This document describes the LLVM bytecode file format. It specifies 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.

The LLVM bytecode format may change in the future, but LLVM will always be backwards compatible with older formats. This document will only describe the most current version of the bytecode format. See Version Differences for the details on how the current version is different from previous versions.

This section describes the general concepts of the bytecode file format without getting into specific layout details. It is recommended that you read this section thoroughly before interpreting the detailed descriptions.

LLVM bytecode files consist simply of a sequence of blocks of bytes using a binary encoding Each block begins with an header of two unsigned integers. The first value identifies the type of block and the second value provides the size of the block in bytes. 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. Note that blocks can be nested within other blocks.

All blocks are variable length, and the block header specifies the size of the block. All blocks begin on a byte index that is aligned to an even 32-bit boundary. That is, the first block is 32-bit aligned because it starts at offset 0. Each block is padded with zero fill bytes to ensure that the next block also starts on a 32-bit boundary.

LLVM Bytecode blocks often contain lists of things of a similar type. For example, a function contains a list of instructions and a function type contains a list of argument types. There are two basic types of lists: length lists (llist), and null terminated lists (zlist), as described below in the Encoding Primitives.

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 bytes) 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.

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.

Most of the values written to LLVM bytecode files are small integers. To minimize the number of bytes written for these 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:

Byte #	Significant Bits	Maximum Value
1	0-6	127
2	7-13	16,383
3	14-20	2,097,151
4	21-27	268,435,455
5	28-34	34,359,738,367
6	35-41	4,398,046,511,103
7	42-48	562,949,953,421,311
8	49-55	72,057,594,037,927,935
9	56-62	9,223,372,036,854,775,807
10	63-69	1,180,591,620,717,411,303,423

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

Signed VBR values are encoded with the standard vbr encoding, but with the sign bit as the low order bit instead of the high order bit. This allows small negative quantities to be encoded efficiently. For example, -3 is encoded as "((3 << 1) | 1)" and 3 is encoded as "(3 << 1) | 0)", emitted with the standard vbr encoding above.

Each field in the bytecode format is encoded into the file using a small set of primitive formats. The table below defines the encoding rules for the various primitives used and gives them each a type name. The type names used in the descriptions of blocks and fields in the Detailed Layoutnext section. Any type name with the suffix _vbr indicates a quantity that is encoded using variable bit rate encoding as described above.

Type	Rule
unsigned	A 32-bit unsigned integer that always occupies four consecutive bytes. The unsigned integer is encoded using LSB first ordering. That is bits 20 through 27 are in the byte with the lowest file offset (little endian).
uint24_vbr	A 24-bit unsigned integer that occupies from one to four bytes using variable bit rate encoding.
uint32_vbr	A 32-bit unsigned integer that occupies from one to five bytes using variable bit rate encoding.
uint64_vbr	A 64-bit unsigned integer that occupies from one to ten bytes using variable bit rate encoding.
int64_vbr	A 64-bit signed integer that occupies from one to ten bytes using the signed variable bit rate encoding.
char	A single unsigned character encoded into one byte
bit(n-m)	A set of bit within some larger integer field. The values of n and m specify the inclusive range of bits that define the subfield. The value for m may be omitted if its the same as n.
float	A floating point value encoded as a 32-bit IEEE value written in little-endian form.
double	A floating point value encoded as a64-bit IEEE value written in little-endian form
string	A uint32_vbr indicating the type of the constant string which also includes its length, immediately followed by the characters of the string. There is no terminating null byte in the string.
data	An arbitrarily long segment of data to which no interpretation is implied. This is used for constant initializers.
llist(x)	A length list of x. This means the list is encoded as an uint32_vbr providing the length of the list, followed by a sequence of that many "x" items. This implies that the reader should iterate the number of times provided by the length.
zlist(x)	A zero-terminated list of x. This means the list is encoded as a sequence of an indeterminate number of "x" items, followed by an uint32_vbr terminating value. This implies that none of the "x" items can have a zero value (or else the list terminates).
block	A block of data that is logically related. A block is an unsigned 32-bit integer that encodes the type of the block in the low 5 bits and the size of the block in the high 27 bits. The length does not include the block header or any alignment bytes at the end of the block. Blocks may compose other blocks.

In the detailed block and field descriptions that follow, a regex like notation is used to describe optional and repeated fields. A very limited subset of regex is used to describe these, as given in the following table:

Character	Meaning
?	The question mark indicates 0 or 1 occurrences of the thing preceding it.
*	The asterisk indicates 0 or more occurrences of the thing preceding it.
+	The plus sign indicates 1 or more occurrences of the thing preceding it.
()	Parentheses are used for grouping.
,	The comma separates sequential fields.

So, for example, consider the following specifications:

with the following interpretations:

1. An optional string. Matches either nothing or a single string
2. One or more pairs of uint32_vbr.
3. Zero or more occurrences of either an unsigned followed by a uint32_vbr or just a uint32_vbr.
4. An optional length list of unsigned values.

The bytecode format uses the notion of a "slot" to reference Types and Values. Since the bytecode file is a direct representation of LLVM's intermediate representation, there is a need to represent pointers in the file. Slots are used for this purpose. For example, if one has the following assembly:

there are two definitions. The definition of %MyVar uses %MyType. In the C++ IR this linkage between %MyVar and %MyType is explicit through the use of C++ pointers. In bytecode, however, there's no ability to store memory addresses. Instead, we compute and write out slot numbers for every Type and Value written to the file.

A slot number is simply an unsigned 32-bit integer encoded in the variable bit rate scheme (see encoding). This ensures that low slot numbers are encoded in one byte. Through various bits of magic LLVM attempts to always keep the slot numbers low. The first attempt is to associate slot numbers with their "type plane". That is, Values of the same type are written to the bytecode file in a list (sequentially). Their order in that list determines their slot number. This means that slot #1 doesn't mean anything unless you also specify for which type you want slot #1. Types are always written to the file first (in the Global Type Pool) and in such a way that both forward and backward references of the types can often be resolved with a single pass through the type pool.

In summary then, a slot number can be thought of as just a vbr encoded index into a list of Type* or Value*. To keep slot numbers low, Value* are indexed by two slot numbers: the "type plane index" (type slot) and the "value index" (value slot).

This section provides the general structure of the LLVM bytecode file format. The bytecode file format requires blocks to be in a certain order and nested in a particular way so that an LLVM module can be constructed efficiently from the contents of the file. This ordering defines a general structure for bytecode files as shown below. The table below shows the order in which all block types may appear. Please note that some of the blocks are optional and some may be repeated. The structure is fairly loose because optional blocks, if empty, are completely omitted from the file.

ID	Parent	Optional?	Repeated?	Level	Block Type	Description
N/A	File	No	No	0	Signature	This contains the file signature (magic number) that identifies the file as LLVM bytecode.
0x01	File	No	No	0	Module	This is the top level block in a bytecode file. It contains all the other blocks.
0x06	Module	No	No	1	Global Type Pool	This block contains all the global (module) level types.
0x05	Module	No	No	1	Module Globals Info	This block contains the type, constness, and linkage for each of the global variables in the module. It also contains the type of the functions and the constant initializers.
0x03	Module	Yes	No	1	Module Constant Pool	This block contains all the global constants except function arguments, global values and constant strings.
0x02	Module	Yes	Yes	1	Function Definitions*	One function block is written for each function in the module. The function block contains the instructions, type constant pool, and symbol table for the function.
0x03	Function	Yes	No	2	Function Constant Pool	Any constants (including types) used solely within the function are emitted here in the function constant pool.
0x07	Function	No	No	2	Instruction List	This block contains all the instructions of the function. The basic blocks are inferred by terminating instructions.
0x04	Function	Yes	No	2	Function Symbol Table	This symbol table provides the names for the function specific values used (basic block labels mostly).
0x04	Module	Yes	No	1	Module Symbol Table	This symbol table provides the names for the various entries in the file that are not function specific (global vars, and functions mostly).

Use the links in the table for details about the contents of each of the block types.

This section provides the detailed layout of the individual block types in the LLVM bytecode file format.

The signature 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.

There are two types of signatures for LLVM bytecode: uncompressed and compressed as shown in the table below.

Type	Uncompressed	Compressed
char	Constant "l" (0x6C)	Constant "l" (0x6C)
char	Constant "l" (0x6C)	Constant "l" (0x6C)
char	Constant "v" (0x76)	Constant "v" (0x76)
char	Constant "m" (0x6D)	Constant "c" (0x63)
char	N/A	'0'=null,'1'=gzip,'2'=bzip2

In other words, the uncompressed signature is just the characters 'llvm' while the compressed signature is the characters 'llvc' followed by an ascii digit ('0', '1', or '2') that indicates the kind of compression used. A value of '0' indicates that null compression was used. This can happen when compression was requested on a platform that wasn't configured for gzip or bzip2. A value of '1' means that the rest of the file is compressed using the gzip algorithm and should be uncompressed before interpretation. A value of '2' means that the rest of the file is compressed using the bzip2 algorithm and should be uncompressed before interpretation. In all cases, the data resulting from uncompression should be interpreted as if it occurred immediately after the 'llvm' signature (i.e. the uncompressed data begins with the Module Block

NOTE: As of LLVM 1.4, all bytecode files produced by the LLVM tools are compressed by default. To disable compression, pass the --disable-compression option to the tool, if it supports it.

The module block contains a small pre-amble and all the other blocks in the file. The table below shows the structure of the module block. Note that it only provides the module identifier, size of the module block, and the format information. Everything else is contained in other blocks, described in other sections.

Type	Field Description
unsigned	Module Block Identifier (0x01)
unsigned	Module Block Size
uint32_vbr	Format Information
block	Global Type Pool
block	Module Globals Info
block	Module Constant Pool
block*	Function Definitions
block	Module Symbol Table

The block header for the module block uses a longer format than the other blocks in a bytecode file. Specifically, instead of encoding the type and size of the block into a 32-bit integer with 5-bits for type and 27-bits for size, the module block header uses two 32-bit unsigned values, one for type, and one for size. While the 2²⁷ byte limit on block size is sufficient for the blocks contained in the module, it isn't sufficient for the module block itself because we want to ensure that bytecode files as large as 2³² bytes are possible. For this reason, the module block (and only the module block) uses a long format header.

The format information field is encoded into a uint32_vbr.

Of particular note, the bytecode format number is simply a 32-bit monotonically increasing integer that identifies the version of the bytecode format (which is not directly related to the LLVM release number). The bytecode versions defined so far are (note that this document only describes the latest version, 2.0):

• #0: LLVM 1.0 & 1.1
• #1: LLVM 1.2
• #2: LLVM 1.2.5 (not released)
• #3: LLVM 1.3
• #4: LLVM 1.3.x (not released)
• #5: LLVM 1.4 through 1.8
• #6: LLVM 1.9
• #7: LLVM 2.0 and newer

The global type pool consists of type definitions. Their order of appearance in the file determines their type 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 type definitions, as shown in the table below.

Type	Field Description
block	Type Pool Identifier (0x06) + Size
llist(type)	A length list of type definitions.

Types in the type pool are defined using a different format for each kind of type, as given in the following sections.

Primitive Types

The primitive types encompass the basic integer and floating point types. They are encoded simply as their TypeID.

Type	Description
uint24_vbr	Type ID for the primitive types (values 1 to 11) 1

Notes:

1. The values for the Type IDs for the primitive types are provided by the definition of the llvm::Type::TypeID enumeration in include/llvm/Type.h. The enumeration gives the following mapping:
   1. bool
   2. ubyte
   3. sbyte
   4. ushort
   5. short
   6. uint
   7. int
   8. ulong
   9. long
   10. float
   11. double

Function Types

Type	Description
uint24_vbr	Type ID for function types (13)
uint24_vbr	Type slot number of function's return type.
llist(uint24_vbr)	Type slot number of each argument's type.
uint32_vbr?	Value 0 if this is a varargs function, missing otherwise.

Structure Types

Type	Description
uint24_vbr	Type ID for structure types (14)
zlist(uint24_vbr)	Slot number of each of the element's fields.

Array Types

Type	Description
uint24_vbr	Type ID for Array Types (15)
uint24_vbr	Type slot number of array's element type.
uint32_vbr	The number of elements in the array.

Pointer Types

Type	Description
uint24_vbr	Type ID For Pointer Types (16)
uint24_vbr	Type slot number of pointer's element type.

Opaque Types

Type	Description
uint24_vbr	Type ID For Opaque Types (17)

Vector Types

Type	Description
uint24_vbr	Type ID for Vector Types (18)
uint24_vbr	Slot number of the vector's element type.
uint32_vbr	The number of elements in the vector.

Packed Structure Types

Type	Description
uint24_vbr	Type ID for packed structure types (19)
zlist(uint24_vbr)	Slot number of each of the element's fields.

The module global info block contains the definitions of all global variables including their initializers and the declaration of all functions. The format is shown in the table below:

Type	Field Description
block	Module global info identifier (0x05) + size
zlist(globalvar)	A zero terminated list of global var definitions occurring in the module.
zlist(funcfield)	A zero terminated list of function definitions occurring in the module.
llist(string)	A length list of strings that specify the names of the libraries that this module depends upon.
string	The target triple for the module (blank means no target triple specified, i.e. a platform-independent module).
string	The data layout string describing the endianness, pointer size, and type alignments for which the module was written (blank means no data layout specified, i.e. a platform-independent module).
llist(string)	A length list of strings that defines a table of section strings for globals. A global's SectionID is an index into this table.
string	The inline asm block for this module.

Global variables are written using an uint32_vbr that encodes information about the global variable, an optional extension vbr, and a an optional initializers for the global var.

The table below provides the bit layout of the first uint32_vbr that describes the global variable.

Type	Description
bit(0)	Is constant?
bit(1)	Has initializer? Note that this bit determines whether the constant initializer field (described below) follows.
bit(2-4)	Linkage type: 0=External, 1=Weak, 2=Appending, 3=Internal, 4=LinkOnce, 5=DllImport, 6=DllExport, 7=ExternWeak
bit(5)	Is Thread Local?
bit(6-31)	Type slot number of type for the global variable.

When the Linkage type is set to 3 (internal) and the initializer field is set to 0 (an invalid combination), an extension word follows the first uint32_vbr which encodes the real linkage and init flag, and can includes more information:

Type	Description
bit(0)	Has initializer? Indicates the real value of the "Has initializer" field for the global.
bit(2-4)	Linkage type: Indicates the real value of the "linkage type" field for the global.
bit(4-8)	The log-base-2 of the alignment for the global.
bit(9)	If this bit is set, a SectionID follows this vbr.
bit(10-12)	Visibility style: 0=Default, 1=Hidden.
bit(13-31)	Currently unassigned.

If the SectionID bit is set above, the following field is included:

Type	Description
uint32_vbr	An optional section ID number, specifying the string to use for the section of the global. This an index (+1) of an entry into the SectionID llist in the Module Global Info block. If this value is 0 or not present, the global has an empty section string.

If the "Has initializer" field is set, the following field is included:

Type	Description
uint32_vbr	An optional value slot number for the global variable's constant initializer.

Functions are written using an uint32_vbr that encodes information about the function and a set of flags. If needed, an extension word may follow this first field.

The table below provides the bit layout of the uint32_vbr that describes the function.

Type	Description
bit(0-3)	Encodes the calling convention number of the function. The CC number of the function is the value of this field minus one.
bit(4)	If this bit is set to 1, the indicated function is external, and there is no Function Definiton Block in the bytecode file for the function. If the function is external and has dllimport or extern_weak linkage additional field in the extension word is used to indicate the actual linkage type.
bit(5-30)	Type slot number of type for the function.
bit(31)	Indicates whether an extension word follows.

If bit(31) is set, an additional uint32_vbr word follows with the following fields:

Type	Description
bit(0-4)	The log-base-2 of the alignment for the function.
bit(5-9)	The top nibble of the calling convention.
bit(10)	If this bit is set, a SectionID follows this vbr.
bit(11-12)	Linkage type for external functions. 0 - External linkage, 1 - DLLImport linkage, 2 - External weak linkage.
bit(13-31)	Currently unassigned.

If the SectionID bit is set above, the following field is included:

Type	Description
uint32_vbr	An optional section ID number, specifying the string to use for the section of the function. This an index (+1) of an entry into the SectionID llist in the Module Global Info block. If this value is 0 or not present, the function has an empty section string.

A constant pool defines as set of constant values. There are actually two types of constant pool blocks: one for modules and one for functions. For modules, the block begins with the constant strings encountered anywhere in the module. For functions, the block begins with types only encountered in the function. In both cases the header is identical. The tables that follow, show the header, module constant pool preamble, function constant pool preamble, and the part common to both function and module constant pools.

Common Block Header

Type	Field Description
block	Constant pool identifier (0x03) + size

Module Constant Pool Preamble (constant strings)

Type	Field Description
uint32_vbr	The number of constant strings that follow.
uint32_vbr	Zero. This identifies the following "plane" as containing the constant strings. This is needed to identify it uniquely from other constant planes that follow.
uint24_vbr+	Type slot number of the constant string's type. Note that the constant string's type implicitly defines the length of the string.

Function Constant Pool Preamble (function types)

The structure of the types for functions is identical to the Global Type Pool. Please refer to that section for the details.

Common Part (other constants)

Type	Field Description
uint32_vbr	Number of entries in this type plane.
uint24_vbr	Type slot number of this plane.
constant+	The definition of a constant (see below).

Constant pool entries come in many shapes and flavors. The sections that follow define the format for each of them. All constants start with a uint32_vbr encoded integer that provides the number of operands for the constant. For primitive, structure, and array constants, this will always be zero to indicate that the form of the constant is solely determined by its type. In this case, we have the following field definitions, based on type:

• Bool. This is written as an uint32_vbr of value 1U or 0U.
• Signed Integers (sbyte,short,int,long) ≤ 64 bits. These are written as an int64_vbr with the corresponding value.
• Unsigned Integers (ubyte,ushort,uint,ulong) ≤ 64 bits. These are written as an uint64_vbr with the corresponding value.
• Integers > 64 bits. These are written as a length followed by a series of 64-bit words. The length specifies the number of words that follow. Any zero-valued high order words are elided. Words with the least significant bits are written to the lowest file offsets (little endian). The length is written as an uint32_vbr. Each word of the value is written as an uint64_vbr.
• Floating Point. Both the float and double types are written literally in binary format.
• Arrays. Arrays are written simply as a list of uint32_vbr encoded value slot numbers to the constant element values.
• Structures. Structures are written simply as a list of uint32_vbr encoded value slot numbers to the constant field values of the structure.

When the number of operands to the constant is one, we have an 'undef' value of the specified type.

Inline Assembler entries are stored in the constant pool, though they are not officially LLVM constants. These entries are marked with a value of "4294967295" (all ones) for the number of operands. They are encoded as follows:

Type	Field Description
string	The asm string.
string	The constraints string.
uint32_vbr	Flags

Currently, the only defined flag, the low bit, indicates whether or not the inline assembler has side effects.

Otherwise, we have a constant expression. The format of the constant expression is specified in the table below, and the number is equal to the number of operands+1.

Type	Field Description
uint32_vbr	Op code of the instruction for the constant expression.
uint32_vbr	The value slot number of the constant value for an operand.1
uint24_vbr	The type slot number for the type of the constant value for an operand.1

Notes:

1. Both these fields are repeatable but only in pairs.

Function definitions contain the linkage, constant pool, instruction list, and symbol table for a function. The following table shows the structure of a function definition.

Type	Field Description
block	Function definition block identifier (0x02) + size
uint32_vbr	The linkage and visibility style field
block	The constant pool block for this function.2
block	The instruction list for the function.
block	The function's symbol table containing only those symbols pertinent to the function (mostly block labels).

Notes:

1. Note that if the linkage type is "External" then none of the other fields will be present as the function is defined elsewhere.

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Reid Spencer and Chris Lattner
The LLVM Compiler Infrastructure
Last modified: $Date$