# MLIR Bytecode Format

This document describes the MLIR bytecode format and its encoding.
This format is versioned and stable: we don't plan to ever break
compatibility, that is a dialect should be able to deserialize any
older bytecode. Similarly, we support back-deployment so that an
older version of the format can be targetted.

That said, it is important to realize that the promises of the
bytecode format are made assuming immutable dialects: the format
allows backward and forward compatibility, but only when nothing
in a dialect changes (operations, types, attributes definitions).

A dialect can opt-in to handle its own versioning through the
`BytecodeDialectInterface`. Some hooks are exposed to the dialect
to allow managing a version encoded into the bytecode file. The
version is loaded lazily and allows to retrieve the version
information while decoding the input IR, and gives an opportunity
to each dialect for which a version is present to perform IR
upgrades post-parsing through the `upgradeFromVersion` method.
There is no restriction on what kind of information a dialect
is allowed to encode to model its versioning.

When emitting bytecode, `mlir-opt --emit-bytecode-version=<version>` selects the
overall bytecode format version. Dialect versions are selected separately with
`--emit-bytecode-dialect-version=<dialect>=<version>`, which may be repeated for
different dialects. For example, `mlir-opt --emit-bytecode
--emit-bytecode-dialect-version=builtin=0` selects the historical builtin dialect
encoding. A dialect must implement `BytecodeDialectInterface::parseVersion` to
accept a version string; the dialect defines its syntax and supported versions.
The resulting version object is passed to the bytecode writer through
`BytecodeWriterConfig::setDialectVersion`. Before emission, the writer invokes
`BytecodeDialectInterface::verifyVersion` for explicitly selected versions so
dialects can reject IR that cannot be represented without losing information.

[TOC]

## Magic Number

MLIR uses the following four-byte magic number to
indicate bytecode files:

'\[‘M’<sub>8</sub>, ‘L’<sub>8</sub>, ‘ï’<sub>8</sub>, ‘R’<sub>8</sub>\]'

In hex:

'\[‘4D’<sub>8</sub>, ‘4C’<sub>8</sub>, ‘EF’<sub>8</sub>, ‘52’<sub>8</sub>\]'

## Format Overview

An MLIR Bytecode file is comprised of a byte stream, with a few simple
structural concepts layered on top.

### Primitives

#### Fixed-Width Integers

```
  byte ::= `0x00`...`0xFF`
```

Fixed width integers are unsigned integers of a known byte size. The values are
stored in little-endian byte order.

TODO: Add larger fixed width integers as necessary.

#### Variable-Width Integers

Variable width integers, or `VarInt`s, provide a compact representation for
integers. Each encoded VarInt consists of one to nine bytes, which together
represent a single 64-bit value. The MLIR bytecode utilizes the "PrefixVarInt"
encoding for VarInts. This encoding is a variant of the
[LEB128 ("Little-Endian Base 128")](https://en.wikipedia.org/wiki/LEB128)
encoding, where each byte of the encoding provides up to 7 bits for the value,
with the remaining bit used to store a tag indicating the number of bytes used
for the encoding. This means that small unsigned integers (less than 2^7) may be
stored in one byte, unsigned integers up to 2^14 may be stored in two bytes,
etc.

The first byte of the encoding includes a length prefix in the low bits. This
prefix is a bit sequence of '0's followed by a terminal '1', or the end of the
byte. The number of '0' bits indicate the number of _additional_ bytes, not
including the prefix byte, used to encode the value. All of the remaining bits
in the first byte, along with all of the bits in the additional bytes, provide
the value of the integer. Below are the various possible encodings of the prefix
byte:

```
xxxxxxx1:  7 value bits, the encoding uses 1 byte
xxxxxx10: 14 value bits, the encoding uses 2 bytes
xxxxx100: 21 value bits, the encoding uses 3 bytes
xxxx1000: 28 value bits, the encoding uses 4 bytes
xxx10000: 35 value bits, the encoding uses 5 bytes
xx100000: 42 value bits, the encoding uses 6 bytes
x1000000: 49 value bits, the encoding uses 7 bytes
10000000: 56 value bits, the encoding uses 8 bytes
00000000: 64 value bits, the encoding uses 9 bytes
```

##### Signed Variable-Width Integers

Signed variable width integer values are encoded in a similar fashion to
[varints](#variable-width-integers), but employ
[zigzag encoding](https://en.wikipedia.org/wiki/Variable-length_quantity#Zigzag_encoding).
This encoding uses the low bit of the value to indicate the sign, which allows
for more efficiently encoding negative numbers. If a negative value were encoded
using a normal [varint](#variable-width-integers), it would be treated as an
extremely large unsigned value. Using zigzag encoding allows for a smaller
number of active bits in the value, leading to a smaller encoding. Below is the
basic computation for generating a zigzag encoding:

```
(value << 1) ^ (value >> 63)
```

#### Strings

Strings are blobs of characters with an associated length.

### Sections

```
section {
  idAndIsAligned: byte // id | (hasAlign << 7)
  length: varint,

  alignment: varint?,
  padding: byte[], // Padding bytes are always `0xCB`.

  data: byte[]
}
```

Sections are a mechanism for grouping data within the bytecode. They enable
delayed processing, which is useful for out-of-order processing of data,
lazy-loading, and more. Each section contains a Section ID, whose high bit
indicates if the section has alignment requirements, a length (which allows for
skipping over the section), and an optional alignment. When an alignment is
present, a variable number of padding bytes (0xCB) may appear before the section
data. The alignment of a section must be a power of 2. 
The input bytecode buffer must satisfy the same alignment requirements as 
those of every section.

## MLIR Encoding

Given the generic structure of MLIR, the bytecode encoding is actually fairly
simplistic. It effectively maps to the core components of MLIR.

### Top Level Structure

The top-level structure of the bytecode contains the 4-byte "magic number", a
version number, a null-terminated producer string, and a list of sections. Each
section is currently only expected to appear once within a bytecode file.

```
bytecode {
  magic: "MLïR",
  version: varint,
  producer: string,
  sections: section[]
}
```

### String Section

```
strings {
  numStrings: varint,
  reverseStringLengths: varint[],
  stringData: byte[]
}
```

The string section contains a table of strings referenced within the bytecode,
more easily enabling string sharing. This section is encoded first with the
total number of strings, followed by the sizes of each of the individual strings
in reverse order. The remaining encoding contains a single blob containing all
of the strings concatenated together.

### Dialect Section

The dialect section of the bytecode contains all of the dialects referenced
within the encoded IR, and some information about the components of those
dialects that were also referenced.

```
dialect_section {
  numDialects: varint,
  dialectNames: dialect_name_group[],
  opNames: dialect_ops_group[]  // ops grouped by dialect
}

dialect_name_group {
  nameAndIsVersioned: varint  // (dialectID << 1) | (hasVersion),
  version: dialect_version_section  // only if versioned
}

dialect_version_section {
  size: varint,
  version: byte[]
}

dialect_ops_group {
  dialect: varint,
  numOpNames: varint,
  opNames: op_name_group[]
}

op_name_group {
  nameAndIsRegistered: varint  // (nameID << 1) | (isRegisteredOp)
}
```

Dialects are encoded as a `varint` containing the index to the name string
within the string section, plus a flag indicating whether the dialect is
versioned. Operation names are encoded in groups by dialect, with each group
containing the dialect, the number of operation names, and the array of indexes
to each name within the string section. The version is encoded as a nested
section for each dialect.

### Attribute/Type Sections

Attributes and types are encoded using two [sections](#sections), one section
(`attr_type_section`) containing the actual encoded representation, and another
section (`attr_type_offset_section`) containing the offsets of each encoded
attribute/type into the previous section. This structure allows for attributes
and types to always be lazily loaded on demand.

```
attr_type_section {
  attrs: attribute[],
  types: type[]
}
attr_type_offset_section {
  numAttrs: varint,
  numTypes: varint,
  offsets: attr_type_offset_group[]
}

attr_type_offset_group {
  dialect: varint,
  numElements: varint,
  offsets: varint[] // (offset << 1) | (hasCustomEncoding)
}

attribute {
  encoding: ...
}
type {
  encoding: ...
}
```

Each `offset` in the `attr_type_offset_section` above is the size of the
encoding for the attribute or type and a flag indicating if the encoding uses
the textual assembly format, or a custom bytecode encoding. We avoid using the
direct offset into the `attr_type_section`, as a smaller relative offsets
provides more effective compression. Attributes and types are grouped by
dialect, with each `attr_type_offset_group` in the offset section containing the
corresponding parent dialect, number of elements, and offsets for each element
within the group.

#### Attribute/Type Encodings

In the abstract, an attribute/type is encoded in one of two possible ways: via
its assembly format, or via a custom dialect defined encoding.

##### Assembly Format Fallback

In the case where a dialect does not define a method for encoding the attribute
or type, the textual assembly format of that attribute or type is used as a
fallback. For example, a type `!bytecode.type<42>` would be encoded as the null
terminated string "!bytecode.type<42>". This ensures that every attribute and
type can be encoded, even if the owning dialect has not yet opted in to a more
efficient serialization.

TODO: We shouldn't redundantly encode the dialect name here, we should use a
reference to the parent dialect instead.

##### Dialect Defined Encoding

As an alternative to the assembly format fallback, dialects may also provide a
custom encoding for their attributes and types. Custom encodings are very
beneficial in that they are significantly smaller and faster to read and write.

Dialects can opt-in to providing custom encodings by implementing the
`BytecodeDialectInterface`. This interface provides hooks, namely
`readAttribute`/`readType` and `writeAttribute`/`writeType`, that will be used
by the bytecode reader and writer. These hooks are provided a reader and writer
implementation that can be used to encode various constructs in the underlying
bytecode format. A unique feature of this interface is that dialects may choose
to only encode a subset of their attributes and types in a custom bytecode
format, which can simplify adding new or experimental components that aren't
fully baked.

When implementing the bytecode interface, dialects are responsible for all
aspects of the encoding. This includes the indicator for which kind of attribute
or type is being encoded; the bytecode reader will only know that it has
encountered an attribute or type of a given dialect, it doesn't encode any
further information. As such, a common encoding idiom is to use a leading
`varint` code to indicate how the attribute or type was encoded.

### Resource Section

Resources are encoded using two [sections](#sections), one section
(`resource_section`) containing the actual encoded representation, and another
section (`resource_offset_section`) containing the offsets of each encoded
resource into the previous section.

```
resource_section {
  resources: resource[]
}
resource {
  value: resource_bool | resource_string | resource_blob
}
resource_bool {
  value: byte
}
resource_string {
  value: varint
}
resource_blob {
  alignment: varint,
  size: varint,
  padding: byte[],
  blob: byte[]
}

resource_offset_section {
  numExternalResourceGroups: varint,
  resourceGroups: resource_group[]
}
resource_group {
  key: varint,
  numResources: varint,
  resources: resource_info[]
}
resource_info {
  key: varint,
  size: varint
  kind: byte,
}
```

Resources are grouped by the provider, either an external entity or a dialect,
with each `resource_group` in the offset section containing the corresponding
provider, number of elements, and info for each element within the group. For
each element, we record the key, the value kind, and the encoded size. We avoid
using the direct offset into the `resource_section`, as a smaller relative
offsets provides more effective compression.

### IR Section

The IR section contains the encoded form of operations within the bytecode.

```
ir_section {
  block: block; // Single block without arguments.
}
```

#### Operation Encoding

```
op {
  name: varint,
  encodingMask: byte,
  location: varint,

  attrDict: varint?,

  numResults: varint?,
  resultTypes: varint[],

  numOperands: varint?,
  operands: varint[],

  numSuccessors: varint?,
  successors: varint[],

  numUseListOrders: varint?,
  useListOrders: uselist[],

  regionEncoding: varint?, // (numRegions << 1) | (isIsolatedFromAbove)

  // regions are stored in a section if isIsolatedFromAbove
  regions: (region | region_section)[]
}

uselist {
  indexInRange: varint?,
  useListEncoding: varint, // (numIndices << 1) | (isIndexPairEncoding)
  indices: varint[]
}
```

The encoding of an operation is important because this is generally the most
commonly appearing structure in the bytecode. A single encoding is used for
every type of operation. Given this prevalence, many of the fields of an
operation are optional. The `encodingMask` field is a bitmask which indicates
which of the components of the operation are present.

##### Location

The location is encoded as the index of the location within the attribute table.

##### Attributes

If the operation has attribues, the index of the operation attribute dictionary
within the attribute table is encoded.

##### Results

If the operation has results, the number of results and the indexes of the
result types within the type table are encoded.

##### Operands

If the operation has operands, the number of operands and the value index of
each operand is encoded. This value index is the relative ordering of the
definition of that value from the start of the first ancestor isolated region.

##### Successors

If the operation has successors, the number of successors and the indexes of the
successor blocks within the parent region are encoded.

##### Use-list orders

The reference use-list order is assumed to be the reverse of the global
enumeration of all the op operands that one would obtain with a pre-order walk
of the IR. This order is naturally obtained by building blocks of operations
op-by-op. However, some transformations may shuffle the use-lists with respect
to this reference ordering. If any of the results of the operation have a
use-list order that is not sorted with respect to the reference use-list order,
an encoding is emitted such that it is possible to reconstruct such order after
parsing the bytecode. The encoding represents an index map from the reference
operand order to the current use-list order. A bit flag is used to detect if
this encoding is of type index-pair or not. When the bit flag is set to zero,
the element at `i` represent the position of the use `i` of the reference list
into the current use-list. When the bit flag is set to `1`, the encoding
represent index pairs `(i, j)`, which indicate that the use at position `i` of
the reference list is mapped to position `j` in the current use-list. When only
less than half of the elements in the current use-list are shuffled with respect
to the reference use-list, the index-pair encoding is used to reduce the
bytecode memory requirements.

##### Regions

If the operation has regions, the number of regions and if the regions are
isolated from above are encoded together in a single varint. Afterwards, each
region is encoded inline.

#### Region Encoding

```
region {
  numBlocks: varint,

  numValues: varint?,
  blocks: block[]
}
```

A region is encoded first with the number of blocks within. If the region is
non-empty, the number of values defined directly within the region are encoded,
followed by the blocks of the region.

#### Block Encoding

```
block {
  encoding: varint, // (numOps << 1) | (hasBlockArgs)
  arguments: block_arguments?, // Optional based on encoding
  ops : op[]
}

block_arguments {
  numArgs: varint?,
  args: block_argument[]
  numUseListOrders: varint?,
  useListOrders: uselist[],
}

block_argument {
  typeAndLocation: varint, // (type << 1) | (hasLocation)
  location: varint? // Optional, else unknown location
}
```

A block is encoded with an array of operations and block arguments. The first
field is an encoding that combines the number of operations in the block, with a
flag indicating if the block has arguments.

Use-list orders are attached to block arguments similarly to how they are
attached to operation results.