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diff --git a/docs/GetElementPtr.rst b/docs/GetElementPtr.rst new file mode 100644 index 0000000..f6f904b --- /dev/null +++ b/docs/GetElementPtr.rst @@ -0,0 +1,538 @@ +.. _gep: + +======================================= +The Often Misunderstood GEP Instruction +======================================= + +.. contents:: + :local: + +Introduction +============ + +This document seeks to dispel the mystery and confusion surrounding LLVM's +`GetElementPtr <LangRef.html#i_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": + +.. 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 <i>must</i> 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 <LangRef.html#tbaa>`_. + +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 <LangRef.html#pointeraliasing>`_. + +* 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. |
