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MLIR Language Reference

MLIR (Multi-Level IR) is a compiler intermediate representation with similarities to traditional three-address SSA representations (like LLVM IR or SIL), but which introduces notions from polyhedral loop optimization as first-class concepts. This hybrid design is optimized to represent, analyze, and transform high level dataflow graphs as well as target-specific code generated for high performance data parallel systems. Beyond its representational capabilities, its single continuous design provides a framework to lower from dataflow graphs to high-performance target-specific code.

This document defines and describes the key concepts in MLIR, and is intended to be a dry reference document - the rationale documentation, glossary, and other content are hosted elsewhere.

MLIR is designed to be used in three different forms: a human-readable textual form suitable for debugging, an in-memory form suitable for programmatic transformations and analysis, and a compact serialized form suitable for storage and transport. The different forms all describe the same semantic content. This document describes the human-readable textual form.

[TOC]

High-Level Structure

MLIR is fundamentally based on a graph-like data structure of nodes, called Operations, and edges, called Values. Each Value is the result of exactly one Operation or Block Argument, and has a Value Type defined by the type system. Operations are contained in Blocks and Blocks are contained in Regions. Operations are also ordered within their containing block and Blocks are ordered in their containing region, although this order may or may not be semantically meaningful in a given kind of region). Operations may also contain regions, enabling hierarchical structures to be represented.

Operations can represent many different concepts, from higher-level concepts like function definitions, function calls, buffer allocations, view or slices of buffers, and process creation, to lower-level concepts like target-independent arithmetic, target-specific instructions, configuration registers, and logic gates. These different concepts are represented by different operations in MLIR and the set of operations usable in MLIR can be arbitrarily extended.

MLIR also provides an extensible framework for transformations on operations, using familiar concepts of compiler Passes. Enabling an arbitrary set of passes on an arbitrary set of operations results in a significant scaling challenge, since each transformation must potentially take into account the semantics of any operation. MLIR addresses this complexity by allowing operation semantics to be described abstractly using Traits and Interfaces, enabling transformations to operate on operations more generically. Traits often describe verification constraints on valid IR, enabling complex invariants to be captured and checked. (see Op vs Operation)

One obvious application of MLIR is to represent an SSA-based IR, like the LLVM core IR, with appropriate choice of operation types to define Modules, Functions, Branches, Memory Allocation, and verification constraints to ensure the SSA Dominance property. MLIR includes a collection of dialects which defines just such structures. However, MLIR is intended to be general enough to represent other compiler-like data structures, such as Abstract Syntax Trees in a language frontend, generated instructions in a target-specific backend, or circuits in a High-Level Synthesis tool.

Here's an example of an MLIR module:

// Compute A*B using an implementation of multiply kernel and print the
// result using a TensorFlow op. The dimensions of A and B are partially
// known. The shapes are assumed to match.
func @mul(%A: tensor<100x?xf32>, %B: tensor<?x50xf32>) -> (tensor<100x50xf32>) {
  // Compute the inner dimension of %A using the dim operation.
  %n = dim %A, 1 : tensor<100x?xf32>

  // Allocate addressable "buffers" and copy tensors %A and %B into them.
  %A_m = alloc(%n) : memref<100x?xf32>
  tensor_store %A to %A_m : memref<100x?xf32>

  %B_m = alloc(%n) : memref<?x50xf32>
  tensor_store %B to %B_m : memref<?x50xf32>

  // Call function @multiply passing memrefs as arguments,
  // and getting returned the result of the multiplication.
  %C_m = call @multiply(%A_m, %B_m)
          : (memref<100x?xf32>, memref<?x50xf32>) -> (memref<100x50xf32>)

  dealloc %A_m : memref<100x?xf32>
  dealloc %B_m : memref<?x50xf32>

  // Load the buffer data into a higher level "tensor" value.
  %C = tensor_load %C_m : memref<100x50xf32>
  dealloc %C_m : memref<100x50xf32>

  // Call TensorFlow built-in function to print the result tensor.
  "tf.Print"(%C){message: "mul result"}
                  : (tensor<100x50xf32) -> (tensor<100x50xf32>)

  return %C : tensor<100x50xf32>
}

// A function that multiplies two memrefs and returns the result.
func @multiply(%A: memref<100x?xf32>, %B: memref<?x50xf32>)
          -> (memref<100x50xf32>)  {
  // Compute the inner dimension of %A.
  %n = dim %A, 1 : memref<100x?xf32>

  // Allocate memory for the multiplication result.
  %C = alloc() : memref<100x50xf32>

  // Multiplication loop nest.
  affine.for %i = 0 to 100 {
     affine.for %j = 0 to 50 {
        store 0 to %C[%i, %j] : memref<100x50xf32>
        affine.for %k = 0 to %n {
           %a_v  = load %A[%i, %k] : memref<100x?xf32>
           %b_v  = load %B[%k, %j] : memref<?x50xf32>
           %prod = mulf %a_v, %b_v : f32
           %c_v  = load %C[%i, %j] : memref<100x50xf32>
           %sum  = addf %c_v, %prod : f32
           store %sum, %C[%i, %j] : memref<100x50xf32>
        }
     }
  }
  return %C : memref<100x50xf32>
}

Notation

MLIR has a simple and unambiguous grammar, allowing it to reliably round-trip through a textual form. This is important for development of the compiler - e.g. for understanding the state of code as it is being transformed and writing test cases.

This document describes the grammar using Extended Backus-Naur Form (EBNF).

This is the EBNF grammar used in this document, presented in yellow boxes.

alternation ::= expr0 | expr1 | expr2  // Either expr0 or expr1 or expr2.
sequence    ::= expr0 expr1 expr2      // Sequence of expr0 expr1 expr2.
repetition0 ::= expr*  // 0 or more occurrences.
repetition1 ::= expr+  // 1 or more occurrences.
optionality ::= expr?  // 0 or 1 occurrence.
grouping    ::= (expr) // Everything inside parens is grouped together.
literal     ::= `abcd` // Matches the literal `abcd`.

Code examples are presented in blue boxes.

// This is an example use of the grammar above:
// This matches things like: ba, bana, boma, banana, banoma, bomana...
example ::= `b` (`an` | `om`)* `a`

Common syntax

The following core grammar productions are used in this document:

// TODO: Clarify the split between lexing (tokens) and parsing (grammar).
digit     ::= [0-9]
hex_digit ::= [0-9a-fA-F]
letter    ::= [a-zA-Z]
id-punct  ::= [$._-]

integer-literal ::= decimal-literal | hexadecimal-literal
decimal-literal ::= digit+
hexadecimal-literal ::= `0x` hex_digit+
float-literal ::= [-+]?[0-9]+[.][0-9]*([eE][-+]?[0-9]+)?
string-literal  ::= `"` [^"\n\f\v\r]* `"`   TODO: define escaping rules

Not listed here, but MLIR does support comments. They use standard BCPL syntax, starting with a // and going until the end of the line.

Identifiers and keywords

Syntax:

// Identifiers
bare-id ::= (letter|[_]) (letter|digit|[_$.])*
bare-id-list ::= bare-id (`,` bare-id)*
value-id ::= `%` suffix-id
suffix-id ::= (digit+ | ((letter|id-punct) (letter|id-punct|digit)*))

symbol-ref-id ::= `@` (suffix-id | string-literal)
value-id-list ::= value-id (`,` value-id)*

// Uses of value, e.g. in an operand list to an operation.
value-use ::= value-id
value-use-list ::= value-use (`,` value-use)*

Identifiers name entities such as values, types and functions, and are chosen by the writer of MLIR code. Identifiers may be descriptive (e.g. %batch_size, @matmul), or may be non-descriptive when they are auto-generated (e.g. %23, @func42). Identifier names for values may be used in an MLIR text file but are not persisted as part of the IR - the printer will give them anonymous names like %42.

MLIR guarantees identifiers never collide with keywords by prefixing identifiers with a sigil (e.g. %, #, @, ^, !). In certain unambiguous contexts (e.g. affine expressions), identifiers are not prefixed, for brevity. New keywords may be added to future versions of MLIR without danger of collision with existing identifiers.

Value identifiers are only in scope for the (nested) region in which they are defined and cannot be accessed or referenced outside of that region. Argument identifiers in mapping functions are in scope for the mapping body. Particular operations may further limit which identifiers are in scope in their regions. For instance, the scope of values in a region with SSA control flow semantics is constrained according to the standard definition of SSA dominance. Another example is the IsolatedFromAbove trait, which restricts directly accessing values defined in containing regions.

Function identifiers and mapping identifiers are associated with Symbols and have scoping rules dependent on symbol attributes.

Dialects

Dialects are the mechanism by which to engage with and extend the MLIR ecosystem. They allow for defining new operations, as well as attributes and types. Each dialect is given a unique namespace that is prefixed to each defined attribute/operation/type. For example, the Affine dialect defines the namespace: affine.

MLIR allows for multiple dialects, even those outside of the main tree, to co-exist together within one module. Dialects are produced and consumed by certain passes. MLIR provides a framework to convert between, and within, different dialects.

A few of the dialects supported by MLIR:

Target specific operations

Dialects provide a modular way in which targets can expose target-specific operations directly through to MLIR. As an example, some targets go through LLVM. LLVM has a rich set of intrinsics for certain target-independent operations (e.g. addition with overflow check) as well as providing access to target-specific operations for the targets it supports (e.g. vector permutation operations). LLVM intrinsics in MLIR are represented via operations that start with an "llvm." name.

Example:

// LLVM: %x = call {i16, i1} @llvm.sadd.with.overflow.i16(i16 %a, i16 %b)
%x:2 = "llvm.sadd.with.overflow.i16"(%a, %b) : (i16, i16) -> (i16, i1)

These operations only work when targeting LLVM as a backend (e.g. for CPUs and GPUs), and are required to align with the LLVM definition of these intrinsics.

Operations

Syntax:

operation            ::= op-result-list? (generic-operation | custom-operation)
                         trailing-location?
generic-operation    ::= string-literal `(` value-use-list? `)`  successor-list?
                         region-list? dictionary-attribute? `:` function-type
custom-operation     ::= bare-id custom-operation-format
op-result-list       ::= op-result (`,` op-result)* `=`
op-result            ::= value-id (`:` integer-literal)
successor-list       ::= `[` successor (`,` successor)* `]`
successor            ::= caret-id (`:` bb-arg-list)?
region-list          ::= `(` region (`,` region)* `)`
dictionary-attribute ::= `{` (attribute-entry (`,` attribute-entry)*)? `}`
trailing-location    ::= (`loc` `(` location `)`)?

MLIR introduces a uniform concept called operations to enable describing many different levels of abstractions and computations. Operations in MLIR are fully extensible (there is no fixed list of operations) and have application-specific semantics. For example, MLIR supports target-independent operations, affine operations, and target-specific machine operations.

The internal representation of an operation is simple: an operation is identified by a unique string (e.g. dim, tf.Conv2d, x86.repmovsb, ppc.eieio, etc), can return zero or more results, take zero or more operands, has a dictionary of attributes, has zero or more successors, and zero or more enclosed regions. The generic printing form includes all these elements literally, with a function type to indicate the types of the results and operands.

Example:

// An operation that produces two results.
// The results of %result can be accessed via the <name> `#` <opNo> syntax.
%result:2 = "foo_div"() : () -> (f32, i32)

// Pretty form that defines a unique name for each result.
%foo, %bar = "foo_div"() : () -> (f32, i32)

// Invoke a TensorFlow function called tf.scramble with two inputs
// and an attribute "fruit".
%2 = "tf.scramble"(%result#0, %bar) {fruit = "banana"} : (f32, i32) -> f32

In addition to the basic syntax above, dialects may register known operations. This allows those dialects to support custom assembly form for parsing and printing operations. In the operation sets listed below, we show both forms.

Builtin Operations

The builtin dialect defines a select few operations that are widely applicable by MLIR dialects, such as a universal conversion cast operation that simplifies inter/intra dialect conversion. This dialect also defines a top-level module operation, that represents a useful IR container.

Blocks

Syntax:

block           ::= block-label operation+
block-label     ::= block-id block-arg-list? `:`
block-id        ::= caret-id
caret-id        ::= `^` suffix-id
value-id-and-type ::= value-id `:` type

// Non-empty list of names and types.
value-id-and-type-list ::= value-id-and-type (`,` value-id-and-type)*

block-arg-list ::= `(` value-id-and-type-list? `)`

A Block is a list of operations. In SSACFG regions, each block represents a compiler basic block where instructions inside the block are executed in order and terminator operations implement control flow branches between basic blocks.

A region with a single block may not include a terminator operation. The enclosing op can opt-out of this requirement with the NoTerminator trait. The top-level ModuleOp is an example of such operation which defined this trait and whose block body does not have a terminator.

Blocks in MLIR take a list of block arguments, notated in a function-like way. Block arguments are bound to values specified by the semantics of individual operations. Block arguments of the entry block of a region are also arguments to the region and the values bound to these arguments are determined by the semantics of the containing operation. Block arguments of other blocks are determined by the semantics of terminator operations, e.g. Branches, which have the block as a successor. In regions with control flow, MLIR leverages this structure to implicitly represent the passage of control-flow dependent values without the complex nuances of PHI nodes in traditional SSA representations. Note that values which are not control-flow dependent can be referenced directly and do not need to be passed through block arguments.

Here is a simple example function showing branches, returns, and block arguments:

func @simple(i64, i1) -> i64 {
^bb0(%a: i64, %cond: i1): // Code dominated by ^bb0 may refer to %a
  cond_br %cond, ^bb1, ^bb2

^bb1:
  br ^bb3(%a: i64)    // Branch passes %a as the argument

^bb2:
  %b = addi %a, %a : i64
  br ^bb3(%b: i64)    // Branch passes %b as the argument

// ^bb3 receives an argument, named %c, from predecessors
// and passes it on to bb4 along with %a. %a is referenced
// directly from its defining operation and is not passed through
// an argument of ^bb3.
^bb3(%c: i64):
  br ^bb4(%c, %a : i64, i64)

^bb4(%d : i64, %e : i64):
  %0 = addi %d, %e : i64
  return %0 : i64   // Return is also a terminator.
}

Context: The "block argument" representation eliminates a number of special cases from the IR compared to traditional "PHI nodes are operations" SSA IRs (like LLVM). For example, the parallel copy semantics of SSA is immediately apparent, and function arguments are no longer a special case: they become arguments to the entry block [more rationale]. Blocks are also a fundamental concept that cannot be represented by operations because values defined in an operation cannot be accessed outside the operation.

Regions

Definition

A region is an ordered list of MLIR Blocks. The semantics within a region is not imposed by the IR. Instead, the containing operation defines the semantics of the regions it contains. MLIR currently defines two kinds of regions: SSACFG regions, which describe control flow between blocks, and Graph regions, which do not require control flow between block. The kinds of regions within an operation are described using the RegionKindInterface.

Regions do not have a name or an address, only the blocks contained in a region do. Regions must be contained within operations and have no type or attributes. The first block in the region is a special block called the 'entry block'. The arguments to the entry block are also the arguments of the region itself. The entry block cannot be listed as a successor of any other block. The syntax for a region is as follows:

region ::= `{` block* `}`

A function body is an example of a region: it consists of a CFG of blocks and has additional semantic restrictions that other types of regions may not have. For example, in a function body, block terminators must either branch to a different block, or return from a function where the types of the return arguments must match the result types of the function signature. Similarly, the function arguments must match the types and count of the region arguments. In general, operations with regions can define these correspondances arbitrarily.

Value Scoping

Regions provide hierarchical encapsulation of programs: it is impossible to reference, i.e. branch to, a block which is not in the same region as the source of the reference, i.e. a terminator operation. Similarly, regions provides a natural scoping for value visibility: values defined in a region don't escape to the enclosing region, if any. By default, operations inside a region can reference values defined outside of the region whenever it would have been legal for operands of the enclosing operation to reference those values, but this can be restricted using traits, such as OpTrait::IsolatedFromAbove, or a custom verifier.

Example:

  "any_op"(%a) ({ // if %a is in-scope in the containing region...
	 // then %a is in-scope here too.
    %new_value = "another_op"(%a) : (i64) -> (i64)
  }) : (i64) -> (i64)

MLIR defines a generalized 'hierarchical dominance' concept that operates across hierarchy and defines whether a value is 'in scope' and can be used by a particular operation. Whether a value can be used by another operation in the same region is defined by the kind of region. A value defined in a region can be used by an operation which has a parent in the same region, if and only if the parent could use the value. A value defined by an argument to a region can always be used by any operation deeply contained in the region. A value defined in a region can never be used outside of the region.

Control Flow and SSACFG Regions

In MLIR, control flow semantics of a region is indicated by RegionKind::SSACFG. Informally, these regions support semantics where operations in a region 'execute sequentially'. Before an operation executes, its operands have well-defined values. After an operation executes, the operands have the same values and results also have well-defined values. After an operation executes, the next operation in the block executes until the operation is the terminator operation at the end of a block, in which case some other operation will execute. The determination of the next instruction to execute is the 'passing of control flow'.

In general, when control flow is passed to an operation, MLIR does not restrict when control flow enters or exits the regions contained in that operation. However, when control flow enters a region, it always begins in the first block of the region, called the entry block. Terminator operations ending each block represent control flow by explicitly specifying the successor blocks of the block. Control flow can only pass to one of the specified successor blocks as in a branch operation, or back to the containing operation as in a return operation. Terminator operations without successors can only pass control back to the containing operation. Within these restrictions, the particular semantics of terminator operations is determined by the specific dialect operations involved. Blocks (other than the entry block) that are not listed as a successor of a terminator operation are defined to be unreachable and can be removed without affecting the semantics of the containing operation.

Although control flow always enters a region through the entry block, control flow may exit a region through any block with an appropriate terminator. The standard dialect leverages this capability to define operations with Single-Entry-Multiple-Exit (SEME) regions, possibly flowing through different blocks in the region and exiting through any block with a return operation. This behavior is similar to that of a function body in most programming languages. In addition, control flow may also not reach the end of a block or region, for example if a function call does not return.

Example:

func @accelerator_compute(i64, i1) -> i64 { // An SSACFG region
^bb0(%a: i64, %cond: i1): // Code dominated by ^bb0 may refer to %a
  cond_br %cond, ^bb1, ^bb2

^bb1:
  // This def for %value does not dominate ^bb2
  %value = "op.convert"(%a) : (i64) -> i64
  br ^bb3(%a: i64)    // Branch passes %a as the argument

^bb2:
  accelerator.launch() { // An SSACFG region
    ^bb0:
      // Region of code nested under "accelerator.launch", it can reference %a but
      // not %value.
      %new_value = "accelerator.do_something"(%a) : (i64) -> ()
  }
  // %new_value cannot be referenced outside of the region

^bb3:
  ...
}

Operations with Multiple Regions

An operation containing multiple regions also completely determines the semantics of those regions. In particular, when control flow is passed to an operation, it may transfer control flow to any contained region. When control flow exits a region and is returned to the containing operation, the containing operation may pass control flow to any region in the same operation. An operation may also pass control flow to multiple contained regions concurrently. An operation may also pass control flow into regions that were specified in other operations, in particular those that defined the values or symbols the given operation uses as in a call operation. This passage of control is generally independent of passage of control flow through the basic blocks of the containing region.

Closure

Regions allow defining an operation that creates a closure, for example by “boxing” the body of the region into a value they produce. It remains up to the operation to define its semantics. Note that if an operation triggers asynchronous execution of the region, it is under the responsibility of the operation caller to wait for the region to be executed guaranteeing that any directly used values remain live.

Graph Regions

In MLIR, graph-like semantics in a region is indicated by RegionKind::Graph. Graph regions are appropriate for concurrent semantics without control flow, or for modeling generic directed graph data structures. Graph regions are appropriate for representing cyclic relationships between coupled values where there is no fundamental order to the relationships. For instance, operations in a graph region may represent independent threads of control with values representing streams of data. As usual in MLIR, the particular semantics of a region is completely determined by its containing operation. Graph regions may only contain a single basic block (the entry block).

Rationale: Currently graph regions are arbitrarily limited to a single basic block, although there is no particular semantic reason for this limitation. This limitation has been added to make it easier to stabilize the pass infrastructure and commonly used passes for processing graph regions to properly handle feedback loops. Multi-block regions may be allowed in the future if use cases that require it arise.

In graph regions, MLIR operations naturally represent nodes, while each MLIR value represents a multi-edge connecting a single source node and multiple destination nodes. All values defined in the region as results of operations are in scope within the region and can be accessed by any other operation in the region. In graph regions, the order of operations within a block and the order of blocks in a region is not semantically meaningful and non-terminator operations may be freely reordered, for instance, by canonicalization. Other kinds of graphs, such as graphs with multiple source nodes and multiple destination nodes, can also be represented by representing graph edges as MLIR operations.

Note that cycles can occur within a single block in a graph region, or between basic blocks.

"test.graph_region"() ({ // A Graph region
  %1 = "op1"(%1, %3) : (i32, i32) -> (i32)  // OK: %1, %3 allowed here
  %2 = "test.ssacfg_region"() ({
	 %5 = "op2"(%1, %2, %3, %4) : (i32, i32, i32, i32) -> (i32) // OK: %1, %2, %3, %4 all defined in the containing region
  }) : () -> (i32)
  %3 = "op2"(%1, %4) : (i32, i32) -> (i32)  // OK: %4 allowed here
  %4 = "op3"(%1) : (i32) -> (i32)
}) : () -> ()

Arguments and Results

The arguments of the first block of a region are treated as arguments of the region. The source of these arguments is defined by the semantics of the parent operation. They may correspond to some of the values the operation itself uses.

Regions produce a (possibly empty) list of values. The operation semantics defines the relation between the region results and the operation results.

Type System

Each value in MLIR has a type defined by the type system. MLIR has an open type system (i.e. there is no fixed list of types), and types may have application-specific semantics. MLIR dialects may define any number of types with no restrictions on the abstractions they represent.

type ::= type-alias | dialect-type | builtin-type

type-list-no-parens ::=  type (`,` type)*
type-list-parens ::= `(` `)`
                   | `(` type-list-no-parens `)`

// This is a common way to refer to a value with a specified type.
ssa-use-and-type ::= ssa-use `:` type

// Non-empty list of names and types.
ssa-use-and-type-list ::= ssa-use-and-type (`,` ssa-use-and-type)*

Type Aliases

type-alias-def ::= '!' alias-name '=' 'type' type
type-alias ::= '!' alias-name

MLIR supports defining named aliases for types. A type alias is an identifier that can be used in the place of the type that it defines. These aliases must be defined before their uses. Alias names may not contain a '.', since those names are reserved for dialect types.

Example:

!avx_m128 = type vector<4 x f32>

// Using the original type.
"foo"(%x) : vector<4 x f32> -> ()

// Using the type alias.
"foo"(%x) : !avx_m128 -> ()

Dialect Types

Similarly to operations, dialects may define custom extensions to the type system.

dialect-namespace ::= bare-id

opaque-dialect-item ::= dialect-namespace '<' string-literal '>'

pretty-dialect-item ::= dialect-namespace '.' pretty-dialect-item-lead-ident
                                              pretty-dialect-item-body?

pretty-dialect-item-lead-ident ::= '[A-Za-z][A-Za-z0-9._]*'
pretty-dialect-item-body ::= '<' pretty-dialect-item-contents+ '>'
pretty-dialect-item-contents ::= pretty-dialect-item-body
                              | '(' pretty-dialect-item-contents+ ')'
                              | '[' pretty-dialect-item-contents+ ']'
                              | '{' pretty-dialect-item-contents+ '}'
                              | '[^[<({>\])}\0]+'

dialect-type ::= '!' opaque-dialect-item
dialect-type ::= '!' pretty-dialect-item

Dialect types can be specified in a verbose form, e.g. like this:

// LLVM type that wraps around llvm IR types.
!llvm<"i32*">

// Tensor flow string type.
!tf.string

// Complex type
!foo<"something<abcd>">

// Even more complex type
!foo<"something<a%%123^^^>>>">

Dialect types that are simple enough can use the pretty format, which is a lighter weight syntax that is equivalent to the above forms:

// Tensor flow string type.
!tf.string

// Complex type
!foo.something<abcd>

Sufficiently complex dialect types are required to use the verbose form for generality. For example, the more complex type shown above wouldn't be valid in the lighter syntax: !foo.something<a%%123^^^>>> because it contains characters that are not allowed in the lighter syntax, as well as unbalanced <> characters.

See here to learn how to define dialect types.

Builtin Types

The builtin dialect defines a set of types that are directly usable by any other dialect in MLIR. These types cover a range from primitive integer and floating-point types, function types, and more.

Attributes

Syntax:

attribute-entry ::= (bare-id | string-literal) `=` attribute-value
attribute-value ::= attribute-alias | dialect-attribute | builtin-attribute

Attributes are the mechanism for specifying constant data on operations in places where a variable is never allowed - e.g. the comparison predicate of a cmpi operation. Each operation has an attribute dictionary, which associates a set of attribute names to attribute values. MLIR's builtin dialect provides a rich set of builtin attribute values out of the box (such as arrays, dictionaries, strings, etc.). Additionally, dialects can define their own dialect attribute values.

The top-level attribute dictionary attached to an operation has special semantics. The attribute entries are considered to be of two different kinds based on whether their dictionary key has a dialect prefix:

inherent attributes are inherent to the definition of an operation's semantics. The operation itself is expected to verify the consistency of these attributes. An example is the predicate attribute of the std.cmpi op. These attributes must have names that do not start with a dialect prefix.
discardable attributes have semantics defined externally to the operation itself, but must be compatible with the operations's semantics. These attributes must have names that start with a dialect prefix. The dialect indicated by the dialect prefix is expected to verify these attributes. An example is the gpu.container_module attribute.

Note that attribute values are allowed to themselves be dictionary attributes, but only the top-level dictionary attribute attached to the operation is subject to the classification above.

Attribute Value Aliases

attribute-alias-def ::= '#' alias-name '=' attribute-value
attribute-alias ::= '#' alias-name

MLIR supports defining named aliases for attribute values. An attribute alias is an identifier that can be used in the place of the attribute that it defines. These aliases must be defined before their uses. Alias names may not contain a '.', since those names are reserved for dialect attributes.

Example:

#map = affine_map<(d0) -> (d0 + 10)>

// Using the original attribute.
%b = affine.apply affine_map<(d0) -> (d0 + 10)> (%a)

// Using the attribute alias.
%b = affine.apply #map(%a)

Dialect Attribute Values

Similarly to operations, dialects may define custom attribute values. The syntactic structure of these values is identical to custom dialect type values, except that dialect attribute values are distinguished with a leading '#', while dialect types are distinguished with a leading '!'.

dialect-attribute-value ::= '#' opaque-dialect-item
dialect-attribute-value ::= '#' pretty-dialect-item

Dialect attribute values can be specified in a verbose form, e.g. like this:

// Complex attribute value.
#foo<"something<abcd>">

// Even more complex attribute value.
#foo<"something<a%%123^^^>>>">

Dialect attribute values that are simple enough can use the pretty format, which is a lighter weight syntax that is equivalent to the above forms:

// Complex attribute
#foo.something<abcd>

Sufficiently complex dialect attribute values are required to use the verbose form for generality. For example, the more complex type shown above would not be valid in the lighter syntax: #foo.something<a%%123^^^>>> because it contains characters that are not allowed in the lighter syntax, as well as unbalanced <> characters.

See here on how to define dialect attribute values.

Builtin Attribute Values

The builtin dialect defines a set of attribute values that are directly usable by any other dialect in MLIR. These types cover a range from primitive integer and floating-point values, attribute dictionaries, dense multi-dimensional arrays, and more.

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MLIR Language Reference

High-Level Structure

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Dialect Attribute Values

Builtin Attribute Values

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