Matt S Trout > Rakudo-Star-2012.08_001 > rakudo-star/parrot/docs/pdds/draft/pdd13_bytecode.pod

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[DRAFT] PDD 13: Bytecode ^

Abstract

This PDD describes the file format for Parrot Bytecode (PBC) files and the interface through which they may be manipulated programmatically.

Synopsis

Parrot bytecode is a binary representation of instructions and data for execution on the virtual machine.

Description

PBC, Parrot bytecode, is the binary format used internally by the Parrot VM to store the data necessary to execute a compiled PIR program. The sequence of instructions making up a Parrot program, a constants table, an annotations table and any ancillary data are stored in a PBC. These files usually have the extension .pbc.

The PBC format is designed so that any valid PBC file can be read and executed by Parrot on any platform, but may be encoded more optimally for a particular platform.

It is possible to add arbitrary annotations to the instruction sequence, for example line numbers in high level languages and other debug data.

PMCs are be used to represent packfiles and packfile segments to provide a programmatic interface, both to Parrot programs and Parrot internals.

Implementation

Packfiles

This section of the documentation describes the format of Parrot packfiles. These contain the bytecode (sequence of instructions), constants table, fixup table, debug data, annotations and possibly more.

Note that, unless otherwise stated, all offsets and lengths are given in terms of Parrot opcodes, not bytes. An opcode corresponds to the word size, defined as long. The ptrsize is silently assumed to be the same as the opcode size.

Packfile Header

PBC files start with a variable length header. All data in this header is stored as strings or in a single byte so endianness and word size need not be considered when reading it.

Note that in this section only, offsets and lengths are in bytes.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 8      | 0xFE 0x50 0x42 0x43 0x0D 0x0A 0x1A 0x0A                |
  |        |        | Parrot "Magic String" to identify a PBC file. In C,    |
  |        |        | this is the string C<\376PBC\r\n\032\n> or             |
  |        |        | C<\xfe\x50\x42\x43\x0d\x0a\x1a\x0a>.                   |
  +--------+--------+--------------------------------------------------------+
  | 8      | 1      | Word size in bytes of words making up the segments of  |
  |        |        | the PBC file. Must be one of:                          |
  |        |        |    0x04 - 4 byte (32-bit) words                        |
  |        |        |    0x08 - 8 byte (64-bit) words                        |
  +--------+--------+--------------------------------------------------------+
  | 9      | 1      | Byte order within the words making up the segments of  |
  |        |        | the PBC file. Must be one of:                          |
  |        |        |    0x00 - Little Endian                                |
  |        |        |    0x01 - Big Endian                                   |
  +--------+--------+--------------------------------------------------------+
  | 10     | 1      | The encoding of floating point numbers in the file.    |
  |        |        | Must be one of:                                        |
  |        |        |    0x00 - IEEE 754 8 byte double                       |
  |        |        |    0x01 - i386 little endian 12 byte long double       |
  |        |        |    0x02 - IEEE 754 16 byte long double                 |
  +--------+--------+--------------------------------------------------------+
  | 11     | 1      | Major version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 0.            |
  +--------+--------+--------------------------------------------------------+
  | 12     | 1      | Minor version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 9.            |
  +--------+--------+--------------------------------------------------------+
  | 13     | 1      | Patch version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 5.            |
  +--------+--------+--------------------------------------------------------+
  | 14     | 1      | Major version number of the bytecode file format. See  |
  |        |        | the section below on bytecode file format version      |
  |        |        | numbers.                                               |
  +--------+--------+--------------------------------------------------------+
  | 15     | 1      | Minor version number of the bytecode file format. See  |
  |        |        | the section below on bytecode file format version      |
  |        |        | numbers.                                               |
  +--------+--------+--------------------------------------------------------+
  | 16     | 1      | The type of the UUID associated with this packfile.    |
  |        |        | Must be one of:                                        |
  |        |        |    0x00 - No UUID                                      |
  |        |        |    0x01 - MD5                                          |
  +--------+--------+--------------------------------------------------------+
  | 17     | 1      | Length of the UUID associated with this packfile. May  |
  |        |        | be zero if the type of the UUID is 0x00. Maximum       |
  |        |        | value is 255.                                          |
  +--------+--------+--------------------------------------------------------+
  | 18     | u      | A UUID of u bytes in length, where u was specified as  |
  |        |        | the length of the UUID in the previous field. Be sure  |
  |        |        | that UUIDs are stored and read as strings. The UUID is |
  |        |        | computed by applying the hash function specified in    |
  |        |        | the UUID type field over the entire packfile not       |
  |        |        | including this header and its trailing zero padding.   |
  +--------+--------+--------------------------------------------------------+
  | 18 + u | n      | Zero-padding to make the total header length a         |
  |        |        | multiple of 16 bytes in length.                        |
  |        |        |    n = u % 16 ? 16 - (u % 16) : 0                      |
  +--------+--------+--------------------------------------------------------+

Everything beyond the header is an opcode, with word length and byte ordering as defined in the header. If the word length and byte ordering of the machine that is reading the PBC file do not match these, it needs to transform the words making up the rest of the packfile.

We should be aware that some systems such as a Sparc/PPC 64-bit use strict 8-byte ptr_alignment per default, and all (opcode_t*)cursor++ or (opcode_t*)cursor += advances must ensure that the cursor ptr is 8-byte aligned. We enforce 16-byte alignment at the start and end of all segments and ptrsize alignment for all items (strings, integers, and opcode_t ops), but not in-between, esp. with 4-byte integers and 4-byte opcode_t pointers.

So we relax pointer alignment strictness on Sparc64, but may add a --64compat option to parrot in the future to produce 8-byte aligned data. Operations on aligned pointers are much faster than on un-aligned pointers.

Directory Format Header

Packfiles contain a directory describing the segments that it contains. This header specifies the format of the directory.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The format of the directory. Must be:                  |
  |        |        |    0x01 - Directory Format 1                           |
  +--------+--------+--------------------------------------------------------+
  | 1      | 3      | Must be:                                               |
  |        |        |    0x00 0x00 0x00 - Reserved                           |
  +--------+--------+--------------------------------------------------------+

Currently only Format 1 exists. In the future, the format of the directory may change. A single version of Parrot may then become capable of generating and reading files of more than one directory format. This header enables Parrot to detect whether it is able to read the directory segment in the packfile.

This header must be followed immediately by a directory segment.

Packfile Segment Header

All segments, regardless of type, start with a 1 opcode segment header. All other segments below are prefixed with this.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The total size of the segment in opcodes, including    |
  |        |        | this header.                                           |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Internal type of the segment                           |
  +--------+--------+--------------------------------------------------------+
  | 2      | 1      | Internal id                                            |
  +--------+--------+--------------------------------------------------------+
  | 3      | 1      | Size of the following op array, 0 if none              |
  +--------+--------+--------------------------------------------------------+

Segment Padding

All segments must have trailing zero (NULL) values appended so they are a multiple of 16 bytes in length. (This allows wordsize support of up to 128 bits.)

Directory Segment

This segment lists the other segments that make up the packfile and where in the file they are located. It must occur immediately after the directory format header. Only one of these segments may occur in a packfile. In the future, a hierarchy of directories may be allowed.

The directory segment adds one additional header after the standard packfile header data, which specifies the number of entries in the directory.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The number of entries in the directory.                |
  +--------+--------+--------------------------------------------------------+

Following this are n variable length entries formatted as described in the following table. Offsets are in words, but are given relative to the start of an individual entry.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The type of the segment. Must be one of the following: |
  |        |        |    0x00 - Reserved (Directory Segment)                 |
  |        |        |    0x01 - Default Segment                              |
  |        |        |    0x02 - Fixup Segment                                |
  |        |        |    0x03 - Constant Table Segment                       |
  |        |        |    0x04 - Bytecode Segment                             |
  |        |        |    0x05 - PIR Debug Segment                            |
  |        |        |    0x06 - Annotations Segment                          |
  +--------+--------+--------------------------------------------------------+
  | 1      | n      | The name of the segment, as a (NULL terminated) ASCII  |
  |        |        | C string. This must be padded with trailing NULL       |
  |        |        | (zero) values to be a full word in size.               |
  +--------+--------+--------------------------------------------------------+
  | n + 1  | 1      | The offset to the segment, relative to the start of    |
  |        |        | the packfile. Specified as a number of words, where    |
  |        |        | the word size is that specified in the header. (Parrot |
  |        |        | may need to do some computation to transform this to   |
  |        |        | an offset in terms of its own word size.) As segments  |
  |        |        | must always be aligned on 16-byte boundaries, this     |
  |        |        | scheme scales up to 128-bit platforms.                 |
  +--------+--------+--------------------------------------------------------+
  | n + 2  | 1      | The length of the segment, including its header, in    |
  |        |        | words. This must match the length stored at the start  |
  |        |        | of the header of the segment the entry is describing.  |
  +--------+--------+--------------------------------------------------------+

Default Segment

The default segment has no additional headers. It will, if possible, be memory mapped. More than one may exist in the packfile, and they are identified by name. They may be used for storing any data that does not fit into any other segment, for example the source code from a high level language (HLL).

Bytecode Segment

This segment has no additional headers. It stores a stream of instructions in bytecode format, with the length given in the last field of the segment header.

Instructions have variable length. Each instruction starts with an operation code (opcode).

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | A valid Parrot opcode, as specified in the opcode      |
  |        |        | list include/parrot/oplib/ops.h.                       |
  +--------+--------+--------------------------------------------------------+

Zero or more operands follow the opcode. All opcodes take a fixed number of operands. An individual operand is always one word in length and may be of one of the following forms.

  +------------------+-------------------------------------------------------+
  | Operand Type     | Description                                           |
  +------------------+-------------------------------------------------------+
  | Register         | An integer specifying a register number.              |
  +------------------+-------------------------------------------------------+
  | Integer Constant | An integer that is the constant itself. That is, the  |
  |                  | constant is stored directly in the instruction        |
  |                  | stream. Storing integer constants of length greater   |
  |                  | than 32 bits has undefined behaviour and should be    |
  |                  | considered unportable.                                |
  +------------------+-------------------------------------------------------+
  | Number Constant  | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+
  | String Constant  | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+
  | PMC Constant     | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+

Constants Segment

This segment stores number, string and PMC constants.

The first element is the number of constants contained.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 2      | 1      | The number of constants in the table.                  |
  +--------+--------+--------------------------------------------------------+

Following this are n constants, each with a single word header specifying the type of constant that follows.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The type of the constant. Must be one of:              |
  |        |        |    0x00 - No constant                                  |
  |        |        |    0x6E - Number constant (ASCII 'n')                  |
  |        |        |    0x73 - String constant (ASCII 's')                  |
  |        |        |    0x70 - PMC constant (ASCII 'p')                     |
  |        |        |    0x6B - Key constant (ASCII 'k')                     |
  +--------+--------+--------------------------------------------------------+

All constants that are not a multiple of the word size in length must be padded with trailing zero bytes up to a word size boundary.

Fixup Segment

The fixup segment maps names of subs to offsets in the bytecode stream.

The number of fixup table entries, n, is given by the last field of the segment header.

This is followed by n fixup table entries, of variable length, that take the following form.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Type of the fixup. Must be:                            |
  |        |        |    0x01 - Subroutine fixup constant string             |
  |        |        |    0x02 - Subroutine fixup ascii string                |
  +--------+--------+--------------------------------------------------------+
  | 1      | -      | The label that is being fixed up. A string constant,   |
  |        |        | stored as an index into the constants table in the 01  |
  |        |        | case, a NULL terminated ASCII string padded to word    |
  |        |        | length with zeroes in the 02.                          |
  +--------+--------+--------------------------------------------------------+
  | -      | 1      | This is an index into the constants table for the sub  |
  |        |        | PMC corresponding to the label.                        |
  +--------+--------+--------------------------------------------------------+

PIR Debug Segment

This segment stores a list of mappings between offsets in the bytecode and filenames, indicating that the bytecode from that point on until the next entry was generated from the PIR found in the given filename

The segment begins with an opcode with n, the number of file mappings. Then come n mappings:

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Offset in the bytecode.                                |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | A string constant holding the filename, stored as an   |
  |        |        | index into the constants table.                        |
  +--------+--------+--------------------------------------------------------+

Annotations Segment

Annotations allow any instruction in the bytecode stream to have zero or more key/value pairs associated with it. These can be retrieved at runtime. High level languages can use annotations to store file names, line numbers, column numbers and any other data, for debug purposes or otherwise, that they need.

The segment comes in three parts:

A list of annotation keys (for example, "line" and "file").
An annotation groups table, used to group together annotations for a particular HLL source file (an annotation group starting clears all active annotations, so they will not spill over between source files; it also allows for faster lookup of annotations).

{{ TODO: Does it clear all annotations, or all annotation groups? }}

A list of indexes into the bytecode stream and key/value pairings (for example, starting at instruction 235, the annotation "line" has value "42").

The last field of the segment header is not used.

The first word in the segment supplies the number of keys.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of annotation key entries that follow.          |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n annotation key entries. There is one entry per key (such as "line" or "file"), but the bytecode may be annotated many times with that key. Key entries take the following format.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Index into the constants table of a string containing  |
  |        |        | the name of the key.                                   |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The type of value that is stored with the key.         |
  |        |        |    0x00 - Integer                                      |
  |        |        |    0x01 - String Constant                              |
  |        |        |    0x02 - Number Constant                              |
  |        |        |    0x03 - PMC Constant                                 |
  +--------+--------+--------------------------------------------------------+

The annotation groups table comes next. This starts with a single integer to specify the number of entries in the table.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of annotation group entries that follow.        |
  +--------+--------+--------------------------------------------------------+

A group entry maps an offset in the bytecode segment to an offset in the list of annotations (that is, offset 0 refers to the first word following this table). The list of offsets into the bytecode segment (and by the definition of this segment, the offsets into the annotations list) must be in ascending order.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Offset into the bytecode segment where the             |
  |        |        | instructions for a particular high level source file   |
  |        |        | start.                                                 |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Offset into the annotations list specifying where the  |
  |        |        | annotations for the given instruction start.           |
  +--------+--------+--------------------------------------------------------+

The rest of the segment is made up of a sequence of bytecode offset to key and value mappings. First comes the number of them that follow:

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of bytecode to keypair mappings that follow.    |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Then there are n entries of the following format:

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Offset into the bytecode segment, in words, of the     |
  |        |        | instruction being annotated. At runtime, this will     |
  |        |        | correspond to the program counter.                     |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The key of the annotation, specified as an index into  |
  |        |        | the zero-based list of keys specified in the first     |
  |        |        | part of the segment. That is, if key "line" was the    |
  |        |        | first entry and "file" the second, they would have     |
  |        |        | indices 0 and 1 respectively.                          |
  +--------+--------+--------------------------------------------------------+
  | 2      | 2      | The value of the annotation. If the annotation type    |
  |        |        | (specified with the key) is an integer, the value is   |
  |        |        | placed directly into this word. Otherwise, an index    |
  |        |        | into the constants table is used.                      |
  +--------+--------+--------------------------------------------------------+

Note that the value of an annotation with a particular key is taken to apply to all following instructions up to the point of a new value being specified for that key with another annotation. This means that if 20 instructions make up the compiled form of a single line of code, only one line annotation is required. Note that this also implies that annotations must be placed in the same order as the instructions.

Packfile PMCs

A packfile can be represented in memory by Parrot as a tree of PMCs. These provide a programmatic way to construct and walk packfiles, both for the Parrot internals and from programs running on the Parrot VM.

{{ TODO... ManagedStruct and UnmanagedStruct may be helpful for these; consider switching these PMCs over to use them at some point. }}

Packfile.pmc

This PMC represents the packfile overall. It will be constructed by the VM when reading a packfile. It implements the following methods and vtable functions.

PackfileSegment.pmc

An abstract PMC that is the base class for all other segments. It has two abstract methods, which are to be implemented by all subclasses. They will not be listed under the method list for other segment PMCs to save space.

PackfileDirectory.pmc (isa PackfileSegment)

This PMC represents a directory segment. Essentially it is an hash of PackfileSegment PMCs. It implements the following methods:

PackfileRawSegment.pmc (isa PackfileSegment)

This PMC presents a segment of a packfile as an array of integers. This is the lowest possible level of access to a segment, and covers both the default and bytecode segment types. It implements the following methods:

PackfileConstantTable.pmc (isa PackfileSegment)

This PMC represents a constants table. It provides access to constants through the keyed integer interface (the interpreter may choose to access underlying structures directly to improve performance, however).

The table of constants can be added to using the keyed set methods; it will grow automatically.

The PMC implements the following methods:

PackfileFixupTable.pmc (isa PackfileSegment)

This PMC provides a keyed integer interface to the fixup table. Each entry in the table is represented by a PackfileFixupEntry PMC. It implements the following methods:

PackfileFixupEntry.pmc

This PMC represents an entry in the fixup table. It implements the following methods.

PackfileAnnotations.pmc (isa PackfileSegment)

This PMC represents the bytecode annotations table. The following methods are implemented:

PackfileAnnotation.pmc

This PMC represents an individual bytecode annotation entry in the annotations segment. It implements the following methods:

Language Notes

None.

Attachments

None.

Footnotes

Changes From Previous Versions

A number of things in this PDD differ from the older implementation, and few items with the more convenient PMC access are not yet implemented. This section details these changes from the old implementation and some of the reasoning behind them.

Packfile Header

The format of the packfile header changed completely, based upon a proposal at http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/1f1af615edec7449/ebfdbb5180a9d813?lnk=gst and the requirement to have a UUID. The old INT field in the previous header format is used nowhere in Parrot and was removed, the parrot patch version number along with the major and minor was added. The opcode type is also gone due to non-use. The opcode type is always long.

The version number now reflects the earliest version of Parrot that is capable of running the bytecode file, to enable cross-version compatibility that will be needed in the future.

Segment Header

Having the type associated with the segment inside the VM is fine, but since it is in the directory segment anyway it seems odd to duplicate it here. Also removed the id (did not seem to be used anywhere) and the second size (always computable by knowing the size of this header, so it appears redundant).

Fixup Segment

We need to support unicode sub names, so fixup labels should be an index into the constants table to the relevant string instead of just a C string as they are now.

Annotations Segment

This is new and replaces and builds upon the debug segment. See here for some on-list discussion:

http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/b0d36dafb42d96c4/4d6ad2ad2243e677?lnk=gst&rnum=2#4d6ad2ad2243e677

Packfile PMCs

This idea will see packfiles and segments within them being represented by PMCs, easing memory management and providing an interface to packfiles for Parrot programs.

Here are mailing list comments that provide one of the motivations or hints of the original proposal.

http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/778ea0ac4c8676f7/b249306b543b040a?lnk=gst&q=packfile+PMCs&rnum=2#b249306b543b040a

References

None.

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