docs/BitCodeFormat.html

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  10 <div class="doc_title"> LLVM Bitcode File Format </div>
  11 <ol>
  12   <li><a href="#abstract">Abstract</a></li>
  13   <li><a href="#overview">Overview</a></li>
  14   <li><a href="#bitstream">Bitstream Format</a>
  15     <ol>
  16     <li><a href="#magic">Magic Numbers</a></li>
  17     <li><a href="#primitives">Primitives</a></li>
  18     <li><a href="#abbrevid">Abbreviation IDs</a></li>
  19     <li><a href="#blocks">Blocks</a></li>
  20     <li><a href="#datarecord">Data Records</a></li>
  21     <li><a href="#abbreviations">Abbreviations</a></li>
  22     <li><a href="#stdblocks">Standard Blocks</a></li>
  23     </ol>
  24   </li>
  25   <li><a href="#llvmir">LLVM IR Encoding</a>
  26     <ol>
  27     <li><a href="#basics">Basics</a></li>
  28     </ol>
  29   </li>
  30 </ol>
  31 <div class="doc_author">
  32   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>.
  33 </p>
  34 </div>
  35
  36 <!-- *********************************************************************** -->
  37 <div class="doc_section"> <a name="abstract">Abstract</a></div>
  38 <!-- *********************************************************************** -->
  39
  40 <div class="doc_text">
  41
  42 <p>This document describes the LLVM bitstream file format and the encoding of
  43 the LLVM IR into it.</p>
  44
  45 </div>
  46
  47 <!-- *********************************************************************** -->
  48 <div class="doc_section"> <a name="overview">Overview</a></div>
  49 <!-- *********************************************************************** -->
  50
  51 <div class="doc_text">
  52
  53 <p>
  54 What is commonly known as the LLVM bitcode file format (also, sometimes
  55 anachronistically known as bytecode) is actually two things: a <a
  56 href="#bitstream">bitstream container format</a>
  57 and an <a href="#llvmir">encoding of LLVM IR</a> into the container format.</p>
  58
  59 <p>
  60 The bitstream format is an abstract encoding of structured data, very
  61 similar to XML in some ways.  Like XML, bitstream files contain tags, and nested
  62 structures, and you can parse the file without having to understand the tags.
  63 Unlike XML, the bitstream format is a binary encoding, and unlike XML it
  64 provides a mechanism for the file to self-describe "abbreviations", which are
  65 effectively size optimizations for the content.</p>
  66
  67 <p>This document first describes the LLVM bitstream format, then describes the
  68 record structure used by LLVM IR files.
  69 </p>
  70
  71 </div>
  72
  73 <!-- *********************************************************************** -->
  74 <div class="doc_section"> <a name="bitstream">Bitstream Format</a></div>
  75 <!-- *********************************************************************** -->
  76
  77 <div class="doc_text">
  78
  79 <p>
  80 The bitstream format is literally a stream of bits, with a very simple
  81 structure.  This structure consists of the following concepts:
  82 </p>
  83
  84 <ul>
  85 <li>A "<a href="#magic">magic number</a>" that identifies the contents of
  86     the stream.</li>
  87 <li>Encoding <a href="#primitives">primitives</a> like variable bit-rate
  88     integers.</li>
  89 <li><a href="#blocks">Blocks</a>, which define nested content.</li>
  90 <li><a href="#datarecord">Data Records</a>, which describe entities within the
  91     file.</li>
  92 <li>Abbreviations, which specify compression optimizations for the file.</li>
  93 </ul>
  94
  95 <p>Note that the <a
  96 href="CommandGuide/html/llvm-bcanalyzer.html">llvm-bcanalyzer</a> tool can be
  97 used to dump and inspect arbitrary bitstreams, which is very useful for
  98 understanding the encoding.</p>
  99
 100 </div>
 101
 102 <!-- ======================================================================= -->
 103 <div class="doc_subsection"><a name="magic">Magic Numbers</a>
 104 </div>
 105
 106 <div class="doc_text">
 107
 108 <p>The first four bytes of the stream identify the encoding of the file.  This
 109 is used by a reader to know what is contained in the file.</p>
 110
 111 </div>
 112
 113 <!-- ======================================================================= -->
 114 <div class="doc_subsection"><a name="primitives">Primitives</a>
 115 </div>
 116
 117 <div class="doc_text">
 118
 119 <p>
 120 A bitstream literally consists of a stream of bits.  This stream is made up of a
 121 number of primitive values that encode a stream of unsigned integer values.
 122 These
 123 integers are are encoded in two ways: either as <a href="#fixedwidth">Fixed
 124 Width Integers</a> or as <a href="#variablewidth">Variable Width
 125 Integers</a>.
 126 </p>
 127
 128 </div>
 129
 130 <!-- _______________________________________________________________________ -->
 131 <div class="doc_subsubsection"> <a name="fixedwidth">Fixed Width Integers</a>
 132 </div>
 133
 134 <div class="doc_text">
 135
 136 <p>Fixed-width integer values have their low bits emitted directly to the file.
 137    For example, a 3-bit integer value encodes 1 as 001.  Fixed width integers
 138    are used when there are a well-known number of options for a field.  For
 139    example, boolean values are usually encoded with a 1-bit wide integer.
 140 </p>
 141
 142 </div>
 143
 144 <!-- _______________________________________________________________________ -->
 145 <div class="doc_subsubsection"> <a name="variablewidth">Variable Width
 146 Integers</a></div>
 147
 148 <div class="doc_text">
 149
 150 <p>Variable-width integer (VBR) values encode values of arbitrary size,
 151 optimizing for the case where the values are small.  Given a 4-bit VBR field,
 152 any 3-bit value (0 through 7) is encoded directly, with the high bit set to
 153 zero.  Values larger than N-1 bits emit their bits in a series of N-1 bit
 154 chunks, where all but the last set the high bit.</p>
 155
 156 <p>For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a
 157 vbr4 value.  The first set of four bits indicates the value 3 (011) with a
 158 continuation piece (indicated by a high bit of 1).  The next word indicates a
 159 value of 24 (011 << 3) with no continuation.  The sum (3+24) yields the value
 160 27.
 161 </p>
 162
 163 </div>
 164
 165 <!-- _______________________________________________________________________ -->
 166 <div class="doc_subsubsection"> <a name="char6">6-bit characters</a></div>
 167
 168 <div class="doc_text">
 169
 170 <p>6-bit characters encode common characters into a fixed 6-bit field.  They
 171 represent the following characters with the following 6-bit values:</p>
 172
 173 <ul>
 174 <li>'a' .. 'z' - 0 .. 25</li>
 175 <li>'A' .. 'Z' - 26 .. 52</li>
 176 <li>'0' .. '9' - 53 .. 61</li>
 177 <li>'.' - 62</li>
 178 <li>'_' - 63</li>
 179 </ul>
 180
 181 <p>This encoding is only suitable for encoding characters and strings that
 182 consist only of the above characters.  It is completely incapable of encoding
 183 characters not in the set.</p>
 184
 185 </div>
 186
 187 <!-- _______________________________________________________________________ -->
 188 <div class="doc_subsubsection"> <a name="wordalign">Word Alignment</a></div>
 189
 190 <div class="doc_text">
 191
 192 <p>Occasionally, it is useful to emit zero bits until the bitstream is a
 193 multiple of 32 bits.  This ensures that the bit position in the stream can be
 194 represented as a multiple of 32-bit words.</p>
 195
 196 </div>
 197
 198
 199 <!-- ======================================================================= -->
 200 <div class="doc_subsection"><a name="abbrevid">Abbreviation IDs</a>
 201 </div>
 202
 203 <div class="doc_text">
 204
 205 <p>
 206 A bitstream is a sequential series of <a href="#blocks">Blocks</a> and
 207 <a href="#datarecord">Data Records</a>.  Both of these start with an
 208 abbreviation ID encoded as a fixed-bitwidth field.  The width is specified by
 209 the current block, as described below.  The value of the abbreviation ID
 210 specifies either a builtin ID (which have special meanings, defined below) or
 211 one of the abbreviation IDs defined by the stream itself.
 212 </p>
 213
 214 <p>
 215 The set of builtin abbrev IDs is:
 216 </p>
 217
 218 <ul>
 219 <li>0 - <a href="#END_BLOCK">END_BLOCK</a> - This abbrev ID marks the end of the
 220     current block.</li>
 221 <li>1 - <a href="#ENTER_SUBBLOCK">ENTER_SUBBLOCK</a> - This abbrev ID marks the
 222     beginning of a new block.</li>
 223 <li>2 - <a href="#DEFINE_ABBREV">DEFINE_ABBREV</a> - This defines a new
 224     abbreviation.</li>
 225 <li>3 - <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a> - This ID specifies the
 226     definition of an unabbreviated record.</li>
 227 </ul>
 228
 229 <p>Abbreviation IDs 4 and above are defined by the stream itself, and specify
 230 an <a href="#abbrev_records">abbreviated record encoding</a>.</p>
 231
 232 </div>
 233
 234 <!-- ======================================================================= -->
 235 <div class="doc_subsection"><a name="blocks">Blocks</a>
 236 </div>
 237
 238 <div class="doc_text">
 239
 240 <p>
 241 Blocks in a bitstream denote nested regions of the stream, and are identified by
 242 a content-specific id number (for example, LLVM IR uses an ID of 12 to represent
 243 function bodies).  Nested blocks capture the hierachical structure of the data
 244 encoded in it, and various properties are associated with blocks as the file is
 245 parsed.  Block definitions allow the reader to efficiently skip blocks
 246 in constant time if the reader wants a summary of blocks, or if it wants to
 247 efficiently skip data they do not understand.  The LLVM IR reader uses this
 248 mechanism to skip function bodies, lazily reading them on demand.
 249 </p>
 250
 251 <p>
 252 When reading and encoding the stream, several properties are maintained for the
 253 block.  In particular, each block maintains:
 254 </p>
 255
 256 <ol>
 257 <li>A current abbrev id width.  This value starts at 2, and is set every time a
 258     block record is entered.  The block entry specifies the abbrev id width for
 259     the body of the block.</li>
 260
 261 <li>A set of abbreviations.  Abbreviations may be defined within a block, or
 262     they may be associated with all blocks of a particular ID.
 263 </li>
 264 </ol>
 265
 266 <p>As sub blocks are entered, these properties are saved and the new sub-block
 267 has its own set of abbreviations, and its own abbrev id width.  When a sub-block
 268 is popped, the saved values are restored.</p>
 269
 270 </div>
 271
 272 <!-- _______________________________________________________________________ -->
 273 <div class="doc_subsubsection"> <a name="ENTER_SUBBLOCK">ENTER_SUBBLOCK
 274 Encoding</a></div>
 275
 276 <div class="doc_text">
 277
 278 <p><tt>[ENTER_SUBBLOCK, blockid<sub>vbr8</sub>, newabbrevlen<sub>vbr4</sub>,
 279      &lt;align32bits&gt;, blocklen<sub>32</sub>]</tt></p>
 280
 281 <p>
 282 The ENTER_SUBBLOCK abbreviation ID specifies the start of a new block record.
 283 The <tt>blockid</tt> value is encoded as a 8-bit VBR identifier, and indicates
 284 the type of block being entered (which is application specific).  The
 285 <tt>newabbrevlen</tt> value is a 4-bit VBR which specifies the
 286 abbrev id width for the sub-block.  The <tt>blocklen</tt> is a 32-bit aligned
 287 value that specifies the size of the subblock, in 32-bit words.  This value
 288 allows the reader to skip over the entire block in one jump.
 289 </p>
 290
 291 </div>
 292
 293 <!-- _______________________________________________________________________ -->
 294 <div class="doc_subsubsection"> <a name="END_BLOCK">END_BLOCK
 295 Encoding</a></div>
 296
 297 <div class="doc_text">
 298
 299 <p><tt>[END_BLOCK, &lt;align32bits&gt;]</tt></p>
 300
 301 <p>
 302 The END_BLOCK abbreviation ID specifies the end of the current block record.
 303 Its end is aligned to 32-bits to ensure that the size of the block is an even
 304 multiple of 32-bits.</p>
 305
 306 </div>
 307
 308
 309
 310 <!-- ======================================================================= -->
 311 <div class="doc_subsection"><a name="datarecord">Data Records</a>
 312 </div>
 313
 314 <div class="doc_text">
 315 <p>
 316 Data records consist of a record code and a number of (up to) 64-bit integer
 317 values.  The interpretation of the code and values is application specific and
 318 there are multiple different ways to encode a record (with an unabbrev record
 319 or with an abbreviation).  In the LLVM IR format, for example, there is a record
 320 which encodes the target triple of a module.  The code is MODULE_CODE_TRIPLE,
 321 and the values of the record are the ascii codes for the characters in the
 322 string.</p>
 323
 324 </div>
 325
 326 <!-- _______________________________________________________________________ -->
 327 <div class="doc_subsubsection"> <a name="UNABBREV_RECORD">UNABBREV_RECORD
 328 Encoding</a></div>
 329
 330 <div class="doc_text">
 331
 332 <p><tt>[UNABBREV_RECORD, code<sub>vbr6</sub>, numops<sub>vbr6</sub>,
 333        op0<sub>vbr6</sub>, op1<sub>vbr6</sub>, ...]</tt></p>
 334
 335 <p>An UNABBREV_RECORD provides a default fallback encoding, which is both
 336 completely general and also extremely inefficient.  It can describe an arbitrary
 337 record, by emitting the code and operands as vbrs.</p>
 338
 339 <p>For example, emitting an LLVM IR target triple as an unabbreviated record
 340 requires emitting the UNABBREV_RECORD abbrevid, a vbr6 for the
 341 MODULE_CODE_TRIPLE code, a vbr6 for the length of the string (which is equal to
 342 the number of operands), and a vbr6 for each character.  Since there are no
 343 letters with value less than 32, each letter would need to be emitted as at
 344 least a two-part VBR, which means that each letter would require at least 12
 345 bits.  This is not an efficient encoding, but it is fully general.</p>
 346
 347 </div>
 348
 349 <!-- _______________________________________________________________________ -->
 350 <div class="doc_subsubsection"> <a name="abbrev_records">Abbreviated Record
 351 Encoding</a></div>
 352
 353 <div class="doc_text">
 354
 355 <p><tt>[&lt;abbrevid&gt;, fields...]</tt></p>
 356
 357 <p>An abbreviated record is a abbreviation id followed by a set of fields that
 358 are encoded according to the <a href="#abbreviations">abbreviation
 359 definition</a>.  This allows records to be encoded significantly more densely
 360 than records encoded with the <a href="#UNABBREV_RECORD">UNABBREV_RECORD</a>
 361 type, and allows the abbreviation types to be specified in the stream itself,
 362 which allows the files to be completely self describing.  The actual encoding
 363 of abbreviations is defined below.
 364 </p>
 365
 366 </div>
 367
 368 <!-- ======================================================================= -->
 369 <div class="doc_subsection"><a name="abbreviations">Abbreviations</a>
 370 </div>
 371
 372 <div class="doc_text">
 373 <p>
 374 Abbreviations are an important form of compression for bitstreams.  The idea is
 375 to specify a dense encoding for a class of records once, then use that encoding
 376 to emit many records.  It takes space to emit the encoding into the file, but
 377 the space is recouped (hopefully plus some) when the records that use it are
 378 emitted.
 379 </p>
 380
 381 <p>
 382 Abbreviations can be determined dynamically per client, per file.  Since the
 383 abbreviations are stored in the bitstream itself, different streams of the same
 384 format can contain different sets of abbreviations if the specific stream does
 385 not need it.  As a concrete example, LLVM IR files usually emit an abbreviation
 386 for binary operators.  If a specific LLVM module contained no or few binary
 387 operators, the abbreviation does not need to be emitted.
 388 </p>
 389 </div>
 390
 391 <!-- _______________________________________________________________________ -->
 392 <div class="doc_subsubsection"><a name="DEFINE_ABBREV">DEFINE_ABBREV
 393  Encoding</a></div>
 394
 395 <div class="doc_text">
 396
 397 <p><tt>[DEFINE_ABBREV, numabbrevops<sub>vbr5</sub>, abbrevop0, abbrevop1,
 398  ...]</tt></p>
 399
 400 <p>An abbreviation definition consists of the DEFINE_ABBREV abbrevid followed
 401 by a VBR that specifies the number of abbrev operands, then the abbrev
 402 operands themselves.  Abbreviation operands come in three forms.  They all start
 403 with a single bit that indicates whether the abbrev operand is a literal operand
 404 (when the bit is 1) or an encoding operand (when the bit is 0).</p>
 405
 406 <ol>
 407 <li>Literal operands - <tt>[1<sub>1</sub>, litvalue<sub>vbr8</sub>]</tt> -
 408 Literal operands specify that the value in the result
 409 is always a single specific value.  This specific value is emitted as a vbr8
 410 after the bit indicating that it is a literal operand.</li>
 411 <li>Encoding info without data - <tt>[0<sub>1</sub>, encoding<sub>3</sub>]</tt>
 412  - Operand encodings that do not have extra data are just emitted as their code.
 413 </li>
 414 <li>Encoding info with data - <tt>[0<sub>1</sub>, encoding<sub>3</sub>,
 415 value<sub>vbr5</sub>]</tt> - Operand encodings that do have extra data are
 416 emitted as their code, followed by the extra data.
 417 </li>
 418 </ol>
 419
 420 <p>The possible operand encodings are:</p>
 421
 422 <ul>
 423 <li>1 - Fixed - The field should be emitted as a <a
 424     href="#fixedwidth">fixed-width value</a>, whose width
 425     is specified by the encoding operand.</li>
 426 <li>2 - VBR - The field should be emitted as a <a
 427     href="#variablewidth">variable-width value</a>, whose width
 428     is specified by the encoding operand.</li>
 429 <li>3 - Array - This field is an array of values.  The element type of the array
 430     is specified by the next encoding operand.</li>
 431 <li>4 - Char6 - This field should be emitted as a <a href="#char6">char6-encoded
 432     value</a>.</li>
 433 </ul>
 434
 435 <p>For example, target triples in LLVM modules are encoded as a record of the
 436 form <tt>[TRIPLE, 'a', 'b', 'c', 'd']</tt>.  Consider if the bitstream emitted
 437 the following abbrev entry:</p>
 438
 439 <ul>
 440 <li><tt>[0, Fixed, 4]</tt></li>
 441 <li><tt>[0, Array]</tt></li>
 442 <li><tt>[0, Char6]</tt></li>
 443 </ul>
 444
 445 <p>When emitting a record with this abbreviation, the above entry would be
 446 emitted as:</p>
 447
 448 <p><tt>[4<sub>abbrevwidth</sub>, 2<sub>4</sub>, 4<sub>vbr6</sub>,
 449    0<sub>6</sub>, 1<sub>6</sub>, 2<sub>6</sub>, 3<sub>6</sub>]</tt></p>
 450
 451 <p>These values are:</p>
 452
 453 <ol>
 454 <li>The first value, 4, is the abbreviation ID for this abbreviation.</li>
 455 <li>The second value, 2, is the code for TRIPLE in LLVM IR files.</li>
 456 <li>The third value, 4, is the length of the array.</li>
 457 <li>The rest of the values are the char6 encoded values for "abcd".</li>
 458 </ol>
 459
 460 <p>With this abbreviation, the triple is emitted with only 37 bits (assuming a
 461 abbrev id width of 3).  Without the abbreviation, significantly more space would
 462 be required to emit the target triple.  Also, since the TRIPLE value is not
 463 emitted as a literal in the abbreviation, the abbreviation can also be used for
 464 any other string value.
 465 </p>
 466
 467 </div>
 468
 469 <!-- ======================================================================= -->
 470 <div class="doc_subsection"><a name="stdblocks">Standard Blocks</a>
 471 </div>
 472
 473 <div class="doc_text">
 474
 475 <p>
 476 In addition to the basic block structure and record encodings, the bitstream
 477 also defines specific builtin block types.  These block types specify how the
 478 stream is to be decoded or other metadata.  In the future, new standard blocks
 479 may be added.
 480 </p>
 481
 482 </div>
 483
 484 <!-- _______________________________________________________________________ -->
 485 <div class="doc_subsubsection"><a name="BLOCKINFO">#0 - BLOCKINFO
 486 Block</a></div>
 487
 488 <div class="doc_text">
 489
 490 <p>The BLOCKINFO block allows the description of metadata for other blocks.  The
 491   currently specified records are:</p>
 492
 493 <ul>
 494 <li><tt>[SETBID (#1), blockid]</tt></li>
 495 <li><tt>[DEFINE_ABBREV, ...]</tt></li>
 496 </ul>
 497
 498 <p>
 499 The SETBID record indicates which block ID is being described.  The standard
 500 DEFINE_ABBREV record specifies an abbreviation.  The abbreviation is associated
 501 with the record ID, and any records with matching ID automatically get the
 502 abbreviation.
 503 </p>
 504
 505 </div>
 506
 507 <!-- *********************************************************************** -->
 508 <div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
 509 <!-- *********************************************************************** -->
 510
 511 <div class="doc_text">
 512
 513 <p>LLVM IR is encoded into a bitstream by defining blocks and records.  It uses
 514 blocks for things like constant pools, functions, symbol tables, etc.  It uses
 515 records for things like instructions, global variable descriptors, type
 516 descriptions, etc.  This document does not describe the set of abbreviations
 517 that the writer uses, as these are fully self-described in the file, and the
 518 reader is not allowed to build in any knowledge of this.</p>
 519
 520 </div>
 521
 522 <!-- ======================================================================= -->
 523 <div class="doc_subsection"><a name="basics">Basics</a>
 524 </div>
 525
 526 <!-- _______________________________________________________________________ -->
 527 <div class="doc_subsubsection"><a name="ir_magic">LLVM IR Magic Number</a></div>
 528
 529 <div class="doc_text">
 530
 531 <p>
 532 The magic number for LLVM IR files is:
 533 </p>
 534
 535 <p><tt>['B'<sub>8</sub>, 'C'<sub>8</sub>, 0x0<sub>4</sub>, 0xC<sub>4</sub>,
 536 0xE<sub>4</sub>, 0xD<sub>4</sub>]</tt></p>
 537
 538 <p>When viewed as bytes, this is "BC 0xC0DE".</p>
 539
 540 </div>
 541
 542 <!-- _______________________________________________________________________ -->
 543 <div class="doc_subsubsection"><a name="ir_signed_vbr">Signed VBRs</a></div>
 544
 545 <div class="doc_text">
 546
 547 <p>
 548 <a href="#variablewidth">Variable Width Integers</a> are an efficient way to
 549 encode arbitrary sized unsigned values, but is an extremely inefficient way to
 550 encode signed values (as signed values are otherwise treated as maximally large
 551 unsigned values).</p>
 552
 553 <p>As such, signed vbr values of a specific width are emitted as follows:</p>
 554
 555 <ul>
 556 <li>Positive values are emitted as vbrs of the specified width, but with their
 557     value shifted left by one.</li>
 558 <li>Negative values are emitted as vbrs of the specified width, but the negated
 559     value is shifted left by one, and the low bit is set.</li>
 560 </ul>
 561
 562 <p>With this encoding, small positive and small negative values can both be
 563 emitted efficiently.</p>
 564
 565 </div>
 566
 567
 568 <!-- _______________________________________________________________________ -->
 569 <div class="doc_subsubsection"><a name="ir_blocks">LLVM IR Blocks</a></div>
 570
 571 <div class="doc_text">
 572
 573 <p>
 574 LLVM IR is defined with the following blocks:
 575 </p>
 576
 577 <ul>
 578 <li>8  - MODULE_BLOCK - This is the top-level block that contains the
 579     entire module, and describes a variety of per-module information.</li>
 580 <li>9  - PARAMATTR_BLOCK - This enumerates the parameter attributes.</li>
 581 <li>10 - TYPE_BLOCK - This describes all of the types in the module.</li>
 582 <li>11 - CONSTANTS_BLOCK - This describes constants for a module or
 583     function.</li>
 584 <li>12 - FUNCTION_BLOCK - This describes a function body.</li>
 585 <li>13 - TYPE_SYMTAB_BLOCK - This describes the type symbol table.</li>
 586 <li>14 - VALUE_SYMTAB_BLOCK - This describes a value symbol table.</li>
 587 </ul>
 588
 589 </div>
 590
 591 <!-- ======================================================================= -->
 592 <div class="doc_subsection"><a name="MODULE_BLOCK">MODULE_BLOCK Contents</a>
 593 </div>
 594
 595 <div class="doc_text">
 596
 597 <p>
 598 </p>
 599
 600 </div>
 601
 602
 603 <!-- *********************************************************************** -->
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