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  14 <h1>
  15   The Often Misunderstood GEP Instruction
  16 </h1>
  17
  18 <ol>
  19   <li><a href="#intro">Introduction</a></li>
  20   <li><a href="#addresses">Address Computation</a>
  21   <ol>
  22     <li><a href="#extra_index">Why is the extra 0 index required?</a></li>
  23     <li><a href="#deref">What is dereferenced by GEP?</a></li>
  24     <li><a href="#firstptr">Why can you index through the first pointer but not
  25       subsequent ones?</a></li>
  26     <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
  27     <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
  28     <li><a href="#vectors">Can GEP index into vector elements?</a>
  29     <li><a href="#addrspace">What effect do address spaces have on GEPs?</a>
  30     <li><a href="#int">How is GEP different from ptrtoint, arithmetic, and inttoptr?</a></li>
  31     <li><a href="#be">I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?</a>
  32     <li><a href="#vla">How does VLA addressing work with GEPs?</a>
  33   </ol></li>
  34   <li><a href="#rules">Rules</a>
  35   <ol>
  36     <li><a href="#bounds">What happens if an array index is out of bounds?</a>
  37     <li><a href="#negative">Can array indices be negative?</a>
  38     <li><a href="#compare">Can I compare two values computed with GEPs?</a>
  39     <li><a href="#types">Can I do GEP with a different pointer type than the type of the underlying object?</a>
  40     <li><a href="#null">Can I cast an object's address to integer and add it to null?</a>
  41     <li><a href="#ptrdiff">Can I compute the distance between two objects, and add that value to one address to compute the other address?</a>
  42     <li><a href="#tbaa">Can I do type-based alias analysis on LLVM IR?</a>
  43     <li><a href="#overflow">What happens if a GEP computation overflows?</a>
  44     <li><a href="#check">How can I tell if my front-end is following the rules?</a>
  45   </ol></li>
  46   <li><a href="#rationale">Rationale</a>
  47   <ol>
  48     <li><a href="#goals">Why is GEP designed this way?</a></li>
  49     <li><a href="#i32">Why do struct member indices always use i32?</a></li>
  50     <li><a href="#uglygep">What's an uglygep?</a>
  51   </ol></li>
  52   <li><a href="#summary">Summary</a></li>
  53 </ol>
  54
  55 <div class="doc_author">
  56   <p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
  57 </div>
  58
  59
  60 <!-- *********************************************************************** -->
  61 <h2><a name="intro">Introduction</a></h2>
  62 <!-- *********************************************************************** -->
  63
  64 <div>
  65   <p>This document seeks to dispel the mystery and confusion surrounding LLVM's
  66   <a href="LangRef.html#i_getelementptr">GetElementPtr</a> (GEP) instruction.
  67   Questions about the wily GEP instruction are
  68   probably the most frequently occurring questions once a developer gets down to
  69   coding with LLVM. Here we lay out the sources of confusion and show that the
  70   GEP instruction is really quite simple.
  71   </p>
  72 </div>
  73
  74 <!-- *********************************************************************** -->
  75 <h2><a name="addresses">Address Computation</a></h2>
  76 <!-- *********************************************************************** -->
  77 <div>
  78   <p>When people are first confronted with the GEP instruction, they tend to
  79   relate it to known concepts from other programming paradigms, most notably C
  80   array indexing and field selection. GEP closely resembles C array indexing
  81   and field selection, however it's is a little different and this leads to
  82   the following questions.</p>
  83
  84 <!-- *********************************************************************** -->
  85 <h3>
  86   <a name="firstptr">What is the first index of the GEP instruction?</a>
  87 </h3>
  88 <div>
  89   <p>Quick answer: The index stepping through the first operand.</p>
  90   <p>The confusion with the first index usually arises from thinking about
  91   the GetElementPtr instruction as if it was a C index operator. They aren't the
  92   same. For example, when we write, in "C":</p>
  93
  94 <div class="doc_code">
  95 <pre>
  96 AType *Foo;
  97 ...
  98 X = &amp;Foo-&gt;F;
  99 </pre>
 100 </div>
 101
 102   <p>it is natural to think that there is only one index, the selection of the
 103   field <tt>F</tt>.  However, in this example, <tt>Foo</tt> is a pointer. That
 104   pointer must be indexed explicitly in LLVM. C, on the other hand, indices
 105   through it transparently.  To arrive at the same address location as the C
 106   code, you would provide the GEP instruction with two index operands. The
 107   first operand indexes through the pointer; the second operand indexes the
 108   field <tt>F</tt> of the structure, just as if you wrote:</p>
 109
 110 <div class="doc_code">
 111 <pre>
 112 X = &amp;Foo[0].F;
 113 </pre>
 114 </div>
 115
 116   <p>Sometimes this question gets rephrased as:</p>
 117   <blockquote><p><i>Why is it okay to index through the first pointer, but
 118       subsequent pointers won't be dereferenced?</i></p></blockquote>
 119   <p>The answer is simply because memory does not have to be accessed to
 120   perform the computation. The first operand to the GEP instruction must be a
 121   value of a pointer type. The value of the pointer is provided directly to
 122   the GEP instruction as an operand without any need for accessing memory. It
 123   must, therefore be indexed and requires an index operand. Consider this
 124   example:</p>
 125
 126 <div class="doc_code">
 127 <pre>
 128 struct munger_struct {
 129   int f1;
 130   int f2;
 131 };
 132 void munge(struct munger_struct *P) {
 133   P[0].f1 = P[1].f1 + P[2].f2;
 134 }
 135 ...
 136 munger_struct Array[3];
 137 ...
 138 munge(Array);
 139 </pre>
 140 </div>
 141
 142   <p>In this "C" example, the front end compiler (llvm-gcc) will generate three
 143   GEP instructions for the three indices through "P" in the assignment
 144   statement.  The function argument <tt>P</tt> will be the first operand of each
 145   of these GEP instructions.  The second operand indexes through that pointer.
 146   The third operand will be the field offset into the
 147   <tt>struct munger_struct</tt> type,  for either the <tt>f1</tt> or
 148   <tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
 149   like:</p>
 150
 151 <div class="doc_code">
 152 <pre>
 153 void %munge(%struct.munger_struct* %P) {
 154 entry:
 155   %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
 156   %tmp = load i32* %tmp
 157   %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
 158   %tmp7 = load i32* %tmp6
 159   %tmp8 = add i32 %tmp7, %tmp
 160   %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
 161   store i32 %tmp8, i32* %tmp9
 162   ret void
 163 }
 164 </pre>
 165 </div>
 166
 167   <p>In each case the first operand is the pointer through which the GEP
 168   instruction starts. The same is true whether the first operand is an
 169   argument, allocated memory, or a global variable. </p>
 170   <p>To make this clear, let's consider a more obtuse example:</p>
 171
 172 <div class="doc_code">
 173 <pre>
 174 %MyVar = uninitialized global i32
 175 ...
 176 %idx1 = getelementptr i32* %MyVar, i64 0
 177 %idx2 = getelementptr i32* %MyVar, i64 1
 178 %idx3 = getelementptr i32* %MyVar, i64 2
 179 </pre>
 180 </div>
 181
 182   <p>These GEP instructions are simply making address computations from the
 183   base address of <tt>MyVar</tt>.  They compute, as follows (using C syntax):
 184   </p>
 185
 186 <div class="doc_code">
 187 <pre>
 188 idx1 = (char*) &amp;MyVar + 0
 189 idx2 = (char*) &amp;MyVar + 4
 190 idx3 = (char*) &amp;MyVar + 8
 191 </pre>
 192 </div>
 193
 194   <p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
 195   0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
 196   memory is accessed to make these computations because the address of
 197   <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
 198   <p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
 199   <tt>%idx3</tt>. They result in the computation of addresses that point to
 200   memory past the end of the <tt>%MyVar</tt> global, which is only one
 201   <tt>i32</tt> long, not three <tt>i32</tt>s long.  While this is legal in LLVM,
 202   it is inadvisable because any load or store with the pointer that results
 203   from these GEP instructions would produce undefined results.</p>
 204 </div>
 205
 206 <!-- *********************************************************************** -->
 207 <h3>
 208   <a name="extra_index">Why is the extra 0 index required?</a>
 209 </h3>
 210 <!-- *********************************************************************** -->
 211 <div>
 212   <p>Quick answer: there are no superfluous indices.</p>
 213   <p>This question arises most often when the GEP instruction is applied to a
 214   global variable which is always a pointer type. For example, consider
 215   this:</p>
 216
 217 <div class="doc_code">
 218 <pre>
 219 %MyStruct = uninitialized global { float*, i32 }
 220 ...
 221 %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
 222 </pre>
 223 </div>
 224
 225   <p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
 226   field of the structure <tt>%MyStruct</tt>. When people first look at it, they
 227   wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
 228   of how globals and GEPs work reveals the need. Becoming aware of the following
 229   facts will dispel the confusion:</p>
 230   <ol>
 231     <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
 232     but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
 233     pointer to a structure containing a pointer to a <tt>float</tt> and an
 234     <tt>i32</tt>.</li>
 235     <li>Point #1 is evidenced by noticing the type of the first operand of
 236     the GEP instruction (<tt>%MyStruct</tt>) which is
 237     <tt>{ float*, i32 }*</tt>.</li>
 238     <li>The first index, <tt>i64 0</tt> is required to step over the global
 239     variable <tt>%MyStruct</tt>.  Since the first argument to the GEP
 240     instruction must always be a value of pointer type, the first index
 241     steps through that pointer. A value of 0 means 0 elements offset from that
 242     pointer.</li>
 243     <li>The second index, <tt>i32 1</tt> selects the second field of the
 244     structure (the <tt>i32</tt>). </li>
 245   </ol>
 246 </div>
 247
 248 <!-- *********************************************************************** -->
 249 <h3>
 250   <a name="deref">What is dereferenced by GEP?</a>
 251 </h3>
 252 <div>
 253   <p>Quick answer: nothing.</p>
 254   <p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
 255   access memory in any way. That's what the Load and Store instructions are for.
 256   GEP is only involved in the computation of addresses. For example, consider
 257   this:</p>
 258
 259 <div class="doc_code">
 260 <pre>
 261 %MyVar = uninitialized global { [40 x i32 ]* }
 262 ...
 263 %idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
 264 </pre>
 265 </div>
 266
 267   <p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
 268   pointer to a structure containing a pointer to an array of 40 ints. The
 269   GEP instruction seems to be accessing the 18th integer of the structure's
 270   array of ints. However, this is actually an illegal GEP instruction. It
 271   won't compile. The reason is that the pointer in the structure <i>must</i>
 272   be dereferenced in order to index into the array of 40 ints. Since the
 273   GEP instruction never accesses memory, it is illegal.</p>
 274   <p>In order to access the 18th integer in the array, you would need to do the
 275   following:</p>
 276
 277 <div class="doc_code">
 278 <pre>
 279 %idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
 280 %arr = load [40 x i32]** %idx
 281 %idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
 282 </pre>
 283 </div>
 284
 285   <p>In this case, we have to load the pointer in the structure with a load
 286   instruction before we can index into the array. If the example was changed
 287   to:</p>
 288
 289 <div class="doc_code">
 290 <pre>
 291 %MyVar = uninitialized global { [40 x i32 ] }
 292 ...
 293 %idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
 294 </pre>
 295 </div>
 296
 297   <p>then everything works fine. In this case, the structure does not contain a
 298   pointer and the GEP instruction can index through the global variable,
 299   into the first field of the structure and access the 18th <tt>i32</tt> in the
 300   array there.</p>
 301 </div>
 302
 303 <!-- *********************************************************************** -->
 304 <h3>
 305   <a name="lead0">Why don't GEP x,0,0,1 and GEP x,1 alias?</a>
 306 </h3>
 307 <div>
 308   <p>Quick Answer: They compute different address locations.</p>
 309   <p>If you look at the first indices in these GEP
 310   instructions you find that they are different (0 and 1), therefore the address
 311   computation diverges with that index. Consider this example:</p>
 312
 313 <div class="doc_code">
 314 <pre>
 315 %MyVar = global { [10 x i32 ] }
 316 %idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
 317 %idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
 318 </pre>
 319 </div>
 320
 321   <p>In this example, <tt>idx1</tt> computes the address of the second integer
 322   in the array that is in the structure in <tt>%MyVar</tt>, that is
 323   <tt>MyVar+4</tt>. The type of <tt>idx1</tt> is <tt>i32*</tt>. However,
 324   <tt>idx2</tt> computes the address of <i>the next</i> structure after
 325   <tt>%MyVar</tt>. The type of <tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its
 326   value is equivalent to <tt>MyVar + 40</tt> because it indexes past the ten
 327   4-byte integers in <tt>MyVar</tt>. Obviously, in such a situation, the
 328   pointers don't alias.</p>
 329
 330 </div>
 331
 332 <!-- *********************************************************************** -->
 333 <h3>
 334   <a name="trail0">Why do GEP x,1,0,0 and GEP x,1 alias?</a>
 335 </h3>
 336 <div>
 337   <p>Quick Answer: They compute the same address location.</p>
 338   <p>These two GEP instructions will compute the same address because indexing
 339   through the 0th element does not change the address. However, it does change
 340   the type. Consider this example:</p>
 341
 342 <div class="doc_code">
 343 <pre>
 344 %MyVar = global { [10 x i32 ] }
 345 %idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
 346 %idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
 347 </pre>
 348 </div>
 349
 350   <p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
 351   its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
 352   <tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
 353 </div>
 354
 355 <!-- *********************************************************************** -->
 356
 357 <h3>
 358   <a name="vectors">Can GEP index into vector elements?</a>
 359 </h3>
 360 <div>
 361   <p>This hasn't always been forcefully disallowed, though it's not recommended.
 362      It leads to awkward special cases in the optimizers, and fundamental
 363      inconsistency in the IR. In the future, it will probably be outright
 364      disallowed.</p>
 365
 366 </div>
 367
 368 <!-- *********************************************************************** -->
 369
 370 <h3>
 371   <a name="addrspace">What effect do address spaces have on GEPs?</a>
 372 </h3>
 373 <div>
 374    <p>None, except that the address space qualifier on the first operand pointer
 375       type always matches the address space qualifier on the result type.</p>
 376
 377 </div>
 378
 379 <!-- *********************************************************************** -->
 380
 381 <h3>
 382   <a name="int">
 383     How is GEP different from ptrtoint, arithmetic, and inttoptr?
 384   </a>
 385 </h3>
 386 <div>
 387   <p>It's very similar; there are only subtle differences.</p>
 388
 389   <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
 390      this is safe on everything LLVM supports (LLVM internally assumes pointers
 391      are never wider than 64 bits in many places), and the optimizer will actually
 392      narrow the i64 arithmetic down to the actual pointer size on targets which
 393      don't support 64-bit arithmetic in most cases. However, there are some cases
 394      where it doesn't do this. With GEP you can avoid this problem.
 395
 396   <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
 397      GEP from one object, address into a different separately allocated
 398      object, and dereference it. IR producers (front-ends) must follow this rule,
 399      and consumers (optimizers, specifically alias analysis) benefit from being
 400      able to rely on it. See the <a href="#rules">Rules</a> section for more
 401      information.</p>
 402
 403   <p>And, GEP is more concise in common cases.</p>
 404
 405   <p>However, for the underlying integer computation implied, there
 406      is no difference.</p>
 407
 408 </div>
 409
 410 <!-- *********************************************************************** -->
 411
 412 <h3>
 413   <a name="be">
 414     I'm writing a backend for a target which needs custom lowering for GEP.
 415     How do I do this?
 416   </a>
 417 </h3>
 418 <div>
 419   <p>You don't. The integer computation implied by a GEP is target-independent.
 420      Typically what you'll need to do is make your backend pattern-match
 421      expressions trees involving ADD, MUL, etc., which are what GEP is lowered
 422      into. This has the advantage of letting your code work correctly in more
 423      cases.</p>
 424
 425   <p>GEP does use target-dependent parameters for the size and layout of data
 426      types, which targets can customize.</p>
 427
 428   <p>If you require support for addressing units which are not 8 bits, you'll
 429      need to fix a lot of code in the backend, with GEP lowering being only a
 430      small piece of the overall picture.</p>
 431
 432 </div>
 433
 434 <!-- *********************************************************************** -->
 435
 436 <h3>
 437   <a name="vla">How does VLA addressing work with GEPs?</a>
 438 </h3>
 439 <div>
 440   <p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
 441      and GEP address computations are guided by an LLVM type.</p>
 442
 443   <p>VLA indices can be implemented as linearized indices. For example, an
 444      expression like X[a][b][c], must be effectively lowered into a form
 445      like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
 446      array reference.</p>
 447
 448   <p>This means if you want to write an analysis which understands array
 449      indices and you want to support VLAs, your code will have to be
 450      prepared to reverse-engineer the linearization. One way to solve this
 451      problem is to use the ScalarEvolution library, which always presents
 452      VLA and non-VLA indexing in the same manner.</p>
 453 </div>
 454
 455 </div>
 456
 457 <!-- *********************************************************************** -->
 458 <h2><a name="rules">Rules</a></h2>
 459 <!-- *********************************************************************** -->
 460 <div>
 461 <!-- *********************************************************************** -->
 462
 463 <h3>
 464   <a name="bounds">What happens if an array index is out of bounds?</a>
 465 </h3>
 466 <div>
 467   <p>There are two senses in which an array index can be out of bounds.</p>
 468
 469   <p>First, there's the array type which comes from the (static) type of
 470      the first operand to the GEP. Indices greater than the number of elements
 471      in the corresponding static array type are valid. There is no problem with
 472      out of bounds indices in this sense. Indexing into an array only depends
 473      on the size of the array element, not the number of elements.</p>
 474
 475   <p>A common example of how this is used is arrays where the size is not known.
 476      It's common to use array types with zero length to represent these. The
 477      fact that the static type says there are zero elements is irrelevant; it's
 478      perfectly valid to compute arbitrary element indices, as the computation
 479      only depends on the size of the array element, not the number of
 480      elements. Note that zero-sized arrays are not a special case here.</p>
 481
 482   <p>This sense is unconnected with <tt>inbounds</tt> keyword. The
 483      <tt>inbounds</tt> keyword is designed to describe low-level pointer
 484      arithmetic overflow conditions, rather than high-level array
 485      indexing rules.
 486
 487   <p>Analysis passes which wish to understand array indexing should not
 488      assume that the static array type bounds are respected.</p>
 489
 490   <p>The second sense of being out of bounds is computing an address that's
 491      beyond the actual underlying allocated object.</p>
 492
 493   <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
 494      undefined if the address is outside the actual underlying allocated
 495      object and not the address one-past-the-end.</p>
 496
 497   <p>Without the <tt>inbounds</tt> keyword, there are no restrictions
 498      on computing out-of-bounds addresses. Obviously, performing a load or
 499      a store requires an address of allocated and sufficiently aligned
 500      memory. But the GEP itself is only concerned with computing addresses.</p>
 501
 502 </div>
 503
 504 <!-- *********************************************************************** -->
 505 <h3>
 506   <a name="negative">Can array indices be negative?</a>
 507 </h3>
 508 <div>
 509   <p>Yes. This is basically a special case of array indices being out
 510      of bounds.</p>
 511
 512 </div>
 513
 514 <!-- *********************************************************************** -->
 515 <h3>
 516   <a name="compare">Can I compare two values computed with GEPs?</a>
 517 </h3>
 518 <div>
 519   <p>Yes. If both addresses are within the same allocated object, or
 520      one-past-the-end, you'll get the comparison result you expect. If either
 521      is outside of it, integer arithmetic wrapping may occur, so the
 522      comparison may not be meaningful.</p>
 523
 524 </div>
 525
 526 <!-- *********************************************************************** -->
 527 <h3>
 528   <a name="types">
 529     Can I do GEP with a different pointer type than the type of
 530     the underlying object?
 531   </a>
 532 </h3>
 533 <div>
 534   <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
 535      pointer type. The types in a GEP serve only to define the parameters for the
 536      underlying integer computation. They need not correspond with the actual
 537      type of the underlying object.</p>
 538
 539   <p>Furthermore, loads and stores don't have to use the same types as the type
 540      of the underlying object. Types in this context serve only to specify
 541      memory size and alignment. Beyond that there are merely a hint to the
 542      optimizer indicating how the value will likely be used.</p>
 543
 544 </div>
 545
 546 <!-- *********************************************************************** -->
 547 <h3>
 548   <a name="null">
 549     Can I cast an object's address to integer and add it to null?
 550   </a>
 551 </h3>
 552 <div>
 553   <p>You can compute an address that way, but if you use GEP to do the add,
 554      you can't use that pointer to actually access the object, unless the
 555      object is managed outside of LLVM.</p>
 556
 557   <p>The underlying integer computation is sufficiently defined; null has a
 558      defined value -- zero -- and you can add whatever value you want to it.</p>
 559
 560   <p>However, it's invalid to access (load from or store to) an LLVM-aware
 561      object with such a pointer. This includes GlobalVariables, Allocas, and
 562      objects pointed to by noalias pointers.</p>
 563
 564   <p>If you really need this functionality, you can do the arithmetic with
 565      explicit integer instructions, and use inttoptr to convert the result to
 566      an address. Most of GEP's special aliasing rules do not apply to pointers
 567      computed from ptrtoint, arithmetic, and inttoptr sequences.</p>
 568
 569 </div>
 570
 571 <!-- *********************************************************************** -->
 572 <h3>
 573   <a name="ptrdiff">
 574     Can I compute the distance between two objects, and add
 575     that value to one address to compute the other address?
 576   </a>
 577 </h3>
 578 <div>
 579   <p>As with arithmetic on null, You can use GEP to compute an address that
 580      way, but you can't use that pointer to actually access the object if you
 581      do, unless the object is managed outside of LLVM.</p>
 582
 583   <p>Also as above, ptrtoint and inttoptr provide an alternative way to do this
 584      which do not have this restriction.</p>
 585
 586 </div>
 587
 588 <!-- *********************************************************************** -->
 589 <h3>
 590   <a name="tbaa">Can I do type-based alias analysis on LLVM IR?</a>
 591 </h3>
 592 <div>
 593   <p>You can't do type-based alias analysis using LLVM's built-in type system,
 594      because LLVM has no restrictions on mixing types in addressing, loads or
 595      stores.</p>
 596
 597   <p>It would be possible to add special annotations to the IR, probably using
 598      metadata, to describe a different type system (such as the C type system),
 599      and do type-based aliasing on top of that. This is a much bigger
 600      undertaking though.</p>
 601
 602 </div>
 603
 604 <!-- *********************************************************************** -->
 605
 606 <h3>
 607   <a name="overflow">What happens if a GEP computation overflows?</a>
 608 </h3>
 609 <div>
 610    <p>If the GEP lacks the <tt>inbounds</tt> keyword, the value is the result
 611       from evaluating the implied two's complement integer computation. However,
 612       since there's no guarantee of where an object will be allocated in the
 613       address space, such values have limited meaning.</p>
 614
 615   <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
 616      undefined (a "<a href="LangRef.html#trapvalues">trap value</a>") if the GEP
 617      overflows (i.e. wraps around the end of the address space).</p>
 618
 619   <p>As such, there are some ramifications of this for inbounds GEPs: scales
 620      implied by array/vector/pointer indices are always known to be "nsw" since
 621      they are signed values that are scaled by the element size.  These values
 622      are also allowed to be negative (e.g. "gep i32 *%P, i32 -1") but the
 623      pointer itself is logically treated as an unsigned value.  This means that
 624      GEPs have an asymmetric relation between the pointer base (which is treated
 625      as unsigned) and the offset applied to it (which is treated as signed). The
 626      result of the additions within the offset calculation cannot have signed
 627      overflow, but when applied to the base pointer, there can be signed
 628      overflow.
 629   </p>
 630
 631
 632 </div>
 633
 634 <!-- *********************************************************************** -->
 635
 636 <h3>
 637   <a name="check">
 638     How can I tell if my front-end is following the rules?
 639   </a>
 640 </h3>
 641 <div>
 642    <p>There is currently no checker for the getelementptr rules. Currently,
 643       the only way to do this is to manually check each place in your front-end
 644       where GetElementPtr operators are created.</p>
 645
 646    <p>It's not possible to write a checker which could find all rule
 647       violations statically. It would be possible to write a checker which
 648       works by instrumenting the code with dynamic checks though. Alternatively,
 649       it would be possible to write a static checker which catches a subset of
 650       possible problems. However, no such checker exists today.</p>
 651
 652 </div>
 653
 654 </div>
 655
 656 <!-- *********************************************************************** -->
 657 <h2><a name="rationale">Rationale</a></h2>
 658 <!-- *********************************************************************** -->
 659 <div>
 660 <!-- *********************************************************************** -->
 661
 662 <h3>
 663   <a name="goals">Why is GEP designed this way?</a>
 664 </h3>
 665 <div>
 666    <p>The design of GEP has the following goals, in rough unofficial
 667       order of priority:</p>
 668    <ul>
 669      <li>Support C, C-like languages, and languages which can be
 670          conceptually lowered into C (this covers a lot).</li>
 671      <li>Support optimizations such as those that are common in
 672          C compilers. In particular, GEP is a cornerstone of LLVM's
 673          <a href="LangRef.html#pointeraliasing">pointer aliasing model</a>.</li>
 674      <li>Provide a consistent method for computing addresses so that
 675          address computations don't need to be a part of load and
 676          store instructions in the IR.</li>
 677      <li>Support non-C-like languages, to the extent that it doesn't
 678          interfere with other goals.</li>
 679      <li>Minimize target-specific information in the IR.</li>
 680    </ul>
 681 </div>
 682
 683 <!-- *********************************************************************** -->
 684 <h3>
 685   <a name="i32">Why do struct member indices always use i32?</a>
 686 </h3>
 687 <div>
 688   <p>The specific type i32 is probably just a historical artifact, however it's
 689      wide enough for all practical purposes, so there's been no need to change it.
 690      It doesn't necessarily imply i32 address arithmetic; it's just an identifier
 691      which identifies a field in a struct. Requiring that all struct indices be
 692      the same reduces the range of possibilities for cases where two GEPs are
 693      effectively the same but have distinct operand types.</p>
 694
 695 </div>
 696
 697 <!-- *********************************************************************** -->
 698
 699 <h3>
 700   <a name="uglygep">What's an uglygep?</a>
 701 </h3>
 702 <div>
 703   <p>Some LLVM optimizers operate on GEPs by internally lowering them into
 704      more primitive integer expressions, which allows them to be combined
 705      with other integer expressions and/or split into multiple separate
 706      integer expressions. If they've made non-trivial changes, translating
 707      back into LLVM IR can involve reverse-engineering the structure of
 708      the addressing in order to fit it into the static type of the original
 709      first operand. It isn't always possibly to fully reconstruct this
 710      structure; sometimes the underlying addressing doesn't correspond with
 711      the static type at all. In such cases the optimizer instead will emit
 712      a GEP with the base pointer casted to a simple address-unit pointer,
 713      using the name "uglygep". This isn't pretty, but it's just as
 714      valid, and it's sufficient to preserve the pointer aliasing guarantees
 715      that GEP provides.</p>
 716
 717 </div>
 718
 719 </div>
 720
 721 <!-- *********************************************************************** -->
 722 <h2><a name="summary">Summary</a></h2>
 723 <!-- *********************************************************************** -->
 724
 725 <div>
 726   <p>In summary, here's some things to always remember about the GetElementPtr
 727   instruction:</p>
 728   <ol>
 729     <li>The GEP instruction never accesses memory, it only provides pointer
 730     computations.</li>
 731     <li>The first operand to the GEP instruction is always a pointer and it must
 732     be indexed.</li>
 733     <li>There are no superfluous indices for the GEP instruction.</li>
 734     <li>Trailing zero indices are superfluous for pointer aliasing, but not for
 735     the types of the pointers.</li>
 736     <li>Leading zero indices are not superfluous for pointer aliasing nor the
 737     types of the pointers.</li>
 738   </ol>
 739 </div>
 740
 741 <!-- *********************************************************************** -->
 742
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