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 +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.
@@ -80,8 +82,9 @@ next in the file.Except for the Header Block all blocks are variable -length. They consume just enough bytes to express their contents.
+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.
@@ -112,6 +115,88 @@ 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.
+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:
+| 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 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.
+| 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). | +
| 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 + bytes using variable bit rate encoding. | +
| int64_vbr | +A 64-bit signed integer that occupies from one to ten + bytes using variable bit rate encoding. | +
| char | +A single unsigned character encoded into one byte | +
| bit | +A single bit within a byte. | +
| 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. Characters are interpreted as unsigned + char and are generally US-ASCII encoded. | +
| data | +An arbitrarily long segment of data to which no + interpretation is implied. This is used for float, double, and constant + initializers. | +
To support cross-platform differences, the bytecode file is aligned on @@ -123,8 +208,8 @@ 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.
+This section provides the detailed layout of the LLVM bytecode file format. +bit and byte level specifics.
The Header Block occurs in every LLVM bytecode file and is always first. It -simply provides a few bytes of data to identify the file, its format, and the -bytecode version. This block is fixed length and always eight bytes, as follows: -
| Byte(s) | Bit(s) | Align? | Type | Field Description | -
|---|---|---|---|---|
| 00 | 00-07 | No | Char | +|
| 00 | - | No | char | Constant "l" (0x6C) | -
| 01 | 00-07 | No | Char | +|
| 01 | - | No | char | Constant "l" (0x6C) | -
| 02 | 00-07 | No | Char | +|
| 02 | - | No | char | Constant "v" (0x76) | -
| 03 | 00-07 | No | Char | +|
| 03 | - | No | 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. 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.
+| 04-07 | 00 | No | Bool | -Target is big endian? | -|||||
| 04-07 | 01 | No | Bool | -Target has long pointers? | -|||||
| 04-07 | 02 | No | Bool | -Target has no endianess? | -|||||
| 04-07 | 03 | No | Bool | -Target has no pointer size? | -|||||
| 04-07 | 04-31 | Yes | Unsigned | -The LLVM bytecode format version number | +Byte(s) | +Bit(s) | +Align? | +Type | +Field Description | +
|---|---|---|---|---|---|---|---|---|---|
| 04-07 | - | No | unsigned | +Module Identifier (0x01) | +|||||
| 08-11 | - | No | unsigned | +Size of the module block in bytes | +|||||
| 12-15 | 00 | Yes | uint32_vbr | +Format Information | +|||||
| '' | 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 | +|||||
| 16-end | - | - | blocks | +The remaining bytes in the block consist + solely of other block types in sequence. |
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.
+| Byte(s) | +Bit(s) | +Align? | +Type | +Field Description | +
|---|---|---|---|---|
| 00-03 | - | No | unsigned | +Type Pool Identifier (0x13) | +
| 04-07 | - | No | unsigned | +Size in bytes of the symbol table block. | +
| 08-111 | - | No | uint32_vbr | +Number of entries in type plane | +
| 12-151 | - | No | uint32_vbr | +Type plane index for following entries | +
| 16-end1,2 | - | No | type | +Each of the type definitions. | +
| 1Maximum length shown,
+ may be smaller 2Repeated field. + | ||||
To be determined.
To be determined.
To be determined.
The module symbol table is a list of +
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.
+| Byte(s) | +Bit(s) | +Align? | +Type | +Field Description | +
|---|---|---|---|---|
| 00-03 | - | No | unsigned | +Symbol Table Identifier (0x13) | +
| 04-07 | - | No | unsigned | +Size in bytes of the symbol table block. | +
| 08-111 | - | No | uint32_vbr | +Number of entries in type plane | +
| 12-151 | - | No | uint32_vbr | +Type plane index for following entries | +
| 16-191,2 | - | No | uint32_vbr | +Slot number of a value. | +
| variable1,2 | - | No | string | +Name of the value in the symbol table. | +
| 1Maximum length shown,
+ may be smaller 2Repeated field. + | ||||