-The bitstream format is an abstract encoding of structured data, like very +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 @@ -99,8 +106,10 @@ understanding the encoding.
The first four bytes of the stream identify the encoding of the file. This -is used by a reader to know what is contained in the file.
+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.
-A bitstream literally consists of a stream of bits. This stream is made up of a -number of primitive values that encode a stream of integer values. These +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. @@ -161,12 +172,12 @@ value of 24 (011 << 3) with no continuation. The sum (3+24) yields the value
6-bit characters encode common characters into a fixed 6-bit field. They
-represent the following characters with the following 6-bit values:
+represent the following characters with the following 6-bit values:
Abbreviation IDs 4 and above are defined by the stream itself.
+Abbreviation IDs 4 and above are defined by the stream itself, and specify +an abbreviated record encoding.
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). Nested blocks capture the hierachical structure of the data +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 @@ -249,8 +264,11 @@ block. In particular, each block maintains: block record is entered. The block entry specifies the abbrev id width for the body of the block. -
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 is application specific). The +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 @@ -303,13 +322,227 @@ multiple of 32-bits.
+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, 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.
+[<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. +
+ +-blah +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, 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).
+ +The possible operand encodings are:
+ +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:
+ +When emitting a record with this abbreviation, the above entry would be +emitted as:
+ +[4abbrevwidth, 24, 4vbr6, + 06, 16, 26, 36]
+ +These values are:
+ +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. +
+ ++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. +
+ +The BLOCKINFO block allows the description of metadata for other blocks. The + currently specified records are:
+ ++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 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.
+ ++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".
+ ++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:
+ +With this encoding, small positive and small negative values can both be +emitted efficiently.
+ ++LLVM IR is defined with the following blocks: +
+ ++