aboutsummaryrefslogtreecommitdiffstats
path: root/docs/BitCodeFormat.rst
blob: 34485b538c54861fda1611c4e763089bfb2ebc28 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
.. role:: raw-html(raw)
   :format: html

========================
LLVM Bitcode File Format
========================

.. contents::
   :local:

Abstract
========

This document describes the LLVM bitstream file format and the encoding of the
LLVM IR into it.

Overview
========

What is commonly known as the LLVM bitcode file format (also, sometimes
anachronistically known as bytecode) is actually two things: a `bitstream
container format`_ and an `encoding of LLVM IR`_ into the container format.

The bitstream format is an abstract encoding of structured data, very similar to
XML in some ways.  Like XML, bitstream files contain tags, and nested
structures, and you can parse the file without having to understand the tags.
Unlike XML, the bitstream format is a binary encoding, and unlike XML it
provides a mechanism for the file to self-describe "abbreviations", which are
effectively size optimizations for the content.

LLVM IR files may be optionally embedded into a `wrapper`_ structure, or in a
`native object file`_. Both of these mechanisms make it easy to embed extra
data along with LLVM IR files.

This document first describes the LLVM bitstream format, describes the wrapper
format, then describes the record structure used by LLVM IR files.

.. _bitstream container format:

Bitstream Format
================

The bitstream format is literally a stream of bits, with a very simple
structure.  This structure consists of the following concepts:

* A "`magic number`_" that identifies the contents of the stream.

* Encoding `primitives`_ like variable bit-rate integers.

* `Blocks`_, which define nested content.

* `Data Records`_, which describe entities within the file.

* Abbreviations, which specify compression optimizations for the file.

Note that the :doc:`llvm-bcanalyzer <CommandGuide/llvm-bcanalyzer>` tool can be
used to dump and inspect arbitrary bitstreams, which is very useful for
understanding the encoding.

.. _magic number:

Magic Numbers
-------------

The first two bytes of a bitcode file are 'BC' (``0x42``, ``0x43``).  The second
two bytes are an application-specific magic number.  Generic bitcode tools can
look at only the first two bytes to verify the file is bitcode, while
application-specific programs will want to look at all four.

.. _primitives:

Primitives
----------

A bitstream literally consists of a stream of bits, which are read in order
starting with the least significant bit of each byte.  The stream is made up of
a number of primitive values that encode a stream of unsigned integer values.
These integers are encoded in two ways: either as `Fixed Width Integers`_ or as
`Variable Width Integers`_.

.. _Fixed Width Integers:
.. _fixed-width value:

Fixed Width Integers
^^^^^^^^^^^^^^^^^^^^

Fixed-width integer values have their low bits emitted directly to the file.
For example, a 3-bit integer value encodes 1 as 001.  Fixed width integers are
used when there are a well-known number of options for a field.  For example,
boolean values are usually encoded with a 1-bit wide integer.

.. _Variable Width Integers:
.. _Variable Width Integer:
.. _variable-width value:

Variable Width Integers
^^^^^^^^^^^^^^^^^^^^^^^

Variable-width integer (VBR) values encode values of arbitrary size, optimizing
for the case where the values are small.  Given a 4-bit VBR field, any 3-bit
value (0 through 7) is encoded directly, with the high bit set to zero.  Values
larger than N-1 bits emit their bits in a series of N-1 bit chunks, where all
but the last set the high bit.

For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a vbr4
value.  The first set of four bits indicates the value 3 (011) with a
continuation piece (indicated by a high bit of 1).  The next word indicates a
value of 24 (011 << 3) with no continuation.  The sum (3+24) yields the value
27.

.. _char6-encoded value:

6-bit characters
^^^^^^^^^^^^^^^^

6-bit characters encode common characters into a fixed 6-bit field.  They
represent the following characters with the following 6-bit values:

::

  'a' .. 'z' ---  0 .. 25
  'A' .. 'Z' --- 26 .. 51
  '0' .. '9' --- 52 .. 61
         '.' --- 62
         '_' --- 63

This encoding is only suitable for encoding characters and strings that consist
only of the above characters.  It is completely incapable of encoding characters
not in the set.

Word Alignment
^^^^^^^^^^^^^^

Occasionally, it is useful to emit zero bits until the bitstream is a multiple
of 32 bits.  This ensures that the bit position in the stream can be represented
as a multiple of 32-bit words.

Abbreviation IDs
----------------

A bitstream is a sequential series of `Blocks`_ and `Data Records`_.  Both of
these start with an abbreviation ID encoded as a fixed-bitwidth field.  The
width is specified by the current block, as described below.  The value of the
abbreviation ID specifies either a builtin ID (which have special meanings,
defined below) or one of the abbreviation IDs defined for the current block by
the stream itself.

The set of builtin abbrev IDs is:

* 0 - `END_BLOCK`_ --- This abbrev ID marks the end of the current block.

* 1 - `ENTER_SUBBLOCK`_ --- This abbrev ID marks the beginning of a new
  block.

* 2 - `DEFINE_ABBREV`_ --- This defines a new abbreviation.

* 3 - `UNABBREV_RECORD`_ --- This ID specifies the definition of an
  unabbreviated record.

Abbreviation IDs 4 and above are defined by the stream itself, and specify an
`abbreviated record encoding`_.

.. _Blocks:

Blocks
------

Blocks in a bitstream denote nested regions of the stream, and are identified by
a content-specific id number (for example, LLVM IR uses an ID of 12 to represent
function bodies).  Block IDs 0-7 are reserved for `standard blocks`_ whose
meaning is defined by Bitcode; block IDs 8 and greater are application
specific. Nested blocks capture the hierarchical structure of the data encoded
in it, and various properties are associated with blocks as the file is parsed.
Block definitions allow the reader to efficiently skip blocks in constant time
if the reader wants a summary of blocks, or if it wants to efficiently skip data
it does not understand.  The LLVM IR reader uses this mechanism to skip function
bodies, lazily reading them on demand.

When reading and encoding the stream, several properties are maintained for the
block.  In particular, each block maintains:

#. A current abbrev id width.  This value starts at 2 at the beginning of the
   stream, and is set every time a block record is entered.  The block entry
   specifies the abbrev id width for the body of the block.

#. A set of abbreviations.  Abbreviations may be defined within a block, in
   which case they are only defined in that block (neither subblocks nor
   enclosing blocks see the abbreviation).  Abbreviations can also be defined
   inside a `BLOCKINFO`_ block, in which case they are defined in all blocks
   that match the ID that the ``BLOCKINFO`` block is describing.

As sub blocks are entered, these properties are saved and the new sub-block has
its own set of abbreviations, and its own abbrev id width.  When a sub-block is
popped, the saved values are restored.

.. _ENTER_SUBBLOCK:

ENTER_SUBBLOCK Encoding
^^^^^^^^^^^^^^^^^^^^^^^

:raw-html:`<tt>`
[ENTER_SUBBLOCK, blockid\ :sub:`vbr8`, newabbrevlen\ :sub:`vbr4`, <align32bits>, blocklen_32]
:raw-html:`</tt>`

The ``ENTER_SUBBLOCK`` abbreviation ID specifies the start of a new block
record.  The ``blockid`` value is encoded as an 8-bit VBR identifier, and
indicates the type of block being entered, which can be a `standard block`_ or
an application-specific block.  The ``newabbrevlen`` value is a 4-bit VBR, which
specifies the abbrev id width for the sub-block.  The ``blocklen`` value is a
32-bit aligned value that specifies the size of the subblock in 32-bit
words. This value allows the reader to skip over the entire block in one jump.

.. _END_BLOCK:

END_BLOCK Encoding
^^^^^^^^^^^^^^^^^^

``[END_BLOCK, <align32bits>]``

The ``END_BLOCK`` abbreviation ID specifies the end of the current block record.
Its end is aligned to 32-bits to ensure that the size of the block is an even
multiple of 32-bits.

.. _Data Records:

Data Records
------------

Data records consist of a record code and a number of (up to) 64-bit integer
values.  The interpretation of the code and values is application specific and
may vary between different block types.  Records can be encoded either using an
unabbrev record, or with an abbreviation.  In the LLVM IR format, for example,
there is a record which encodes the target triple of a module.  The code is
``MODULE_CODE_TRIPLE``, and the values of the record are the ASCII codes for the
characters in the string.

.. _UNABBREV_RECORD:

UNABBREV_RECORD Encoding
^^^^^^^^^^^^^^^^^^^^^^^^

:raw-html:`<tt>`
[UNABBREV_RECORD, code\ :sub:`vbr6`, numops\ :sub:`vbr6`, op0\ :sub:`vbr6`, op1\ :sub:`vbr6`, ...]
:raw-html:`</tt>`

An ``UNABBREV_RECORD`` provides a default fallback encoding, which is both
completely general and extremely inefficient.  It can describe an arbitrary
record by emitting the code and operands as VBRs.

For example, emitting an LLVM IR target triple as an unabbreviated record
requires emitting the ``UNABBREV_RECORD`` abbrevid, a vbr6 for the
``MODULE_CODE_TRIPLE`` code, a vbr6 for the length of the string, which is equal
to the number of operands, and a vbr6 for each character.  Because there are no
letters with values less than 32, each letter would need to be emitted as at
least a two-part VBR, which means that each letter would require at least 12
bits.  This is not an efficient encoding, but it is fully general.

.. _abbreviated record encoding:

Abbreviated Record Encoding
^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[<abbrevid>, fields...]``

An abbreviated record is a abbreviation id followed by a set of fields that are
encoded according to the `abbreviation definition`_.  This allows records to be
encoded significantly more densely than records encoded with the
`UNABBREV_RECORD`_ type, and allows the abbreviation types to be specified in
the stream itself, which allows the files to be completely self describing.  The
actual encoding of abbreviations is defined below.

The record code, which is the first field of an abbreviated record, may be
encoded in the abbreviation definition (as a literal operand) or supplied in the
abbreviated record (as a Fixed or VBR operand value).

.. _abbreviation definition:

Abbreviations
-------------

Abbreviations are an important form of compression for bitstreams.  The idea is
to specify a dense encoding for a class of records once, then use that encoding
to emit many records.  It takes space to emit the encoding into the file, but
the space is recouped (hopefully plus some) when the records that use it are
emitted.

Abbreviations can be determined dynamically per client, per file. Because the
abbreviations are stored in the bitstream itself, different streams of the same
format can contain different sets of abbreviations according to the needs of the
specific stream.  As a concrete example, LLVM IR files usually emit an
abbreviation for binary operators.  If a specific LLVM module contained no or
few binary operators, the abbreviation does not need to be emitted.

.. _DEFINE_ABBREV:

DEFINE_ABBREV Encoding
^^^^^^^^^^^^^^^^^^^^^^

:raw-html:`<tt>`
[DEFINE_ABBREV, numabbrevops\ :sub:`vbr5`, abbrevop0, abbrevop1, ...]
:raw-html:`</tt>`

A ``DEFINE_ABBREV`` record adds an abbreviation to the list of currently defined
abbreviations in the scope of this block.  This definition only exists inside
this immediate block --- it is not visible in subblocks or enclosing blocks.
Abbreviations are implicitly assigned IDs sequentially starting from 4 (the
first application-defined abbreviation ID).  Any abbreviations defined in a
``BLOCKINFO`` record for the particular block type receive IDs first, in order,
followed by any abbreviations defined within the block itself.  Abbreviated data
records reference this ID to indicate what abbreviation they are invoking.

An abbreviation definition consists of the ``DEFINE_ABBREV`` abbrevid followed
by a VBR that specifies the number of abbrev operands, then the abbrev operands
themselves.  Abbreviation operands come in three forms.  They all start with a
single bit that indicates whether the abbrev operand is a literal operand (when
the bit is 1) or an encoding operand (when the bit is 0).

#. Literal operands --- :raw-html:`<tt>` [1\ :sub:`1`, litvalue\
   :sub:`vbr8`] :raw-html:`</tt>` --- Literal operands specify that the value in
   the result is always a single specific value.  This specific value is emitted
   as a vbr8 after the bit indicating that it is a literal operand.

#. Encoding info without data --- :raw-html:`<tt>` [0\ :sub:`1`, encoding\
   :sub:`3`] :raw-html:`</tt>` --- Operand encodings that do not have extra data
   are just emitted as their code.

#. Encoding info with data --- :raw-html:`<tt>` [0\ :sub:`1`, encoding\
   :sub:`3`, value\ :sub:`vbr5`] :raw-html:`</tt>` --- Operand encodings that do
   have extra data are emitted as their code, followed by the extra data.

The possible operand encodings are:

* Fixed (code 1): The field should be emitted as a `fixed-width value`_, whose
  width is specified by the operand's extra data.

* VBR (code 2): The field should be emitted as a `variable-width value`_, whose
  width is specified by the operand's extra data.

* Array (code 3): This field is an array of values.  The array operand has no
  extra data, but expects another operand to follow it, indicating the element
  type of the array.  When reading an array in an abbreviated record, the first
  integer is a vbr6 that indicates the array length, followed by the encoded
  elements of the array.  An array may only occur as the last operand of an
  abbreviation (except for the one final operand that gives the array's
  type).

* Char6 (code 4): This field should be emitted as a `char6-encoded value`_.
  This operand type takes no extra data. Char6 encoding is normally used as an
  array element type.

* Blob (code 5): This field is emitted as a vbr6, followed by padding to a
  32-bit boundary (for alignment) and an array of 8-bit objects.  The array of
  bytes is further followed by tail padding to ensure that its total length is a
  multiple of 4 bytes.  This makes it very efficient for the reader to decode
  the data without having to make a copy of it: it can use a pointer to the data
  in the mapped in file and poke directly at it.  A blob may only occur as the
  last operand of an abbreviation.

For example, target triples in LLVM modules are encoded as a record of the form
``[TRIPLE, 'a', 'b', 'c', 'd']``.  Consider if the bitstream emitted the
following abbrev entry:

::

  [0, Fixed, 4]
  [0, Array]
  [0, Char6]

When emitting a record with this abbreviation, the above entry would be emitted
as:

:raw-html:`<tt><blockquote>`
[4\ :sub:`abbrevwidth`, 2\ :sub:`4`, 4\ :sub:`vbr6`, 0\ :sub:`6`, 1\ :sub:`6`, 2\ :sub:`6`, 3\ :sub:`6`]
:raw-html:`</blockquote></tt>`

These values are:

#. The first value, 4, is the abbreviation ID for this abbreviation.

#. The second value, 2, is the record code for ``TRIPLE`` records within LLVM IR
   file ``MODULE_BLOCK`` blocks.

#. The third value, 4, is the length of the array.

#. The rest of the values are the char6 encoded values for ``"abcd"``.

With this abbreviation, the triple is emitted with only 37 bits (assuming a
abbrev id width of 3).  Without the abbreviation, significantly more space would
be required to emit the target triple.  Also, because the ``TRIPLE`` value is
not emitted as a literal in the abbreviation, the abbreviation can also be used
for any other string value.

.. _standard blocks:
.. _standard block:

Standard Blocks
---------------

In addition to the basic block structure and record encodings, the bitstream
also defines specific built-in block types.  These block types specify how the
stream is to be decoded or other metadata.  In the future, new standard blocks
may be added.  Block IDs 0-7 are reserved for standard blocks.

.. _BLOCKINFO:

#0 - BLOCKINFO Block
^^^^^^^^^^^^^^^^^^^^

The ``BLOCKINFO`` block allows the description of metadata for other blocks.
The currently specified records are:

::

  [SETBID (#1), blockid]
  [DEFINE_ABBREV, ...]
  [BLOCKNAME, ...name...]
  [SETRECORDNAME, RecordID, ...name...]

The ``SETBID`` record (code 1) indicates which block ID is being described.
``SETBID`` records can occur multiple times throughout the block to change which
block ID is being described.  There must be a ``SETBID`` record prior to any
other records.

Standard ``DEFINE_ABBREV`` records can occur inside ``BLOCKINFO`` blocks, but
unlike their occurrence in normal blocks, the abbreviation is defined for blocks
matching the block ID we are describing, *not* the ``BLOCKINFO`` block
itself.  The abbreviations defined in ``BLOCKINFO`` blocks receive abbreviation
IDs as described in `DEFINE_ABBREV`_.

The ``BLOCKNAME`` record (code 2) can optionally occur in this block.  The
elements of the record are the bytes of the string name of the block.
llvm-bcanalyzer can use this to dump out bitcode files symbolically.

The ``SETRECORDNAME`` record (code 3) can also optionally occur in this block.
The first operand value is a record ID number, and the rest of the elements of
the record are the bytes for the string name of the record.  llvm-bcanalyzer can
use this to dump out bitcode files symbolically.

Note that although the data in ``BLOCKINFO`` blocks is described as "metadata,"
the abbreviations they contain are essential for parsing records from the
corresponding blocks.  It is not safe to skip them.

.. _wrapper:

Bitcode Wrapper Format
======================

Bitcode files for LLVM IR may optionally be wrapped in a simple wrapper
structure.  This structure contains a simple header that indicates the offset
and size of the embedded BC file.  This allows additional information to be
stored alongside the BC file.  The structure of this file header is:

:raw-html:`<tt><blockquote>`
[Magic\ :sub:`32`, Version\ :sub:`32`, Offset\ :sub:`32`, Size\ :sub:`32`, CPUType\ :sub:`32`]
:raw-html:`</blockquote></tt>`

Each of the fields are 32-bit fields stored in little endian form (as with the
rest of the bitcode file fields).  The Magic number is always ``0x0B17C0DE`` and
the version is currently always ``0``.  The Offset field is the offset in bytes
to the start of the bitcode stream in the file, and the Size field is the size
in bytes of the stream. CPUType is a target-specific value that can be used to
encode the CPU of the target.

.. _native object file:

Native Object File Wrapper Format
=================================

Bitcode files for LLVM IR may also be wrapped in a native object file
(i.e. ELF, COFF, Mach-O).  The bitcode must be stored in a section of the
object file named ``.llvmbc``.  This wrapper format is useful for accommodating
LTO in compilation pipelines where intermediate objects must be native object
files which contain metadata in other sections.

Not all tools support this format.

.. _encoding of LLVM IR:

LLVM IR Encoding
================

LLVM IR is encoded into a bitstream by defining blocks and records.  It uses
blocks for things like constant pools, functions, symbol tables, etc.  It uses
records for things like instructions, global variable descriptors, type
descriptions, etc.  This document does not describe the set of abbreviations
that the writer uses, as these are fully self-described in the file, and the
reader is not allowed to build in any knowledge of this.

Basics
------

LLVM IR Magic Number
^^^^^^^^^^^^^^^^^^^^

The magic number for LLVM IR files is:

:raw-html:`<tt><blockquote>`
[0x0\ :sub:`4`, 0xC\ :sub:`4`, 0xE\ :sub:`4`, 0xD\ :sub:`4`]
:raw-html:`</blockquote></tt>`

When combined with the bitcode magic number and viewed as bytes, this is
``"BC 0xC0DE"``.

.. _Signed VBRs:

Signed VBRs
^^^^^^^^^^^

`Variable Width Integer`_ encoding is an efficient way to encode arbitrary sized
unsigned values, but is an extremely inefficient for encoding signed values, as
signed values are otherwise treated as maximally large unsigned values.

As such, signed VBR values of a specific width are emitted as follows:

* Positive values are emitted as VBRs of the specified width, but with their
  value shifted left by one.

* Negative values are emitted as VBRs of the specified width, but the negated
  value is shifted left by one, and the low bit is set.

With this encoding, small positive and small negative values can both be emitted
efficiently. Signed VBR encoding is used in ``CST_CODE_INTEGER`` and
``CST_CODE_WIDE_INTEGER`` records within ``CONSTANTS_BLOCK`` blocks.
It is also used for phi instruction operands in `MODULE_CODE_VERSION`_ 1.

LLVM IR Blocks
^^^^^^^^^^^^^^

LLVM IR is defined with the following blocks:

* 8 --- `MODULE_BLOCK`_ --- This is the top-level block that contains the entire
  module, and describes a variety of per-module information.

* 9 --- `PARAMATTR_BLOCK`_ --- This enumerates the parameter attributes.

* 10 --- `TYPE_BLOCK`_ --- This describes all of the types in the module.

* 11 --- `CONSTANTS_BLOCK`_ --- This describes constants for a module or
  function.

* 12 --- `FUNCTION_BLOCK`_ --- This describes a function body.

* 13 --- `TYPE_SYMTAB_BLOCK`_ --- This describes the type symbol table.

* 14 --- `VALUE_SYMTAB_BLOCK`_ --- This describes a value symbol table.

* 15 --- `METADATA_BLOCK`_ --- This describes metadata items.

* 16 --- `METADATA_ATTACHMENT`_ --- This contains records associating metadata
  with function instruction values.

.. _MODULE_BLOCK:

MODULE_BLOCK Contents
---------------------

The ``MODULE_BLOCK`` block (id 8) is the top-level block for LLVM bitcode files,
and each bitcode file must contain exactly one. In addition to records
(described below) containing information about the module, a ``MODULE_BLOCK``
block may contain the following sub-blocks:

* `BLOCKINFO`_
* `PARAMATTR_BLOCK`_
* `TYPE_BLOCK`_
* `TYPE_SYMTAB_BLOCK`_
* `VALUE_SYMTAB_BLOCK`_
* `CONSTANTS_BLOCK`_
* `FUNCTION_BLOCK`_
* `METADATA_BLOCK`_

.. _MODULE_CODE_VERSION:

MODULE_CODE_VERSION Record
^^^^^^^^^^^^^^^^^^^^^^^^^^

``[VERSION, version#]``

The ``VERSION`` record (code 1) contains a single value indicating the format
version. Versions 0 and 1 are supported at this time. The difference between
version 0 and 1 is in the encoding of instruction operands in
each `FUNCTION_BLOCK`_.

In version 0, each value defined by an instruction is assigned an ID
unique to the function. Function-level value IDs are assigned starting from
``NumModuleValues`` since they share the same namespace as module-level
values. The value enumerator resets after each function. When a value is
an operand of an instruction, the value ID is used to represent the operand.
For large functions or large modules, these operand values can be large.

The encoding in version 1 attempts to avoid large operand values
in common cases. Instead of using the value ID directly, operands are
encoded as relative to the current instruction. Thus, if an operand
is the value defined by the previous instruction, the operand
will be encoded as 1.

For example, instead of

.. code-block:: llvm

  #n = load #n-1
  #n+1 = icmp eq #n, #const0
  br #n+1, label #(bb1), label #(bb2)

version 1 will encode the instructions as

.. code-block:: llvm

  #n = load #1
  #n+1 = icmp eq #1, (#n+1)-#const0
  br #1, label #(bb1), label #(bb2)

Note in the example that operands which are constants also use
the relative encoding, while operands like basic block labels
do not use the relative encoding.

Forward references will result in a negative value.
This can be inefficient, as operands are normally encoded
as unsigned VBRs. However, forward references are rare, except in the
case of phi instructions. For phi instructions, operands are encoded as
`Signed VBRs`_ to deal with forward references.


MODULE_CODE_TRIPLE Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[TRIPLE, ...string...]``

The ``TRIPLE`` record (code 2) contains a variable number of values representing
the bytes of the ``target triple`` specification string.

MODULE_CODE_DATALAYOUT Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[DATALAYOUT, ...string...]``

The ``DATALAYOUT`` record (code 3) contains a variable number of values
representing the bytes of the ``target datalayout`` specification string.

MODULE_CODE_ASM Record
^^^^^^^^^^^^^^^^^^^^^^

``[ASM, ...string...]``

The ``ASM`` record (code 4) contains a variable number of values representing
the bytes of ``module asm`` strings, with individual assembly blocks separated
by newline (ASCII 10) characters.

.. _MODULE_CODE_SECTIONNAME:

MODULE_CODE_SECTIONNAME Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[SECTIONNAME, ...string...]``

The ``SECTIONNAME`` record (code 5) contains a variable number of values
representing the bytes of a single section name string. There should be one
``SECTIONNAME`` record for each section name referenced (e.g., in global
variable or function ``section`` attributes) within the module. These records
can be referenced by the 1-based index in the *section* fields of ``GLOBALVAR``
or ``FUNCTION`` records.

MODULE_CODE_DEPLIB Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[DEPLIB, ...string...]``

The ``DEPLIB`` record (code 6) contains a variable number of values representing
the bytes of a single dependent library name string, one of the libraries
mentioned in a ``deplibs`` declaration.  There should be one ``DEPLIB`` record
for each library name referenced.

MODULE_CODE_GLOBALVAR Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[GLOBALVAR, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal, unnamed_addr, dllstorageclass]``

The ``GLOBALVAR`` record (code 7) marks the declaration or definition of a
global variable. The operand fields are:

* *pointer type*: The type index of the pointer type used to point to this
  global variable

* *isconst*: Non-zero if the variable is treated as constant within the module,
  or zero if it is not

* *initid*: If non-zero, the value index of the initializer for this variable,
  plus 1.

.. _linkage type:

* *linkage*: An encoding of the linkage type for this variable:
  * ``external``: code 0
  * ``weak``: code 1
  * ``appending``: code 2
  * ``internal``: code 3
  * ``linkonce``: code 4
  * ``dllimport``: code 5
  * ``dllexport``: code 6
  * ``extern_weak``: code 7
  * ``common``: code 8
  * ``private``: code 9
  * ``weak_odr``: code 10
  * ``linkonce_odr``: code 11
  * ``available_externally``: code 12
  * deprecated : code 13
  * deprecated : code 14

* alignment*: The logarithm base 2 of the variable's requested alignment, plus 1

* *section*: If non-zero, the 1-based section index in the table of
  `MODULE_CODE_SECTIONNAME`_ entries.

.. _visibility:

* *visibility*: If present, an encoding of the visibility of this variable:
  * ``default``: code 0
  * ``hidden``: code 1
  * ``protected``: code 2

* *threadlocal*: If present, an encoding of the thread local storage mode of the
  variable:
  * ``not thread local``: code 0
  * ``thread local; default TLS model``: code 1
  * ``localdynamic``: code 2
  * ``initialexec``: code 3
  * ``localexec``: code 4

* *unnamed_addr*: If present and non-zero, indicates that the variable has
  ``unnamed_addr``

.. _bcdllstorageclass:

* *dllstorageclass*: If present, an encoding of the DLL storage class of this variable:

  * ``default``: code 0
  * ``dllimport``: code 1
  * ``dllexport``: code 2

.. _FUNCTION:

MODULE_CODE_FUNCTION Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[FUNCTION, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc, prefix, dllstorageclass]``

The ``FUNCTION`` record (code 8) marks the declaration or definition of a
function. The operand fields are:

* *type*: The type index of the function type describing this function

* *callingconv*: The calling convention number:
  * ``ccc``: code 0
  * ``fastcc``: code 8
  * ``coldcc``: code 9
  * ``webkit_jscc``: code 12
  * ``anyregcc``: code 13
  * ``preserve_mostcc``: code 14
  * ``preserve_allcc``: code 15
  * ``x86_stdcallcc``: code 64
  * ``x86_fastcallcc``: code 65
  * ``arm_apcscc``: code 66
  * ``arm_aapcscc``: code 67
  * ``arm_aapcs_vfpcc``: code 68

* isproto*: Non-zero if this entry represents a declaration rather than a
  definition

* *linkage*: An encoding of the `linkage type`_ for this function

* *paramattr*: If nonzero, the 1-based parameter attribute index into the table
  of `PARAMATTR_CODE_ENTRY`_ entries.

* *alignment*: The logarithm base 2 of the function's requested alignment, plus
  1

* *section*: If non-zero, the 1-based section index in the table of
  `MODULE_CODE_SECTIONNAME`_ entries.

* *visibility*: An encoding of the `visibility`_ of this function

* *gc*: If present and nonzero, the 1-based garbage collector index in the table
  of `MODULE_CODE_GCNAME`_ entries.

* *unnamed_addr*: If present and non-zero, indicates that the function has
  ``unnamed_addr``

* *prefix*: If non-zero, the value index of the prefix data for this function,
  plus 1.

* *dllstorageclass*: An encoding of the
  :ref:`dllstorageclass<bcdllstorageclass>` of this function

MODULE_CODE_ALIAS Record
^^^^^^^^^^^^^^^^^^^^^^^^

``[ALIAS, alias type, aliasee val#, linkage, visibility, dllstorageclass]``

The ``ALIAS`` record (code 9) marks the definition of an alias. The operand
fields are

* *alias type*: The type index of the alias

* *aliasee val#*: The value index of the aliased value

* *linkage*: An encoding of the `linkage type`_ for this alias

* *visibility*: If present, an encoding of the `visibility`_ of the alias

* *dllstorageclass*: If present, an encoding of the
  :ref:`dllstorageclass<bcdllstorageclass>` of the alias

MODULE_CODE_PURGEVALS Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[PURGEVALS, numvals]``

The ``PURGEVALS`` record (code 10) resets the module-level value list to the
size given by the single operand value. Module-level value list items are added
by ``GLOBALVAR``, ``FUNCTION``, and ``ALIAS`` records.  After a ``PURGEVALS``
record is seen, new value indices will start from the given *numvals* value.

.. _MODULE_CODE_GCNAME:

MODULE_CODE_GCNAME Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[GCNAME, ...string...]``

The ``GCNAME`` record (code 11) contains a variable number of values
representing the bytes of a single garbage collector name string. There should
be one ``GCNAME`` record for each garbage collector name referenced in function
``gc`` attributes within the module. These records can be referenced by 1-based
index in the *gc* fields of ``FUNCTION`` records.

.. _PARAMATTR_BLOCK:

PARAMATTR_BLOCK Contents
------------------------

The ``PARAMATTR_BLOCK`` block (id 9) contains a table of entries describing the
attributes of function parameters. These entries are referenced by 1-based index
in the *paramattr* field of module block `FUNCTION`_ records, or within the
*attr* field of function block ``INST_INVOKE`` and ``INST_CALL`` records.

Entries within ``PARAMATTR_BLOCK`` are constructed to ensure that each is unique
(i.e., no two indicies represent equivalent attribute lists).

.. _PARAMATTR_CODE_ENTRY:

PARAMATTR_CODE_ENTRY Record
^^^^^^^^^^^^^^^^^^^^^^^^^^^

``[ENTRY, paramidx0, attr0, paramidx1, attr1...]``

The ``ENTRY`` record (code 1) contains an even number of values describing a
unique set of function parameter attributes. Each *paramidx* value indicates
which set of attributes is represented, with 0 representing the return value
attributes, 0xFFFFFFFF representing function attributes, and other values
representing 1-based function parameters. Each *attr* value is a bitmap with the
following interpretation:

* bit 0: ``zeroext``
* bit 1: ``signext``
* bit 2: ``noreturn``
* bit 3: ``inreg``
* bit 4: ``sret``
* bit 5: ``nounwind``
* bit 6: ``noalias``
* bit 7: ``byval``
* bit 8: ``nest``
* bit 9: ``readnone``
* bit 10: ``readonly``
* bit 11: ``noinline``
* bit 12: ``alwaysinline``
* bit 13: ``optsize``
* bit 14: ``ssp``
* bit 15: ``sspreq``
* bits 16-31: ``align n``
* bit 32: ``nocapture``
* bit 33: ``noredzone``
* bit 34: ``noimplicitfloat``
* bit 35: ``naked``
* bit 36: ``inlinehint``
* bits 37-39: ``alignstack n``, represented as the logarithm
  base 2 of the requested alignment, plus 1

.. _TYPE_BLOCK:

TYPE_BLOCK Contents
-------------------

The ``TYPE_BLOCK`` block (id 10) contains records which constitute a table of
type operator entries used to represent types referenced within an LLVM
module. Each record (with the exception of `NUMENTRY`_) generates a single type
table entry, which may be referenced by 0-based index from instructions,
constants, metadata, type symbol table entries, or other type operator records.

Entries within ``TYPE_BLOCK`` are constructed to ensure that each entry is
unique (i.e., no two indicies represent structurally equivalent types).

.. _TYPE_CODE_NUMENTRY:
.. _NUMENTRY:

TYPE_CODE_NUMENTRY Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[NUMENTRY, numentries]``

The ``NUMENTRY`` record (code 1) contains a single value which indicates the
total number of type code entries in the type table of the module. If present,
``NUMENTRY`` should be the first record in the block.

TYPE_CODE_VOID Record
^^^^^^^^^^^^^^^^^^^^^

``[VOID]``

The ``VOID`` record (code 2) adds a ``void`` type to the type table.

TYPE_CODE_HALF Record
^^^^^^^^^^^^^^^^^^^^^

``[HALF]``

The ``HALF`` record (code 10) adds a ``half`` (16-bit floating point) type to
the type table.

TYPE_CODE_FLOAT Record
^^^^^^^^^^^^^^^^^^^^^^

``[FLOAT]``

The ``FLOAT`` record (code 3) adds a ``float`` (32-bit floating point) type to
the type table.

TYPE_CODE_DOUBLE Record
^^^^^^^^^^^^^^^^^^^^^^^

``[DOUBLE]``

The ``DOUBLE`` record (code 4) adds a ``double`` (64-bit floating point) type to
the type table.

TYPE_CODE_LABEL Record
^^^^^^^^^^^^^^^^^^^^^^

``[LABEL]``

The ``LABEL`` record (code 5) adds a ``label`` type to the type table.

TYPE_CODE_OPAQUE Record
^^^^^^^^^^^^^^^^^^^^^^^

``[OPAQUE]``

The ``OPAQUE`` record (code 6) adds an ``opaque`` type to the type table. Note
that distinct ``opaque`` types are not unified.

TYPE_CODE_INTEGER Record
^^^^^^^^^^^^^^^^^^^^^^^^

``[INTEGER, width]``

The ``INTEGER`` record (code 7) adds an integer type to the type table. The
single *width* field indicates the width of the integer type.

TYPE_CODE_POINTER Record
^^^^^^^^^^^^^^^^^^^^^^^^

``[POINTER, pointee type, address space]``

The ``POINTER`` record (code 8) adds a pointer type to the type table. The
operand fields are

* *pointee type*: The type index of the pointed-to type

* *address space*: If supplied, the target-specific numbered address space where
  the pointed-to object resides. Otherwise, the default address space is zero.

TYPE_CODE_FUNCTION Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[FUNCTION, vararg, ignored, retty, ...paramty... ]``

The ``FUNCTION`` record (code 9) adds a function type to the type table. The
operand fields are

* *vararg*: Non-zero if the type represents a varargs function

* *ignored*: This value field is present for backward compatibility only, and is
  ignored

* *retty*: The type index of the function's return type

* *paramty*: Zero or more type indices representing the parameter types of the
  function

TYPE_CODE_STRUCT Record
^^^^^^^^^^^^^^^^^^^^^^^

``[STRUCT, ispacked, ...eltty...]``

The ``STRUCT`` record (code 10) adds a struct type to the type table. The
operand fields are

* *ispacked*: Non-zero if the type represents a packed structure

* *eltty*: Zero or more type indices representing the element types of the
  structure

TYPE_CODE_ARRAY Record
^^^^^^^^^^^^^^^^^^^^^^

``[ARRAY, numelts, eltty]``

The ``ARRAY`` record (code 11) adds an array type to the type table.  The
operand fields are

* *numelts*: The number of elements in arrays of this type

* *eltty*: The type index of the array element type

TYPE_CODE_VECTOR Record
^^^^^^^^^^^^^^^^^^^^^^^

``[VECTOR, numelts, eltty]``

The ``VECTOR`` record (code 12) adds a vector type to the type table.  The
operand fields are

* *numelts*: The number of elements in vectors of this type

* *eltty*: The type index of the vector element type

TYPE_CODE_X86_FP80 Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[X86_FP80]``

The ``X86_FP80`` record (code 13) adds an ``x86_fp80`` (80-bit floating point)
type to the type table.

TYPE_CODE_FP128 Record
^^^^^^^^^^^^^^^^^^^^^^

``[FP128]``

The ``FP128`` record (code 14) adds an ``fp128`` (128-bit floating point) type
to the type table.

TYPE_CODE_PPC_FP128 Record
^^^^^^^^^^^^^^^^^^^^^^^^^^

``[PPC_FP128]``

The ``PPC_FP128`` record (code 15) adds a ``ppc_fp128`` (128-bit floating point)
type to the type table.

TYPE_CODE_METADATA Record
^^^^^^^^^^^^^^^^^^^^^^^^^

``[METADATA]``

The ``METADATA`` record (code 16) adds a ``metadata`` type to the type table.

.. _CONSTANTS_BLOCK:

CONSTANTS_BLOCK Contents
------------------------

The ``CONSTANTS_BLOCK`` block (id 11) ...

.. _FUNCTION_BLOCK:

FUNCTION_BLOCK Contents
-----------------------

The ``FUNCTION_BLOCK`` block (id 12) ...

In addition to the record types described below, a ``FUNCTION_BLOCK`` block may
contain the following sub-blocks:

* `CONSTANTS_BLOCK`_
* `VALUE_SYMTAB_BLOCK`_
* `METADATA_ATTACHMENT`_

.. _TYPE_SYMTAB_BLOCK:

TYPE_SYMTAB_BLOCK Contents
--------------------------

The ``TYPE_SYMTAB_BLOCK`` block (id 13) contains entries which map between
module-level named types and their corresponding type indices.

.. _TST_CODE_ENTRY:

TST_CODE_ENTRY Record
^^^^^^^^^^^^^^^^^^^^^

``[ENTRY, typeid, ...string...]``

The ``ENTRY`` record (code 1) contains a variable number of values, with the
first giving the type index of the designated type, and the remaining values
giving the character codes of the type name. Each entry corresponds to a single
named type.

.. _VALUE_SYMTAB_BLOCK:

VALUE_SYMTAB_BLOCK Contents
---------------------------

The ``VALUE_SYMTAB_BLOCK`` block (id 14) ...

.. _METADATA_BLOCK:

METADATA_BLOCK Contents
-----------------------

The ``METADATA_BLOCK`` block (id 15) ...

.. _METADATA_ATTACHMENT:

METADATA_ATTACHMENT Contents
----------------------------

The ``METADATA_ATTACHMENT`` block (id 16) ...