LLVM Bitcode File Format

1. Abstract
2. Overview
3. Bitstream Format
   1. Magic Numbers
   2. Primitives
   3. Abbreviation IDs
   4. Blocks
   5. Data Records
   6. Abbreviations
   7. Standard Blocks
4. LLVM IR Encoding
   1. Basics

Written by Chris Lattner and Joshua Haberman.

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.

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

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 llvm-bcanalyzer tool can be used to dump and inspect arbitrary bitstreams, which is very useful for understanding the encoding.

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

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 are encoded in two ways: either as Fixed Width Integers or as Variable Width Integers.

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 (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

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 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 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 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:

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

`[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 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` 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 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 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 Encoding

`[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.

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.

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. 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 Encoding

`[DEFINE_ABBREV, numabbrevopsvbr5, abbrevop0, abbrevop1, ...]`

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 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).

1. Literal operands - `[11, litvaluevbr8]` - 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.
2. Encoding info without data - `[01, encoding3]` - Operand encodings that do not have extra data are just emitted as their code.
3. Encoding info with data - `[01, encoding3, valuevbr5]` - Operand encodings that do have extra data are emitted as their code, followed by the extra data.

The possible operand encodings are:

- 1 - Fixed - The field should be emitted as a fixed-width value, whose width is specified by the operand's extra data.
- 2 - VBR - The field should be emitted as a variable-width value, whose width is specified by the operand's extra data.
- 3 - Array - This field is an array of values. The array operand has no extra data, but expects another operand to follow it which indicates 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).
- 4 - Char6 - This field should be emitted as a char6-encoded value. This operand type takes no extra data.

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:

`[4abbrevwidth, 24, 4vbr6, 06, 16, 26, 36]`

These values are:

1. The first value, 4, is the abbreviation ID for this abbreviation.
2. The second value, 2, is the code for TRIPLE in LLVM IR files.
3. The third value, 4, is the length of the array.
4. 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, since 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

In addition to the basic block structure and record encodings, the bitstream also defines specific builtin 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.

#0 - BLOCKINFO Block

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

- `[SETBID (#1), blockid]`
- `[DEFINE_ABBREV, ...]`

The SETBID record 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.

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.

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:

`[0x04, 0xC4, 0xE4, 0xD4]`

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

Signed VBRs

Variable Width Integers are an efficient way to encode arbitrary sized unsigned values, but is an extremely inefficient way to encode 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.

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.

MODULE_BLOCK Contents