %idx2 is also - MyVar+40 but its type is { [10 x i32] }*.

- - - -

- Can GEP index into vector elements? -

This hasn't always been forcefully disallowed, though it's not recommended. - It leads to awkward special cases in the optimizers, and fundamental - inconsistency in the IR. In the future, it will probably be outright - disallowed.

- -

- - - -

- 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.

- -

- - - -

- - 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. See the Rules section for more - information.

- -

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.

- -

- - - -

- 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.

- -

Rules

- - -

- 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.

- -

LLVM's type-based alias analysis pass uses metadata to describe a different - type system (such as the C type system), and performs type-based aliasing - on top of that. Further details are in the - language reference.

- -

- - - -

- What happens if a GEP computation overflows? -

If the GEP lacks the inbounds keyword, the 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.

- -

If the GEP has the inbounds keyword, the result value is - undefined (a "trap value") if the GEP - overflows (i.e. wraps around the end of the address space).

- -

As such, there are some ramifications of this for inbounds GEPs: scales - implied by array/vector/pointer indices are always known to be "nsw" since - they are signed values that are scaled by the element size. These values - are also allowed to be negative (e.g. "gep i32 *%P, i32 -1") but the - pointer itself is logically treated as an unsigned value. This means that - GEPs have an asymmetric relation between the pointer base (which is treated - as unsigned) and the offset applied to it (which is treated as signed). The - result of the additions within the offset calculation cannot have signed - overflow, but when applied to the base pointer, there can be signed - overflow. -

- - -

- - - -

- - How can I tell if my front-end is following the rules? - -

There is currently no checker for the getelementptr rules. Currently, - the only way to do this is to manually check each place in your front-end - where GetElementPtr operators are created.

- -

It's not possible to write a checker which could find all rule - violations statically. It would be possible to write a checker which - works by instrumenting the code with dynamic checks though. Alternatively, - it would be possible to write a static checker which catches a subset of - possible problems. However, no such checker exists today.

- -

Rationale

- - -

- 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. In particular, GEP is a cornerstone of LLVM's - pointer aliasing model.
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.

- - -

- 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.

- -

- - - -

- 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.

- -

Summary

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

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

`_ (GEP) instruction. Questions +about the wily GEP instruction are probably the most frequently occurring +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. + +Address Computation +=================== + +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. GEP closely resembles C array indexing and field +selection, however it's is a little different and this leads to the following +questions. + +What is the first index of the GEP instruction? +----------------------------------------------- + +Quick answer: The index stepping through the first operand. + +The confusion with the first index usually arises from thinking about the +GetElementPtr instruction as if it was a C index operator. They aren't the +same. For example, when we write, in "C": + +.. code-block:: c++ + + AType *Foo; + ... + X = &Foo->F; + +it is natural to think that there is only one index, the selection of the field +``F``. However, in this example, ``Foo`` is a pointer. That pointer +must be indexed explicitly in LLVM. C, on the other hand, indices through it +transparently. To arrive at the same address location as the C code, you would +provide the GEP instruction with two index operands. The first operand indexes +through the pointer; the second operand indexes the field ``F`` of the +structure, just as if you wrote: + +.. code-block:: c++ + + X = &Foo[0].F; + +Sometimes this question gets rephrased as: + +.. _GEP index through first pointer: + + *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 as an operand without any need for accessing memory. It must, +therefore be indexed and requires an index operand. Consider this example: + +.. code-block:: c++ + + struct munger_struct { + int f1; + int f2; + }; + void munge(struct munger_struct *P) { + P[0].f1 = P[1].f1 + P[2].f2; + } + ... + munger_struct Array[3]; + ... + munge(Array); + +In this "C" example, the front end compiler (llvm-gcc) will generate three GEP +instructions for the three indices through "P" in the assignment statement. The +function argument ``P`` will be the first operand of each of these GEP +instructions. The second operand indexes through that pointer. The third +operand will be the field offset into the ``struct munger_struct`` type, for +either the ``f1`` or ``f2`` field. So, in LLVM assembly the ``munge`` function +looks like: + +.. code-block:: llvm + + void %munge(%struct.munger_struct* %P) { + entry: + %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0 + %tmp = load i32* %tmp + %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1 + %tmp7 = load i32* %tmp6 + %tmp8 = add i32 %tmp7, %tmp + %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0 + store i32 %tmp8, i32* %tmp9 + ret void + } + +In each case the first operand is the pointer through which the GEP instruction +starts. The same is true whether the first operand is an argument, allocated +memory, or a global variable. + +To make this clear, let's consider a more obtuse example: + +.. code-block:: llvm + + %MyVar = uninitialized global i32 + ... + %idx1 = getelementptr i32* %MyVar, i64 0 + %idx2 = getelementptr i32* %MyVar, i64 1 + %idx3 = getelementptr i32* %MyVar, i64 2 + +These GEP instructions are simply making address computations from the base +address of ``MyVar``. They compute, as follows (using C syntax): + +.. code-block:: c++ + + idx1 = (char*) &MyVar + 0 + idx2 = (char*) &MyVar + 4 + idx3 = (char*) &MyVar + 8 + +Since the type ``i32`` 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. + +The obtuse part of this example is in the cases of ``%idx2`` and ``%idx3``. They +result in the computation of addresses that point to memory past the end of the +``%MyVar`` global, which is only one ``i32`` long, not three ``i32``\s long. +While this is legal in LLVM, it is inadvisable because any load or store with +the pointer that results from these GEP instructions would produce undefined +results. + +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: + +.. code-block:: llvm + + %MyStruct = uninitialized global { float*, i32 } + ... + %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1 + +The GEP above yields an ``i32*`` by indexing the ``i32`` typed field of the +structure ``%MyStruct``. When people first look at it, they wonder why the ``i64 +0`` index is needed. However, a closer inspection of how globals and GEPs work +reveals the need. Becoming aware of the following facts will dispel the +confusion: + +#. The type of ``%MyStruct`` is *not* ``{ float*, i32 }`` but rather ``{ float*, + i32 }*``. That is, ``%MyStruct`` is a pointer to a structure containing a + pointer to a ``float`` and an ``i32``. + +#. Point #1 is evidenced by noticing the type of the first operand of the GEP + instruction (``%MyStruct``) which is ``{ float*, i32 }*``. + +#. The first index, ``i64 0`` is required to step over the global variable + ``%MyStruct``. Since the first argument to the GEP instruction must always + be a value of pointer type, the first index steps through that pointer. A + value of 0 means 0 elements offset from that pointer. + +#. The second index, ``i32 1`` selects the second field of the structure (the + ``i32``). + +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 and Store instructions are for. GEP is +only involved in the computation of addresses. For example, consider this: + +.. code-block:: llvm + + %MyVar = uninitialized global { [40 x i32 ]* } + ... + %idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 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 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: + +.. code-block:: llvm + + %idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0 + %arr = load [40 x i32]** %idx + %idx = getelementptr [40 x i32]* %arr, i64 0, i64 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: + +.. code-block:: llvm + + %MyVar = uninitialized global { [40 x i32 ] } + ... + %idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 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, into the +first field of the structure and access the 18th ``i32`` in the array there. + +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: + +.. code-block:: llvm + + %MyVar = global { [10 x i32 ] } + %idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1 + %idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 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 ``i32*``. However, ``idx2`` computes the address of *the next* +structure after ``%MyVar``. The type of ``idx2`` is ``{ [10 x i32] }*`` 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: + +.. code-block:: llvm + + %MyVar = global { [10 x i32 ] } + %idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0 + %idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1 + +In this example, the value of ``%idx1`` is ``%MyVar+40`` and its type is +``i32*``. The value of ``%idx2`` is also ``MyVar+40`` but its type is ``{ [10 x +i32] }*``. + +Can GEP index into vector elements? +----------------------------------- + +This hasn't always been forcefully disallowed, though it's not recommended. It +leads to awkward special cases in the optimizers, and fundamental inconsistency +in the IR. In the future, it will probably be outright disallowed. + +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. + +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. See the `Rules`_ section for more information. + +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. + +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. + +.. _Rules: + +Rules +===== + +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. + +LLVM's type-based alias analysis pass uses metadata to describe a different type +system (such as the C type system), and performs type-based aliasing on top of +that. Further details are in the `language reference `_. + +What happens if a GEP computation overflows? +-------------------------------------------- + +If the GEP lacks the ``inbounds`` keyword, the 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. + +If the GEP has the ``inbounds`` keyword, the result value is undefined (a "trap +value") if the GEP overflows (i.e. wraps around the end of the address space). + +As such, there are some ramifications of this for inbounds GEPs: scales implied +by array/vector/pointer indices are always known to be "nsw" since they are +signed values that are scaled by the element size. These values are also +allowed to be negative (e.g. "``gep i32 *%P, i32 -1``") but the pointer itself +is logically treated as an unsigned value. This means that GEPs have an +asymmetric relation between the pointer base (which is treated as unsigned) and +the offset applied to it (which is treated as signed). The result of the +additions within the offset calculation cannot have signed overflow, but when +applied to the base pointer, there can be signed overflow. + +How can I tell if my front-end is following the rules? +------------------------------------------------------ + +There is currently no checker for the getelementptr rules. Currently, the only +way to do this is to manually check each place in your front-end where +GetElementPtr operators are created. + +It's not possible to write a checker which could find all rule violations +statically. It would be possible to write a checker which works by instrumenting +the code with dynamic checks though. Alternatively, it would be possible to +write a static checker which catches a subset of possible problems. However, no +such checker exists today. + +Rationale +========= + +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. In + particular, GEP is a cornerstone of LLVM's `pointer aliasing + model `_. + +* 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. + +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. + +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. + +Summary +======= + +In summary, here's some things to always remember about the GetElementPtr +instruction: + + +#. The GEP instruction never accesses memory, it only provides pointer + computations. + +#. The first operand to the GEP instruction is always a pointer and it must be + indexed. + +#. There are no superfluous indices for the GEP instruction. + +#. Trailing zero indices are superfluous for pointer aliasing, but not for the + types of the pointers. + +#. Leading zero indices are not superfluous for pointer aliasing nor the types + of the pointers. diff --git a/docs/design_and_overview.rst b/docs/design_and_overview.rst index c272fbfb74..ea684155e0 100644 --- a/docs/design_and_overview.rst +++ b/docs/design_and_overview.rst @@ -3,6 +3,11 @@ LLVM Design & Overview ====================== +.. toctree:: + :hidden: + + GetElementPtr + * `LLVM Language Reference Manual `_ Defines the LLVM intermediate representation. @@ -25,7 +30,7 @@ LLVM Design & Overview More details (quite old now). -* `GetElementPtr FAQ `_ +* :ref:`gep` Answers to some very frequent questions about LLVM's most frequently misunderstood instruction. -- cgit v1.2.3

- The Often Misunderstood GEP Instruction -

Introduction

Address Computation

- What is the first index of the GEP instruction? -

- Why is the extra 0 index required? -

- What is dereferenced by GEP? -

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

- Why do GEP x,1,0,0 and GEP x,1 alias? -

- Can GEP index into vector elements? -

- What effect do address spaces have on GEPs? -

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

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

- How does VLA addressing work with GEPs? -

Rules

- What happens if an array index is out of bounds? -

- Can array indices be negative? -

- Can I compare two values computed with GEPs? -

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

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

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

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

- What happens if a GEP computation overflows? -

- - How can I tell if my front-end is following the rules? - -

Rationale

- Why is GEP designed this way? -

- Why do struct member indices always use i32? -

- What's an uglygep? -

Summary