This document describes the LLVM bitstream file format and the encoding of the LLVM IR into it.
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, like 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.
This document first describes the LLVM bitstream format, then describes the record structure used by LLVM IR files.
The bitstream format is literally a stream of bits, with a very simple structure. This structure consists of the following concepts:
Note that the llvm-bcanalyzer tool can be used to dump and inspect arbitrary bitstreams, which is very useful for understanding the encoding.
The first four bytes of the stream identify the encoding of the file. This is used by a reader to know what is contained in the file.
A bitstream literally consists of a stream of bits. This stream is made up of a number of primitive values that encode a stream of integer values. These integers are are encoded in two ways: either as Fixed Width Integers or as Variable 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 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.
6-bit characters encode common characters into a fixed 6-bit field. They represent the following characters with the following 6-bit values:
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.
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.
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 by the stream itself.
The set of builtin abbrev IDs is:
Abbreviation IDs 4 and above are defined by the stream itself, and specify an abbreviated record encoding.
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). Nested blocks capture the hierachical 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 they do 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:
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, blockidvbr8, newabbrevlenvbr4, <align32bits>, blocklen32]
The ENTER_SUBBLOCK abbreviation ID specifies the start of a new block record. The blockid value is encoded as a 8-bit VBR identifier, and indicates the type of block being entered (which is application specific). The newabbrevlen value is a 4-bit VBR which specifies the abbrev id width for the sub-block. The blocklen 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, <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 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 there are multiple different ways to encode a record (with 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, codevbr6, numopsvbr6, op0vbr6, op1vbr6, ...]
An UNABBREV_RECORD provides a default fallback encoding, which is both completely general and also 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. Since there are no letters with value 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.
[<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.
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. Since the abbreviations are stored in the bitstream itself, different streams of the same format can contain different sets of abbreviations if the specific stream does not need it. 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, numabbrevopsvbr5, abbrevop0, abbrevop1, ...]
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).