Warning: This is a work in progress.
This document describes the LLVM bytecode -file format. It specifies the binary encoding rules of the bytecode file format +
This document describes the LLVM bytecode file format as of version 1.3. +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.
This section describes the general concepts of the bytecode file format @@ -73,48 +84,42 @@ older formats. This document only describes the most current version of the bytecode format.
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):
-All blocks are variable length, and the block header specifies the size of -the block. All blocks are rounded aligned to even 32-bit boundaries, so they -always start and end of this boundary. Each block begins with an integer -identifier and the length of the block, which does not include the padding -bytes needed for alignment.
+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.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 -contains a list of instructions (the terminator instructions implicitly define -the end of the basic blocks).
-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. -
+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, and null terminated lists, as described here:
+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: -
- -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.
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 below). 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 handled -specially and 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.
-Slot numbers are also kept small by rearranging their order. Because of the -structure of LLVM, certain values are much more likely to be used frequently -in the body of a function. For this reason, a compaction table is provided in -the body of a function if its use would make the function body smaller. -Suppose you have a function body that uses just the types "int*" and "{double}" -but uses them thousands of time. Its worthwhile to ensure that the slot number -for these types are low so they can be encoded in a single byte (via vbr). -This is exactly what the compaction table does.
-| Type | -Rule | +Rule | |
|---|---|---|---|
| unsigned | -A 32-bit unsigned integer that always occupies four + | 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). |
| uint_vbr | -A 32-bit unsigned integer that occupies from one to five + | uint_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 + | 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 + | 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 | +char | +A single unsigned character encoded into one byte |
| bit | -A single bit within a byte. | +bit | +A single bit within some larger integer field. |
| string | -A uint_vbr indicating the length of the character string + | string | +A uint_vbr indicating the length of the character string 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 + | data | +An arbitrarily long segment of data to which no interpretation is implied. This is used for float, double, and constant initializers. | +
| block | +A block of data that is logically related. A block + begins with an unsigned that provides the block + identifier (constant value) and an unsigned that + provides the length of the block. Blocks may compose other blocks. + |
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. +
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 handled +specially and 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.
+Slot numbers are also kept small by rearranging their order. Because of the +structure of LLVM, certain values are much more likely to be used frequently +in the body of a function. For this reason, a compaction table is provided in +the body of a function if its use would make the function body smaller. +Suppose you have a function body that uses just the types "int*" and "{double}" +but uses them thousands of time. Its worthwhile to ensure that the slot number +for these types are low so they can be encoded in a single byte (via vbr). +This is exactly what the compaction table does.
This section provides the general layout of the LLVM bytecode file format. + The detailed layout can be found in the next section. +
+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 | +
|---|---|---|---|---|---|
| N/A | File | No | No | 0 | +Signature | +
| 0x01 | File | No | No | 0 | +Module | +
| 0x15 | Module | No | No | 1 | ++ Global Type Pool | +
| 0x14 | Module | No | No | 1 | ++ Module Globals Info | +
| 0x12 | Module | Yes | No | 1 | ++ Module Constant Pool | +
| 0x11 | Module | Yes | Yes | 1 | ++ Function Definitions | +
| 0x12 | Function | Yes | No | 2 | ++ Function Constant Pool | +
| 0x33 | Function | Yes | No | 2 | ++ Compaction Table | +
| 0x32 | Function | No | No | 2 | ++ Instruction List | +
| 0x13 | Function | Yes | No | 2 | ++ Function Symbol Table | +
| 0x13 | Module | Yes | No | 1 | ++ Module Symbol Table | +
Use the links in the table or see Block Types for +details about the contents of each of the block types.
+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:
-The descriptions of the bytecode format that follow describe the order, type +and bit fields in detail. These descriptions are provided in tabular form. +Each table has four columns that specify:
+| Byte(s) | -Bit(s) | -Align? | Type | -Field Description | +Field Description | |
|---|---|---|---|---|---|---|
| 00 | - | No | char | -Constant "l" (0x6C) | +char | +Constant "l" (0x6C) |
| 01 | - | No | char | -Constant "l" (0x6C) | +char | +Constant "l" (0x6C) |
| 02 | - | No | char | -Constant "v" (0x76) | +char | +Constant "v" (0x76) |
| 03 | - | No | char | -Constant "m" (0x6D) | +char | +Constant "m" (0x6D) |
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 (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):
- -The table below shows the format of the module block header. It is defined -by blocks described in other sections.
+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.| Byte(s) | -Bit(s) | -Align? | Type | -Field Description | +Field Description | |
|---|---|---|---|---|---|---|
| 04-07 | - | No | unsigned | -Module Identifier (0x01) | +unsigned | +Module Identifier (0x01) |
| 08-11 | - | No | unsigned | -Size of the module block in bytes | +unsigned | +Size of the module block in bytes |
| 12-15 | 00 | Yes | uint32_vbr | -Format Information | +uint32_vbr | +Format Information |
| '' | 0 | - | bit | -Big Endian? | +block | +Global Type Pool |
| '' | 1 | - | bit | -Pointers Are 64-bit? | +block | +Module Globals Info |
| '' | 2 | - | bit | -Has No Endianess? | +block | +Module Constant Pool |
| '' | 3 | - | bit | -Has No Pointer Size? | +block | +Function Definitions |
| '' | 4-31 | - | bit | -Bytecode Format Version | -||
| 16-end | - | - | blocks | -The remaining bytes in the block consist - solely of other block types in sequence. | +block | +Module Symbol Table |
The format information field is encoded into a 32-bit vbr-encoded unsigned +integer as shown in the following table.
+| Bit(s) | +Type | +Description | +
|---|---|---|
| 0 | bit | +Big Endian? | +
| 1 | bit | +Pointers Are 64-bit? | +
| 2 | bit | +Has No Endianess? | +
| 3 | bit | +Has No Pointer Size? | +
| 4-31 | bit | +Bytecode Format Version | +
+Of particular note, the bytecode format number is simply a 28-bit +monotonically increase 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, 1.3):
+Note that we plan to eventually expand the target description capabilities -of bytecode files to target -triples.
- +of bytecode files to target triples. +The global type pool consists of type definitions. Their order of appearance in the file determines their slot number (0 based). Slot numbers are used to @@ -423,48 +533,161 @@ 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 +
The type pool is simply a list of type definitions, as shown in the table below.
| Byte(s) | -Bit(s) | -Align? | Type | -Field Description | +Field Description | |
|---|---|---|---|---|---|---|
| 00-03 | - | No | unsigned | -Type Pool Identifier (0x13) | +unsigned | +Type Pool Identifier (0x13) |
| 04-07 | - | No | unsigned | -Size in bytes of the symbol table block. | +unsigned | +Size in bytes of the symbol table block. |
| 08-111 | - | No | uint32_vbr | -Number of entries in type plane | +uint32_vbr | +Number of entries in type plane |
| 12-151 | - | No | uint32_vbr | -Type plane index for following entries | +type | +Each of the type definitions (see below)1 | +
|
+ 1Repeated field. + |
+ ||||||
Types in the type pool are defined using a different format for each +basic type of type as given in the following sections.
+The primitive types encompass the basic integer and floating point types
+| Type | +Description | |||||
|---|---|---|---|---|---|---|
| 16-end1,2 | - | No | type | -Each of the type definitions. | +uint32_vbr | +Type ID For The Primitive (1-11)1 |
| 1Maximum length shown,
- may be smaller 2Repeated field. + |
+ 1See the definition of Type::TypeID in Type.h for the numeric
+ equivalents of the primitive type ids. + |
+ |||||
| Type | +Description | +
|---|---|
| uint32_vbr | +Type ID for function types (13) | +
| uint32_vbr | +Slot number of function's return type. | +
| uint32_vbr | +The number of arguments in the function. | +
| uint32_vbr | +Slot number of each argument's type.1 | +
| uint32_vbr | +Value 0 if this is a varargs function.2 | +
|
+ 1Repeated field. + 2Optional field. + |
+ |
| Type | +Description | +
|---|---|
| uint32_vbr | +Type ID for structure types (14) | +
| uint32_vbr | +Slot number of each of the element's fields.1 | +
| uint32_vbr | +Null Terminator (VoidTy type id) | +
|
+ 1Repeated field. + |
+ |
| Type | +Description | +
|---|---|
| uint32_vbr | +Type ID for Array Types (15) | +
| uint32_vbr | +Slot number of array's element type. | +
| uint32_vbr | +The number of elements in the array. |
| Type | +Description | +
|---|---|
| uint32_vbr | +Type ID For Pointer Types (16) | +
| uint32_vbr | +Slot number of pointer's element type. | +
| Type | +Description | +
|---|---|
| uint32_vbr | +Type ID For Opaque Types (17) | +
To be determined.
To be determined.
To be determined.
To be determined.
+To be determined.
TBD: How version 1.2 differs from version 1.3
+In version 1.2, the Type class in the LLVM IR derives from the Value class. + This is not the case in version 1.3. Consequently, in version 1.2 the notion + of a "Type Type" was used to write out values that were Types. The types + always occuped plane 12 (corresponding to the TypeTyID) of any type planed + set of values. In 1.3 this representation is not convenient because the + TypeTyID (12) is not present and its value is now used for LabelTyID. + Consequently, the data structures written that involve types do so by writing + all the types first and then each of the value planes according to those + types. In version 1.2, the types would have been written intermingled with + the values.
+In version 1.2, the getelementptr instruction required a ubyte type index + for accessing a structure field and a long type index for accessing an array + element. Consequently, it was only possible to access structures of 255 or + fewer elements. Starting in version 1.3, this restriction was lifted. + Structures must now be indexed with int or uint types. Arrays must now be + indexed with long or ulong types. This requirement was needed so that LLVM + could compile several test cases that used large numbers of fields in their + structures. The consequence of this was that the bytecode format had to + change in order to accommodate the larger range of structure indices.
In version 1.1, the zero value for primitives was explicitly encoded into + the bytecode format. Since these zero values are constant values in the + LLVM IR and never change, there is no reason to explicitly encode them. This + explicit encoding was removed in version 1.2.
+TBD: How version 1.1 differs from version 1.2
+In version 1.1, the Module Global Info block was not aligned causing the + next block to be read in on an unaligned boundary. This problem was corrected + in version 1.2.
TBD: How version 1.0 differs from version 1.1
+None. Version 1.0 and 1.1 bytecode formats are identical.