-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 provides a mechanism for the file to self-describe "abbreviations", which are effectively size optimizations for the content.
-This document first describes the LLVM bitstream format, then describes the -record structure used by LLVM IR files. +
LLVM IR files may be optionally embedded into a wrapper structure that makes it easy to embed extra data +along with LLVM IR files.
+ +This document first describes the LLVM bitstream format, describes the +wrapper format, then describes the record structure used by LLVM IR files.
@@ -100,8 +121,10 @@ 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. @@ -164,13 +189,15 @@ 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:
-+'a' .. 'z' — 0 .. 25 +'A' .. 'Z' — 26 .. 51 +'0' .. '9' — 52 .. 61 + '.' — 62 + '_' — 63 ++
This encoding is only suitable for encoding characters and strings that consist only of the above characters. It is completely incapable of encoding @@ -202,7 +229,7 @@ A bitstream is a sequential series of Blocks and abbreviation ID encoded as a fixed-bitwidth field. The width is specified by the current block, as described below. The value of the abbreviation ID specifies either a builtin ID (which have special meanings, defined below) or -one of the abbreviation IDs defined by the stream itself. +one of the abbreviation IDs defined for the current block by the stream itself.
@@ -210,14 +237,14 @@ The set of builtin abbrev IDs is:
Abbreviation IDs 4 and above are defined by the stream itself, and specify @@ -234,11 +261,13 @@ 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 hierarchical 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 -efficiently skip data they do not understand. The LLVM IR reader uses this +efficiently skip data it does not understand. The LLVM IR reader uses this mechanism to skip function bodies, lazily reading them on demand.
@@ -248,18 +277,25 @@ block. In particular, each block maintains:As sub blocks are entered, these properties are saved and the new sub-block -has its own set of abbreviations, and its own abbrev id width. When a sub-block -is popped, the saved values are restored.
++As sub blocks are entered, these properties are saved and the new sub-block has +its own set of abbreviations, and its own abbrev id width. When a sub-block is +popped, the saved values are restored. +
-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 -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 -allows the reader to skip over the entire block in one jump. +The ENTER_SUBBLOCK abbreviation ID specifies the start of a new block +record. The blockid value is encoded as an 8-bit VBR identifier, and +indicates 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 value is a 32-bit aligned value +that specifies the size of the subblock in 32-bit words. This value allows the +reader to skip over the entire block in one jump.
@@ -293,9 +330,10 @@ Encoding[END_BLOCK, <align32bits>]
-The END_BLOCK abbreviation ID specifies the end of the current block record. -Its end is aligned to 32-bits to ensure that the size of the block is an even -multiple of 32-bits.
+The END_BLOCK abbreviation ID specifies the end of the current block +record. Its end is aligned to 32-bits to ensure that the size of the block is +an even multiple of 32-bits. + @@ -307,13 +345,15 @@ 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.
+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 may vary between different block types. +Records can be encoded either using 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.
++An UNABBREV_RECORD provides a default fallback encoding, which is both +completely general and 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.
++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. Because there +are no letters with values 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. +
@@ -348,15 +392,21 @@ Encoding[<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. +
+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.
+The record code, which is the first field of an abbreviated record, +may be encoded in the abbreviation definition (as a literal +operand) or supplied in the abbreviated record (as a Fixed or VBR +operand value).
+ @@ -373,10 +423,11 @@ emitted.-Abbreviations can be determined dynamically per client, per file. Since the +Abbreviations can be determined dynamically per client, per file. Because 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 +format can contain different sets of abbreviations according to the needs +of the specific stream. +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.
@@ -391,28 +442,214 @@ operators, the abbreviation does not need to be emitted.[DEFINE_ABBREV, numabbrevopsvbr5, abbrevop0, abbrevop1, ...]
-An abbreviation definition consists of the DEFINE_ABBREV abbrevid followed -by a VBR that specifies the number of abbrev operands, then the abbrev +
+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 for the particular block type +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).
+(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: +
+ ++[0, Fixed, 4] +[0, Array] +[0, Char6] ++
+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, because 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 built-in 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: +
+ ++[SETBID (#1), blockid] +[DEFINE_ABBREV, ...] +[BLOCKNAME, ...name...] +[SETRECORDNAME, RecordID, ...name...] ++
+The SETBID record (code 1) 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. +
+ +The BLOCKNAME record (code 2) can optionally occur in this block. The elements of +the record are the bytes of the string name of the block. llvm-bcanalyzer can use +this to dump out bitcode files symbolically.
+ +The SETRECORDNAME record (code 3) can also optionally occur in this block. The +first operand value is a record ID number, and the rest of the elements of the record are +the bytes for the string name of the record. llvm-bcanalyzer can use +this to dump out bitcode files symbolically.
+ ++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. +
+ ++Bitcode files for LLVM IR may optionally be wrapped in a simple wrapper +structure. This structure contains a simple header that indicates the offset +and size of the embedded BC file. This allows additional information to be +stored alongside the BC file. The structure of this file header is: +
+ ++[Magic32, Version32, Offset32, +Size32, CPUType32] +
++Each of the fields are 32-bit fields stored in little endian form (as with +the rest of the bitcode file fields). The Magic number is always +0x0B17C0DE and the version is currently always 0. The Offset +field is the offset in bytes to the start of the bitcode stream in the file, and +the Size field is the size in bytes of the stream. CPUType is a target-specific +value that can be used to encode the CPU of the target. +
+ ++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 Integer encoding is an efficient way to +encode arbitrary sized unsigned values, but is an extremely inefficient for +encoding 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. Signed VBR encoding is used in +CST_CODE_INTEGER and CST_CODE_WIDE_INTEGER records +within CONSTANTS_BLOCK blocks. +
+ ++LLVM IR is defined with the following blocks: +
+ +The MODULE_BLOCK block (id 8) is the top-level block for LLVM +bitcode files, and each bitcode file must contain exactly one. In +addition to records (described below) containing information +about the module, a MODULE_BLOCK block may contain the +following sub-blocks: +
+ + + +[VERSION, version#]
+ +The VERSION record (code 1) contains a single value +indicating the format version. Only version 0 is supported at this +time.
+[TRIPLE, ...string...]
+ +The TRIPLE record (code 2) contains a variable number of +values representing the bytes of the target triple +specification string.
+[DATALAYOUT, ...string...]
+ +The DATALAYOUT record (code 3) contains a variable number of +values representing the bytes of the target datalayout +specification string.
+[ASM, ...string...]
+ +The ASM record (code 4) contains a variable number of +values representing the bytes of module asm strings, with +individual assembly blocks separated by newline (ASCII 10) characters.
+[SECTIONNAME, ...string...]
+ +The SECTIONNAME record (code 5) contains a variable number +of values representing the bytes of a single section name +string. There should be one SECTIONNAME record for each +section name referenced (e.g., in global variable or function +section attributes) within the module. These records can be +referenced by the 1-based index in the section fields of +GLOBALVAR or FUNCTION records.
+[DEPLIB, ...string...]
+ +The DEPLIB record (code 6) contains a variable number of +values representing the bytes of a single dependent library name +string, one of the libraries mentioned in a deplibs +declaration. There should be one DEPLIB record for each +library name referenced.
+[GLOBALVAR, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal]
+ +The GLOBALVAR record (code 7) marks the declaration or +definition of a global variable. The operand fields are:
+ +[FUNCTION, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc]
+ +The FUNCTION record (code 8) marks the declaration or +definition of a function. The operand fields are:
+ +[ALIAS, alias type, aliasee val#, linkage, visibility]
+ +The ALIAS record (code 9) marks the definition of an +alias. The operand fields are
+ +[PURGEVALS, numvals]
+ +The PURGEVALS record (code 10) resets the module-level +value list to the size given by the single operand value. Module-level +value list items are added by GLOBALVAR, FUNCTION, +and ALIAS records. After a PURGEVALS record is seen, +new value indices will start from the given numvals value.
+[GCNAME, ...string...]
+ +The GCNAME record (code 11) contains a variable number of +values representing the bytes of a single garbage collector name +string. There should be one GCNAME record for each garbage +collector name referenced in function gc attributes within +the module. These records can be referenced by 1-based index in the gc +fields of FUNCTION records.
+The PARAMATTR_BLOCK block (id 9) ... +
+ +[ENTRY, paramidx0, attr0, paramidx1, attr1...]
+ +The ENTRY record (code 1) ... +
+The TYPE_BLOCK block (id 10) ... +
+ +The CONSTANTS_BLOCK block (id 11) ... +
+ +The FUNCTION_BLOCK block (id 12) ... +
+ +In addition to the record types described below, a +FUNCTION_BLOCK block may contain the following sub-blocks: +
+ + + +The TYPE_SYMTAB_BLOCK block (id 13) ... +
+ +The VALUE_SYMTAB_BLOCK block (id 14) ... +
+ +The METADATA_BLOCK block (id 15) ... +
+ +The METADATA_ATTACHMENT block (id 16) ... +
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