Written by Reid Spencer and Chris Lattner

This document is an (after the fact) specification of the LLVM bytecode file format. It documents the binary encoding rules of the bytecode file format so that equivalent systems can encode bytecode files correctly. The LLVM bytecode representation is used to store the intermediate representation on disk in compacted form.

This section describes the general concepts of the bytecode file format without getting into bit and byte level specifics.

LLVM bytecode files consist simply of a sequence of blocks of bytes. Each block begins with an identification value that determines the type of the next block. The possible types of blocks are described below in the section Block Types. The block identifier is used because it is possible for entire blocks to be omitted from the file if they are empty. The block identifier helps the reader determine which kind of block is next in the file.

The following block identifiers are currently in use (from llvm/Bytecode/Format.h):

1. Module (0x01).
2. Function (0x11).
3. ConstantPool (0x12).
4. SymbolTable (0x13).
5. ModuleGlobalInfo (0x14).
6. GlobalTypePlane (0x15).
7. BasicBlock (0x31).
8. InstructionList (0x32).
9. CompactionTable (0x33).

All blocks are variable length. They consume just enough bytes to express their contents. Each block begins with an integer identifier and the length of the block.

Most blocks are constructed of lists of information. Lists can be constructed of other lists, etc. This decomposition of information follows the containment hierarchy of the LLVM Intermediate Representation. For example, a function is composed of a list of basic blocks. Each basic block is composed of a set of instructions. This list of list nesting and hierarchy is maintained in the bytecode file.

A list is encoded into the file simply by encoding the number of entries as an integer followed by each of the entries. The reader knows when the list is done because it will have filled the list with the required numbe of entries.

Fields are units of information that LLVM knows how to write atomically. Most fields have a uniform length or some kind of length indication built into their encoding. For example, a constant string (array of SByte or UByte) is written simply as the length followed by the characters. Although this is similar to a list, constant strings are treated atomically and are thus fields.

Fields use a condensed bit format specific to the type of information they must contain. As few bits as possible are written for each field. The sections that follow will provide the details on how these fields are written and how the bits are to be interpreted.

Each field that can be put out is encoded into the file using a small set of primitives. The rules for these primitives are described below.

Variable Bit Rate Encoding

To minimize the number of bytes written for small quantities, an encoding scheme similar to UTF-8 is used to write integer data. The scheme is known as variable bit rate (vbr) encoding. In this encoding, the high bit of each byte is used to indicate if more bytes follow. If (byte & 0x80) is non-zero in any given byte, it means there is another byte immediately following that also contributes to the value. For the final byte (byte & 0x80) is false (the high bit is not set). In each byte only the low seven bits contribute to the value. Consequently 32-bit quantities can take from one to five bytes to encode. In general, smaller quantities will encode in fewer bytes, as follows:

Note that in practice, the tenth byte could only encode bits 63 and 64 since the maximum quantity to use this encoding is a 64-bit integer.

The table below defines the encoding rules for type names used in the descriptions of blocks and fields in the next section. Any type name with the suffix _vbr indicate a quantity that is encoded using variable bit rate encoding as described above.

To support cross-platform differences, the bytecode file is aligned on certain boundaries. This means that a small amount of padding (at most 3 bytes) will be added to ensure that the next entry is aligned to a 32-bit boundary.

This section provides the detailed layout of the LLVM bytecode file format. bit and byte level specifics.

The descriptions of the bytecode format that follow describe the bit fields in detail. These descriptions are provided in tabular form. Each table has four columns that specify:

1. Byte(s). The offset in bytes of the field from the start of its container (block, list, other field).
2. Bit(s). The offset in bits of the field from the start of the byte field. Bits are always little endian. That is, bit addresses with smaller values have smaller address (i.e. 2⁰ is at bit 0, 2¹ at 1, etc.)
3. Align? Indicates if this field is aligned to 32 bits or not. This indicates where the next field starts, always on a 32 bit boundary.
4. Type. The basic type of information contained in the field.
5. Description. Descripts the contents of the field.

The bytecode format encodes the intermediate representation into groups of bytes known as blocks. The blocks are written sequentially to the file in the following order:

1. Signature. This block contains the file signature (magic number) that identifies the file as LLVM bytecode.
2. Module Block. This is the top level block in a bytecode file. It contains all the other blocks.
3. Global Type Pool. This block contains all the global (module) level types.
4. Module Info. This block contains the types of the global variables and functions in the module as well as the constant initializers for the global variables
5. Constants. This block contains all the global constants except function arguments, global values and constant strings.
6. Functions. One function block is written for each function in the module.
7. Symbol Table. The module level symbol table that provides names for the various other entries in the file is the final block written.

The signature block occurs in every LLVM bytecode file and is always first. It simply provides a few bytes of data to identify the file as being an LLVM bytecode file. This block is always four bytes in length and differs from the other blocks because there is no identifier and no block length at the start of the block. Essentially, this block is just the "magic number" for the file.

The module block contains a small pre-amble and all the other blocks in the file. Of particular note, the bytecode format number is simply a 28-bit monotonically increase integer that identifiers the version of the bytecode format. While the bytecode format version is not related to the LLVM release (it doesn't automatically get increased with each new LLVM release), there is a definite correspondence between the bytecode format version and the LLVM release.

The table below shows the format of the module block header. The blocks it contains are detailed in other sections.

The global type pool consists of type definitions. Their order of appearnce in the file determines their slot number (0 based). Slot numbers are used to replace pointers in the intermediate representation. Each slot number uniquely identifies one entry in a type plane (a collection of values of the same type). Since all values have types and are associated with the order in which the type pool is written, the global type pool must be written as the first block of a module. If it is not, attempts to read the file will fail because both forward and backward type resolution will not be possible.

The type pool is simply a list of types definitions, as shown in the table below.

To be determined.

To be determined.

To be determined.

A symbol table can be put out in conjunction with a module or a function. A symbol table is a list of type planes. Each type plane starts with the number of entries in the plane and the type plane's slot number (so the type can be looked up in the global type pool). For each entry in a type plane, the slot number of the value and the name associated with that value are written. The format is given in the table below.