1======================================= 2The Often Misunderstood GEP Instruction 3======================================= 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11This document seeks to dispel the mystery and confusion surrounding LLVM's 12`GetElementPtr <LangRef.html#i_getelementptr>`_ (GEP) instruction. Questions 13about the wily GEP instruction are probably the most frequently occurring 14questions once a developer gets down to coding with LLVM. Here we lay out the 15sources of confusion and show that the GEP instruction is really quite simple. 16 17Address Computation 18=================== 19 20When people are first confronted with the GEP instruction, they tend to relate 21it to known concepts from other programming paradigms, most notably C array 22indexing and field selection. GEP closely resembles C array indexing and field 23selection, however it is a little different and this leads to the following 24questions. 25 26What is the first index of the GEP instruction? 27----------------------------------------------- 28 29Quick answer: The index stepping through the first operand. 30 31The confusion with the first index usually arises from thinking about the 32GetElementPtr instruction as if it was a C index operator. They aren't the 33same. For example, when we write, in "C": 34 35.. code-block:: c++ 36 37 AType *Foo; 38 ... 39 X = &Foo->F; 40 41it is natural to think that there is only one index, the selection of the field 42``F``. However, in this example, ``Foo`` is a pointer. That pointer 43must be indexed explicitly in LLVM. C, on the other hand, indices through it 44transparently. To arrive at the same address location as the C code, you would 45provide the GEP instruction with two index operands. The first operand indexes 46through the pointer; the second operand indexes the field ``F`` of the 47structure, just as if you wrote: 48 49.. code-block:: c++ 50 51 X = &Foo[0].F; 52 53Sometimes this question gets rephrased as: 54 55.. _GEP index through first pointer: 56 57 *Why is it okay to index through the first pointer, but subsequent pointers 58 won't be dereferenced?* 59 60The answer is simply because memory does not have to be accessed to perform the 61computation. The first operand to the GEP instruction must be a value of a 62pointer type. The value of the pointer is provided directly to the GEP 63instruction as an operand without any need for accessing memory. It must, 64therefore be indexed and requires an index operand. Consider this example: 65 66.. code-block:: c++ 67 68 struct munger_struct { 69 int f1; 70 int f2; 71 }; 72 void munge(struct munger_struct *P) { 73 P[0].f1 = P[1].f1 + P[2].f2; 74 } 75 ... 76 munger_struct Array[3]; 77 ... 78 munge(Array); 79 80In this "C" example, the front end compiler (Clang) will generate three GEP 81instructions for the three indices through "P" in the assignment statement. The 82function argument ``P`` will be the first operand of each of these GEP 83instructions. The second operand indexes through that pointer. The third 84operand will be the field offset into the ``struct munger_struct`` type, for 85either the ``f1`` or ``f2`` field. So, in LLVM assembly the ``munge`` function 86looks like: 87 88.. code-block:: llvm 89 90 void %munge(%struct.munger_struct* %P) { 91 entry: 92 %tmp = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 1, i32 0 93 %tmp = load i32* %tmp 94 %tmp6 = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 2, i32 1 95 %tmp7 = load i32* %tmp6 96 %tmp8 = add i32 %tmp7, %tmp 97 %tmp9 = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 0, i32 0 98 store i32 %tmp8, i32* %tmp9 99 ret void 100 } 101 102In each case the first operand is the pointer through which the GEP instruction 103starts. The same is true whether the first operand is an argument, allocated 104memory, or a global variable. 105 106To make this clear, let's consider a more obtuse example: 107 108.. code-block:: llvm 109 110 %MyVar = uninitialized global i32 111 ... 112 %idx1 = getelementptr i32, i32* %MyVar, i64 0 113 %idx2 = getelementptr i32, i32* %MyVar, i64 1 114 %idx3 = getelementptr i32, i32* %MyVar, i64 2 115 116These GEP instructions are simply making address computations from the base 117address of ``MyVar``. They compute, as follows (using C syntax): 118 119.. code-block:: c++ 120 121 idx1 = (char*) &MyVar + 0 122 idx2 = (char*) &MyVar + 4 123 idx3 = (char*) &MyVar + 8 124 125Since the type ``i32`` is known to be four bytes long, the indices 0, 1 and 2 126translate into memory offsets of 0, 4, and 8, respectively. No memory is 127accessed to make these computations because the address of ``%MyVar`` is passed 128directly to the GEP instructions. 129 130The obtuse part of this example is in the cases of ``%idx2`` and ``%idx3``. They 131result in the computation of addresses that point to memory past the end of the 132``%MyVar`` global, which is only one ``i32`` long, not three ``i32``\s long. 133While this is legal in LLVM, it is inadvisable because any load or store with 134the pointer that results from these GEP instructions would produce undefined 135results. 136 137Why is the extra 0 index required? 138---------------------------------- 139 140Quick answer: there are no superfluous indices. 141 142This question arises most often when the GEP instruction is applied to a global 143variable which is always a pointer type. For example, consider this: 144 145.. code-block:: llvm 146 147 %MyStruct = uninitialized global { float*, i32 } 148 ... 149 %idx = getelementptr { float*, i32 }, { float*, i32 }* %MyStruct, i64 0, i32 1 150 151The GEP above yields an ``i32*`` by indexing the ``i32`` typed field of the 152structure ``%MyStruct``. When people first look at it, they wonder why the ``i64 1530`` index is needed. However, a closer inspection of how globals and GEPs work 154reveals the need. Becoming aware of the following facts will dispel the 155confusion: 156 157#. The type of ``%MyStruct`` is *not* ``{ float*, i32 }`` but rather ``{ float*, 158 i32 }*``. That is, ``%MyStruct`` is a pointer to a structure containing a 159 pointer to a ``float`` and an ``i32``. 160 161#. Point #1 is evidenced by noticing the type of the first operand of the GEP 162 instruction (``%MyStruct``) which is ``{ float*, i32 }*``. 163 164#. The first index, ``i64 0`` is required to step over the global variable 165 ``%MyStruct``. Since the first argument to the GEP instruction must always 166 be a value of pointer type, the first index steps through that pointer. A 167 value of 0 means 0 elements offset from that pointer. 168 169#. The second index, ``i32 1`` selects the second field of the structure (the 170 ``i32``). 171 172What is dereferenced by GEP? 173---------------------------- 174 175Quick answer: nothing. 176 177The GetElementPtr instruction dereferences nothing. That is, it doesn't access 178memory in any way. That's what the Load and Store instructions are for. GEP is 179only involved in the computation of addresses. For example, consider this: 180 181.. code-block:: llvm 182 183 %MyVar = uninitialized global { [40 x i32 ]* } 184 ... 185 %idx = getelementptr { [40 x i32]* }, { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17 186 187In this example, we have a global variable, ``%MyVar`` that is a pointer to a 188structure containing a pointer to an array of 40 ints. The GEP instruction seems 189to be accessing the 18th integer of the structure's array of ints. However, this 190is actually an illegal GEP instruction. It won't compile. The reason is that the 191pointer in the structure *must* be dereferenced in order to index into the 192array of 40 ints. Since the GEP instruction never accesses memory, it is 193illegal. 194 195In order to access the 18th integer in the array, you would need to do the 196following: 197 198.. code-block:: llvm 199 200 %idx = getelementptr { [40 x i32]* }, { [40 x i32]* }* %, i64 0, i32 0 201 %arr = load [40 x i32]** %idx 202 %idx = getelementptr [40 x i32], [40 x i32]* %arr, i64 0, i64 17 203 204In this case, we have to load the pointer in the structure with a load 205instruction before we can index into the array. If the example was changed to: 206 207.. code-block:: llvm 208 209 %MyVar = uninitialized global { [40 x i32 ] } 210 ... 211 %idx = getelementptr { [40 x i32] }, { [40 x i32] }*, i64 0, i32 0, i64 17 212 213then everything works fine. In this case, the structure does not contain a 214pointer and the GEP instruction can index through the global variable, into the 215first field of the structure and access the 18th ``i32`` in the array there. 216 217Why don't GEP x,0,0,1 and GEP x,1 alias? 218---------------------------------------- 219 220Quick Answer: They compute different address locations. 221 222If you look at the first indices in these GEP instructions you find that they 223are different (0 and 1), therefore the address computation diverges with that 224index. Consider this example: 225 226.. code-block:: llvm 227 228 %MyVar = global { [10 x i32] } 229 %idx1 = getelementptr { [10 x i32] }, { [10 x i32] }* %MyVar, i64 0, i32 0, i64 1 230 %idx2 = getelementptr { [10 x i32] }, { [10 x i32] }* %MyVar, i64 1 231 232In this example, ``idx1`` computes the address of the second integer in the 233array that is in the structure in ``%MyVar``, that is ``MyVar+4``. The type of 234``idx1`` is ``i32*``. However, ``idx2`` computes the address of *the next* 235structure after ``%MyVar``. The type of ``idx2`` is ``{ [10 x i32] }*`` and its 236value is equivalent to ``MyVar + 40`` because it indexes past the ten 4-byte 237integers in ``MyVar``. Obviously, in such a situation, the pointers don't 238alias. 239 240Why do GEP x,1,0,0 and GEP x,1 alias? 241------------------------------------- 242 243Quick Answer: They compute the same address location. 244 245These two GEP instructions will compute the same address because indexing 246through the 0th element does not change the address. However, it does change the 247type. Consider this example: 248 249.. code-block:: llvm 250 251 %MyVar = global { [10 x i32] } 252 %idx1 = getelementptr { [10 x i32] }, { [10 x i32] }* %MyVar, i64 1, i32 0, i64 0 253 %idx2 = getelementptr { [10 x i32] }, { [10 x i32] }* %MyVar, i64 1 254 255In this example, the value of ``%idx1`` is ``%MyVar+40`` and its type is 256``i32*``. The value of ``%idx2`` is also ``MyVar+40`` but its type is ``{ [10 x 257i32] }*``. 258 259Can GEP index into vector elements? 260----------------------------------- 261 262This hasn't always been forcefully disallowed, though it's not recommended. It 263leads to awkward special cases in the optimizers, and fundamental inconsistency 264in the IR. In the future, it will probably be outright disallowed. 265 266What effect do address spaces have on GEPs? 267------------------------------------------- 268 269None, except that the address space qualifier on the first operand pointer type 270always matches the address space qualifier on the result type. 271 272How is GEP different from ``ptrtoint``, arithmetic, and ``inttoptr``? 273--------------------------------------------------------------------- 274 275It's very similar; there are only subtle differences. 276 277With ptrtoint, you have to pick an integer type. One approach is to pick i64; 278this is safe on everything LLVM supports (LLVM internally assumes pointers are 279never wider than 64 bits in many places), and the optimizer will actually narrow 280the i64 arithmetic down to the actual pointer size on targets which don't 281support 64-bit arithmetic in most cases. However, there are some cases where it 282doesn't do this. With GEP you can avoid this problem. 283 284Also, GEP carries additional pointer aliasing rules. It's invalid to take a GEP 285from one object, address into a different separately allocated object, and 286dereference it. IR producers (front-ends) must follow this rule, and consumers 287(optimizers, specifically alias analysis) benefit from being able to rely on 288it. See the `Rules`_ section for more information. 289 290And, GEP is more concise in common cases. 291 292However, for the underlying integer computation implied, there is no 293difference. 294 295 296I'm writing a backend for a target which needs custom lowering for GEP. How do I do this? 297----------------------------------------------------------------------------------------- 298 299You don't. The integer computation implied by a GEP is target-independent. 300Typically what you'll need to do is make your backend pattern-match expressions 301trees involving ADD, MUL, etc., which are what GEP is lowered into. This has the 302advantage of letting your code work correctly in more cases. 303 304GEP does use target-dependent parameters for the size and layout of data types, 305which targets can customize. 306 307If you require support for addressing units which are not 8 bits, you'll need to 308fix a lot of code in the backend, with GEP lowering being only a small piece of 309the overall picture. 310 311How does VLA addressing work with GEPs? 312--------------------------------------- 313 314GEPs don't natively support VLAs. LLVM's type system is entirely static, and GEP 315address computations are guided by an LLVM type. 316 317VLA indices can be implemented as linearized indices. For example, an expression 318like ``X[a][b][c]``, must be effectively lowered into a form like 319``X[a*m+b*n+c]``, so that it appears to the GEP as a single-dimensional array 320reference. 321 322This means if you want to write an analysis which understands array indices and 323you want to support VLAs, your code will have to be prepared to reverse-engineer 324the linearization. One way to solve this problem is to use the ScalarEvolution 325library, which always presents VLA and non-VLA indexing in the same manner. 326 327.. _Rules: 328 329Rules 330===== 331 332What happens if an array index is out of bounds? 333------------------------------------------------ 334 335There are two senses in which an array index can be out of bounds. 336 337First, there's the array type which comes from the (static) type of the first 338operand to the GEP. Indices greater than the number of elements in the 339corresponding static array type are valid. There is no problem with out of 340bounds indices in this sense. Indexing into an array only depends on the size of 341the array element, not the number of elements. 342 343A common example of how this is used is arrays where the size is not known. 344It's common to use array types with zero length to represent these. The fact 345that the static type says there are zero elements is irrelevant; it's perfectly 346valid to compute arbitrary element indices, as the computation only depends on 347the size of the array element, not the number of elements. Note that zero-sized 348arrays are not a special case here. 349 350This sense is unconnected with ``inbounds`` keyword. The ``inbounds`` keyword is 351designed to describe low-level pointer arithmetic overflow conditions, rather 352than high-level array indexing rules. 353 354Analysis passes which wish to understand array indexing should not assume that 355the static array type bounds are respected. 356 357The second sense of being out of bounds is computing an address that's beyond 358the actual underlying allocated object. 359 360With the ``inbounds`` keyword, the result value of the GEP is undefined if the 361address is outside the actual underlying allocated object and not the address 362one-past-the-end. 363 364Without the ``inbounds`` keyword, there are no restrictions on computing 365out-of-bounds addresses. Obviously, performing a load or a store requires an 366address of allocated and sufficiently aligned memory. But the GEP itself is only 367concerned with computing addresses. 368 369Can array indices be negative? 370------------------------------ 371 372Yes. This is basically a special case of array indices being out of bounds. 373 374Can I compare two values computed with GEPs? 375-------------------------------------------- 376 377Yes. If both addresses are within the same allocated object, or 378one-past-the-end, you'll get the comparison result you expect. If either is 379outside of it, integer arithmetic wrapping may occur, so the comparison may not 380be meaningful. 381 382Can I do GEP with a different pointer type than the type of the underlying object? 383---------------------------------------------------------------------------------- 384 385Yes. There are no restrictions on bitcasting a pointer value to an arbitrary 386pointer type. The types in a GEP serve only to define the parameters for the 387underlying integer computation. They need not correspond with the actual type of 388the underlying object. 389 390Furthermore, loads and stores don't have to use the same types as the type of 391the underlying object. Types in this context serve only to specify memory size 392and alignment. Beyond that there are merely a hint to the optimizer indicating 393how the value will likely be used. 394 395Can I cast an object's address to integer and add it to null? 396------------------------------------------------------------- 397 398You can compute an address that way, but if you use GEP to do the add, you can't 399use that pointer to actually access the object, unless the object is managed 400outside of LLVM. 401 402The underlying integer computation is sufficiently defined; null has a defined 403value --- zero --- and you can add whatever value you want to it. 404 405However, it's invalid to access (load from or store to) an LLVM-aware object 406with such a pointer. This includes ``GlobalVariables``, ``Allocas``, and objects 407pointed to by noalias pointers. 408 409If you really need this functionality, you can do the arithmetic with explicit 410integer instructions, and use inttoptr to convert the result to an address. Most 411of GEP's special aliasing rules do not apply to pointers computed from ptrtoint, 412arithmetic, and inttoptr sequences. 413 414Can I compute the distance between two objects, and add that value to one address to compute the other address? 415--------------------------------------------------------------------------------------------------------------- 416 417As with arithmetic on null, you can use GEP to compute an address that way, but 418you can't use that pointer to actually access the object if you do, unless the 419object is managed outside of LLVM. 420 421Also as above, ptrtoint and inttoptr provide an alternative way to do this which 422do not have this restriction. 423 424Can I do type-based alias analysis on LLVM IR? 425---------------------------------------------- 426 427You can't do type-based alias analysis using LLVM's built-in type system, 428because LLVM has no restrictions on mixing types in addressing, loads or stores. 429 430LLVM's type-based alias analysis pass uses metadata to describe a different type 431system (such as the C type system), and performs type-based aliasing on top of 432that. Further details are in the `language reference <LangRef.html#tbaa>`_. 433 434What happens if a GEP computation overflows? 435-------------------------------------------- 436 437If the GEP lacks the ``inbounds`` keyword, the value is the result from 438evaluating the implied two's complement integer computation. However, since 439there's no guarantee of where an object will be allocated in the address space, 440such values have limited meaning. 441 442If the GEP has the ``inbounds`` keyword, the result value is undefined (a "trap 443value") if the GEP overflows (i.e. wraps around the end of the address space). 444 445As such, there are some ramifications of this for inbounds GEPs: scales implied 446by array/vector/pointer indices are always known to be "nsw" since they are 447signed values that are scaled by the element size. These values are also 448allowed to be negative (e.g. "``gep i32 *%P, i32 -1``") but the pointer itself 449is logically treated as an unsigned value. This means that GEPs have an 450asymmetric relation between the pointer base (which is treated as unsigned) and 451the offset applied to it (which is treated as signed). The result of the 452additions within the offset calculation cannot have signed overflow, but when 453applied to the base pointer, there can be signed overflow. 454 455How can I tell if my front-end is following the rules? 456------------------------------------------------------ 457 458There is currently no checker for the getelementptr rules. Currently, the only 459way to do this is to manually check each place in your front-end where 460GetElementPtr operators are created. 461 462It's not possible to write a checker which could find all rule violations 463statically. It would be possible to write a checker which works by instrumenting 464the code with dynamic checks though. Alternatively, it would be possible to 465write a static checker which catches a subset of possible problems. However, no 466such checker exists today. 467 468Rationale 469========= 470 471Why is GEP designed this way? 472----------------------------- 473 474The design of GEP has the following goals, in rough unofficial order of 475priority: 476 477* Support C, C-like languages, and languages which can be conceptually lowered 478 into C (this covers a lot). 479 480* Support optimizations such as those that are common in C compilers. In 481 particular, GEP is a cornerstone of LLVM's `pointer aliasing 482 model <LangRef.html#pointeraliasing>`_. 483 484* Provide a consistent method for computing addresses so that address 485 computations don't need to be a part of load and store instructions in the IR. 486 487* Support non-C-like languages, to the extent that it doesn't interfere with 488 other goals. 489 490* Minimize target-specific information in the IR. 491 492Why do struct member indices always use ``i32``? 493------------------------------------------------ 494 495The specific type i32 is probably just a historical artifact, however it's wide 496enough for all practical purposes, so there's been no need to change it. It 497doesn't necessarily imply i32 address arithmetic; it's just an identifier which 498identifies a field in a struct. Requiring that all struct indices be the same 499reduces the range of possibilities for cases where two GEPs are effectively the 500same but have distinct operand types. 501 502What's an uglygep? 503------------------ 504 505Some LLVM optimizers operate on GEPs by internally lowering them into more 506primitive integer expressions, which allows them to be combined with other 507integer expressions and/or split into multiple separate integer expressions. If 508they've made non-trivial changes, translating back into LLVM IR can involve 509reverse-engineering the structure of the addressing in order to fit it into the 510static type of the original first operand. It isn't always possibly to fully 511reconstruct this structure; sometimes the underlying addressing doesn't 512correspond with the static type at all. In such cases the optimizer instead will 513emit a GEP with the base pointer casted to a simple address-unit pointer, using 514the name "uglygep". This isn't pretty, but it's just as valid, and it's 515sufficient to preserve the pointer aliasing guarantees that GEP provides. 516 517Summary 518======= 519 520In summary, here's some things to always remember about the GetElementPtr 521instruction: 522 523 524#. The GEP instruction never accesses memory, it only provides pointer 525 computations. 526 527#. The first operand to the GEP instruction is always a pointer and it must be 528 indexed. 529 530#. There are no superfluous indices for the GEP instruction. 531 532#. Trailing zero indices are superfluous for pointer aliasing, but not for the 533 types of the pointers. 534 535#. Leading zero indices are not superfluous for pointer aliasing nor the types 536 of the pointers. 537