Create and begin writing glossary.
This creates a central place in the documentation where MLIR-specific terminology is defined. See discussion on the MLIR forum (https://groups.google.com/a/tensorflow.org/g/mlir/c/5YXDSdu76Hk). PiperOrigin-RevId: 280220365 Change-Id: I370d5bd7bc329b81212d9cca37f107dd0d8243be
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third_party/mlir/g3doc/Glossary.md
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# MLIR Glossary
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This glossary contains definitions of MLIR-specific terminology. It is intended
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to be a quick reference document. For terms which are well-documented elsewhere,
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definitions are kept brief and the header links to the more in-depth
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documentation.
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<!-- When contributing, please ensure that entries remain in alphabetical order. -->
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[Block](LangRef.md#blocks) {#block}
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: A sequential list of operations without control flow.
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: Also called a [basic block](https://en.wikipedia.org/wiki/Basic_block).
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Conversion {#conversion}
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: The transformation of code represented in one dialect into a semantically
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equivalent representation in another dialect (i.e. inter-dialect conversion)
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or the same dialect (i.e. intra-dialect conversion).
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: In the context of MLIR, conversion is distinct from
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[translation](#translation). Conversion refers to a transformation between
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(or within) dialects, but all still within MLIR, whereas translation refers
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to a transformation between MLIR and an external representation.
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[Declarative Rewrite Rule](DeclarativeRewrites.md) (DRR) {#drr}
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: A [rewrite rule](https://en.wikipedia.org/wiki/Graph_rewriting) which can be
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defined declaratively (e.g. through specification in a
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[TableGen](https://llvm.org/docs/TableGen/) record). At compiler build time,
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these rules are expanded into an equivalent `mlir::RewritePattern` subclass.
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[Dialect](LangRef.md#dialects) {#dialect}
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: A dialect is a grouping of functionality which can be used to extend the
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MLIR system.
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: A dialect creates a unique `namespace` within which new
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[operations](#operation), [attributes](LangRef.md#attributes), and
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[types](LangRef.md#type-system) are defined. This is the fundamental method
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by which to extend MLIR.
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: In this way, MLIR is a meta-IR: its extensible framework allows it to be
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leveraged in many different ways (e.g. at different levels of the
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compilation process). Dialects provide an abstraction for the different uses
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of MLIR while recognizing that they are all a part of the meta-IR that is
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MLIR.
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: The tutorial provides an example of
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[interfacing with MLIR](Tutorials/Toy/Ch-2.md#interfacing-with-mlir) in this
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way.
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: (Note that we have intentionally selected the term "dialect" instead of
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"language", as the latter would wrongly suggest that these different
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namespaces define entirely distinct IRs.)
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Export {#export}
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: To transform code represented in MLIR into a semantically equivalent
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representation which is external to MLIR.
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: The tool that performs such a transformation is called an exporter.
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: See also: [translation](#translation).
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[Function](LangRef.md#functions) {#function}
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: An [operation](#operation) with a name containing one [region](#region).
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: The region of a function is not allowed to implicitly capture values defined
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outside of the function, and all external references must use function
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arguments or attributes that establish a symbolic connection.
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Import {#import}
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: To transform code represented in an external representation into a
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semantically equivalent representation in MLIR.
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: The tool that performs such a transformation is called an importer.
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: See also: [translation](#translation).
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Legalization {#legalization}
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: The process of transforming operations into a semantically equivalent
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representation which adheres to the requirements set by the
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[conversion target](DialectConversion.md#conversion-target).
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: That is, legalization is accomplished if and only if the new representation
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contains only operations which are legal, as specified in the conversion
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target.
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Lowering {#lowering}
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: The process of transforming a higher-level representation of an operation
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into a lower-level, but semantically equivalent, representation.
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: In MLIR, this is typically accomplished through
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[dialect conversion](DialectConversion.md). This provides a framework by
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which to define the requirements of the lower-level representation, called
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the [conversion target](DialectConversion.md#conversion-target), by
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specifying which operations are legal versus illegal after lowering.
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: See also: [legalization](#legalization).
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[Module](LangRef.md#module) {#module}
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: An [operation](#operation) which contains a single region containing a
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single block that is comprised of operations.
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: This provides an organizational structure for MLIR operations, and is the
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expected top-level operation in the IR: the textual parser returns a Module.
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[Operation](LangRef.md#operations) (op) {#operation}
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: A unit of code in MLIR. Operations are the building blocks for all code and
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computations represented by MLIR. They are fully extensible (there is no
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fixed list of operations) and have application-specific semantics.
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: An operation can have zero or more [regions](#region). Note that this
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creates a nested IR structure, as regions consist of blocks, which in turn,
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consist of a list of operations.
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[Region](LangRef.md#regions) {#region}
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: A [CFG](https://en.wikipedia.org/wiki/Control-flow_graph) of MLIR
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[blocks](#block).
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Round-trip {#round-trip}
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: The process of converting from a source format to a target format and then
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back to the source format.
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: This is a good way of gaining confidence that the target format richly
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models the source format. This is particularly relevant in the MLIR context,
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since MLIR's multi-level nature allows for easily writing target dialects
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that model a source format (such as TensorFlow GraphDef or another non-MLIR
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format) faithfully and have a simple conversion procedure. Further
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cleanup/lowering can be done entirely within the MLIR representation. This
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separation - making the [importer](#importer) as simple as possible and
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performing all further cleanups/lowering in MLIR - has proven to be a useful
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design pattern.
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[Terminator operation](LangRef.md#terminator-operations) {#terminator-operations}
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: An [operation](#operation) which *must* terminate a [block](#block).
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Terminator operations are a special category of operations.
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Transitive lowering {#transitive-lowering}
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: An A->B->C [lowering](#lowering); that is, a lowering in which multiple
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patterns may be applied in order to fully transform an illegal operation
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into a set of legal ones.
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: This provides the flexibility that the [conversion](#conversion) framework
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may perform the lowering in multiple stages of applying patterns (which may
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utilize intermediate patterns not in the conversion target) in order to
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fully legalize an operation. This is accomplished through
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[partial conversion](DialectConversion.md#modes-of-conversion).
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Translation {#translation}
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: The transformation of code represented in an external (non-MLIR)
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representation into a semantically equivalent representation in MLIR (i.e.
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[importing](#import)), or the inverse (i.e. [exporting](#export)).
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: In the context of MLIR, translation is distinct from
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[conversion](#conversion). Translation refers to a transformation between
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MLIR and an external representation, whereas conversion refers to a
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transformation within MLIR (between or within dialects).
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6
third_party/mlir/g3doc/LangRef.md
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6
third_party/mlir/g3doc/LangRef.md
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@ -12,8 +12,8 @@ continuous design provides a framework to lower from dataflow graphs to
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high-performance target-specific code.
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This document defines and describes the key concepts in MLIR, and is intended to
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be a dry reference document - [rationale documentation](Rationale.md) and other
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content is hosted elsewhere.
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be a dry reference document - the [rationale documentation](Rationale.md),
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[glossary](Glossary.md), and other content are hosted elsewhere.
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MLIR is designed to be used in three different forms: a human-readable textual
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form suitable for debugging, an in-memory form suitable for programmatic
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@ -285,7 +285,7 @@ printing operations. In the operation sets listed below, we show both forms.
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### Terminator Operations
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These are a special class of operations that *must* terminate a block, for
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These are a special category of operations that *must* terminate a block, for
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example [branches](Dialects/Standard.md#terminator-operations). These operations
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may also have a list of successors ([blocks](#blocks) and their arguments).
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