MLIR

Multi-Level IR Compiler Framework

'llvm' Dialect

This dialect wraps the LLVM IR types and instructions into MLIR types and operations. It provides several additional operations that are necessary to cover for the differences in the IR structure (e.g., MLIR does not have phi operations and LLVM IR does not have a constant operation).

In this document, we use “LLVM IR” to designate the intermediate representation of LLVM and “LLVM IR dialect” to refer to the MLIR dialect reflecting LLVM instructions and types.

Context and Module Association 

The LLVM IR dialect object contains an LLVM Context and an LLVM Module that it uses to define, print, parse and manage LLVM IR types. These objects can be obtained from the dialect object using .getLLVMContext() and getLLVMModule(). All LLVM IR objects that interact with the LLVM IR dialect must exist in the dialect’s context.

Types 

The LLVM IR dialect defines a single MLIR type, LLVM::LLVMType, that can wrap any existing LLVM IR type. Its syntax is as follows

type ::= `!llvm<"` llvm-canonical-type `">
llvm-canonical-type ::= <canonical textual representation defined by LLVM>

For example, one can use primitive types !llvm.i32, pointer types !llvm<"i8*">, vector types !llvm<"<4 x float>"> or structure types !llvm<"{i32, float}">. The parsing and printing of the canonical form are delegated to the LLVM assembly parser and printer.

LLVM IR dialect types contain an llvm::Type* object that can be obtained by calling .getUnderlyingType() and used in LLVM API calls directly. These objects are allocated within the LLVM context associated with the LLVM IR dialect and may be linked to the properties of the associated LLVM module.

LLVM IR dialect type can be constructed from any llvm::Type* that is associated with the LLVM context of the dialect. In this document, we use the term “wrapped LLVM IR type” to refer to the LLVM IR dialect type containing a specific LLVM IR type.

Operations 

All operations in the LLVM IR dialect have a custom form in MLIR. The mnemonic of an operation is that used in LLVM IR prefixed with “llvm.”.

LLVM functions 

MLIR functions are defined by an operation that is not built into the IR itself. The LLVM IR dialect provides an llvm.func operation to define functions compatible with LLVM IR. These functions have wrapped LLVM IR function type but use MLIR syntax to express it. They are required to have exactly one result type. LLVM function operation is intended to capture additional properties of LLVM functions, such as linkage and calling convention, that may be modeled differently by the built-in MLIR function.

// The type of @bar is !llvm<"i64 (i64)">
llvm.func @bar(%arg0: !llvm.i64) -> !llvm.i64 {
  llvm.return %arg0 : !llvm.i64
}

// Type type of @foo is !llvm<"void (i64)">
// !llvm.void type is omitted
llvm.func @foo(%arg0: !llvm.i64) {
  llvm.return
}

// A function with `internal` linkage.
llvm.func internal @internal_func() {
  llvm.return
}

Attribute pass-through 

An LLVM IR dialect function provides a mechanism to forward function-level attributes to LLVM IR using the passthrough attribute. This is an array attribute containing either string attributes or array attributes. In the former case, the value of the string is interpreted as the name of LLVM IR function attribute. In the latter case, the array is expected to contain exactly two string attributes, the first corresponding to the name of LLVM IR function attribute, and the second corresponding to its value. Note that even integer LLVM IR function attributes have their value represented in the string form.

Example:

llvm.func @func() attributes {
  passthrough = ["noinline",           // value-less attribute
                 ["alignstack", "4"],  // integer attribute with value
                 ["other", "attr"]]    // attribute unknown to LLVM
} {
  llvm.return
}

If the attribute is not known to LLVM IR, it will be attached as a string attribute.

Linkage 

An LLVM IR dialect function has a linkage attribute derived from LLVM IR linkage types . Linkage is specified by the same keyword as in LLVM IR and is located between llvm.func and the symbol name. If no linkage keyword is present, external linkage is assumed by default.

LLVM IR operations 

The following operations are currently supported. The semantics of these operations corresponds to the semantics of the similarly-named LLVM IR instructions.

Integer binary arithmetic operations 

Take two arguments of wrapped LLVM IR integer type, produce one value of the same type.

  • add
  • sub
  • mul
  • udiv
  • sdiv
  • urem
  • srem

Examples:

// Integer addition.
%0 = llvm.add %a, %b : !llvm.i32

// Unsigned integer division.
%1 = llvm.udiv %a, %b : !llvm.i32

Floating point binary arithmetic operations 

Take two arguments of wrapped LLVM IR floating point type, produce one value of the same type.

  • fadd
  • fsub
  • fmul
  • fdiv
  • frem

Examples:

// Float addition.
%0 = llvm.fadd %a, %b : !llvm.float

// Float division.
%1 = llvm.fdiv %a, %b : !llvm.float
  • <r> = alloca <size> x <type>
  • <r> = getelementptr <address>[<index> (, <index>)+]
  • <r> = load <address>
  • store <value>, <address>

In these operations, <size> must be a value of wrapped LLVM IR integer type, <address> must be a value of wrapped LLVM IR pointer type, and <value> must be a value of wrapped LLVM IR type that corresponds to the pointer type of <address>.

The index operands are integer values whose semantics is identical to the non-pointer arguments of LLVM IR’s getelementptr.

Examples:

// Allocate an array of 4 floats on stack
%c4 = llvm.mlir.constant(4) : !llvm.i64
%0 = llvm.alloca %c4 x !llvm.float : (!llvm.i64) -> !llvm<"float*">

// Get the second element of the array (note 0-based indexing).
%c1 = llvm.mlir.constant(1) : !llvm.i64
%1 = llvm.getelementptr %0[%c1] : (!llvm<"float*">, !llvm.i64)
                                   -> !llvm<"float*">

// Store a constant into this element.
%cf = llvm.mlir.constant(42.0 : f32) : !llvm.float
llvm.store %cf, %1 : !llvm<"float*">

// Load the value from this element.
%3 = llvm.load %1 : !llvm<"float*">

Operations on values of aggregate type. 

  • <value> = extractvalue <struct>[<index> (, <index>)+]
  • <struct> = insertvalue <value>, <struct>[<index> (, <index>)+]

In these operations, <struct> must be a value of wrapped LLVM IR structure type and <value> must be a value that corresponds to one of the (nested) structure element types.

Note the use of integer literals to designate subscripts, which is made possible by extractvalue and insertvalue must have constant subscripts. Internally, they are modeled as array attributes.

Examples:

// Get the value third element of the second element of a structure.
%0 = llvm.extractvalue %s[1, 2] : !llvm<"{i32, {i1, i8, i16}">

// Insert the value to the third element of the second element of a structure.
// Note that this returns a new structure-typed value.
%1 = llvm.insertvalue %0, %s[1, 2] : !llvm<"{i32, {i1, i8, i16}">

Terminator operations. 

Branch operations:

  • br [<successor>(<operands>)]
  • cond_br <condition> [<true-successor>(<true-operands>), <false-successor>(<false-operands>)]

In order to comply with MLIR design, branch operations in the LLVM IR dialect pass arguments to basic blocks. Successors must be valid block MLIR identifiers and operand lists for each of them must have the same types as the arguments of the respective blocks. <condition> must be a wrapped LLVM IR i1 type.

Since LLVM IR uses the name of the predecessor basic block to identify the sources of a PHI node, it is invalid for two entries of the PHI node to indicate different values coming from the same block. Therefore, cond_br in the LLVM IR dialect disallows its successors to be the same block if this block has arguments.

Examples:

// Branch without arguments.
^bb0:
  llvm.br ^bb0

// Branch and pass arguments.
^bb1(%arg: !llvm.i32):
  llvm.br ^bb1(%arg : !llvm.i32)

// Conditionally branch and pass arguments to one of the blocks.
llvm.cond_br %cond, ^bb0, %bb1(%arg : !llvm.i32)

// It's okay to use the same block without arguments, but probably useless.
llvm.cond_br %cond, ^bb0, ^bb0

// ERROR: Passing different arguments to the same block in a conditional branch.
llvm.cond_br %cond, ^bb1(%0 : !llvm.i32), ^bb1(%1 : !llvm.i32)

Call operations:

  • <r> = call(<operands>)
  • call(<operands>)

In LLVM IR, functions may return either 0 or 1 value. LLVM IR dialect implements this behavior by providing a variadic call operation for 0- and 1-result functions. Even though MLIR supports multi-result functions, LLVM IR dialect disallows them.

The call instruction supports both direct and indirect calls. Direct calls start with a function name (@-prefixed) and indirect calls start with an SSA value (%-prefixed). The direct callee, if present, is stored as a function attribute callee. The trailing type of the instruction is always the MLIR function type, which may be different from the indirect callee that has the wrapped LLVM IR function type.

Examples:

// Direct call without arguments and with one result.
%0 = llvm.call @foo() : () -> (!llvm.float)

// Direct call with arguments and without a result.
llvm.call @bar(%0) : (!llvm.float) -> ()

// Indirect call with an argument and without a result.
llvm.call %1(%0) : (!llvm.float) -> ()

Miscellaneous operations. 

Integer comparisons: icmp "predicate" <lhs>, <rhs>. The following predicate values are supported:

  • eq - equality comparison;
  • ne - inequality comparison;
  • slt - signed less-than comparison
  • sle - signed less-than-or-equal comparison
  • sgt - signed greater-than comparison
  • sge - signed greater-than-or-equal comparison
  • ult - unsigned less-than comparison
  • ule - unsigned less-than-or-equal comparison
  • ugt - unsigned greater-than comparison
  • uge - unsigned greater-than-or-equal comparison

Bitwise reinterpretation: bitcast <value>.

Selection: select <condition>, <lhs>, <rhs>.

Auxiliary MLIR Operations for Constants and Globals 

LLVM IR has broad support for first-class constants, which is not the case for MLIR. Instead, constants are defined in MLIR as regular SSA values produced by operations with specific traits. The LLVM dialect provides a set of operations that model LLVM IR constants. These operations do not correspond to LLVM IR instructions and are therefore prefixed with llvm.mlir.

Inline constants can be created by llvm.mlir.constant, which currently supports integer, float, string or elements attributes (constant structs are not currently supported). LLVM IR constant expressions are expected to be constructed as sequences of regular operations on SSA values produced by llvm.mlir.constant. Additionally, MLIR provides semantically-charged operations llvm.mlir.undef and llvm.mlir.null for the corresponding constants.

LLVM IR globals can be defined using llvm.mlir.global at the module level, except for functions that are defined with llvm.func. Globals, both variables and functions, can be accessed by taking their address with the llvm.mlir.addressof operation, which produces a pointer to the named global, unlike the llvm.mlir.constant that produces the value of the same type as the constant.

llvm.mlir.addressof 

Creates an SSA value containing a pointer to a global variable or constant defined by llvm.mlir.global. The global value can be defined after its first referenced. If the global value is a constant, storing into it is not allowed.

Examples:

func @foo() {
  // Get the address of a global variable.
  %0 = llvm.mlir.addressof @const : !llvm<"i32*">

  // Use it as a regular pointer.
  %1 = llvm.load %0 : !llvm<"i32*">

  // Get the address of a function.
  %2 = llvm.mlir.addressof @foo : !llvm<"void ()*">

  // The function address can be used for indirect calls.
  llvm.call %2() : () -> ()
}

// Define the global.
llvm.mlir.global @const(42 : i32) : !llvm.i32

llvm.mlir.constant 

Unlike LLVM IR, MLIR does not have first-class constant values. Therefore, all constants must be created as SSA values before being used in other operations. llvm.mlir.constant creates such values for scalars and vectors. It has a mandatory value attribute, which may be an integer, floating point attribute; dense or sparse attribute containing integers or floats. The type of the attribute is one of the corresponding MLIR builtin types. It may be omitted for i64 and f64 types that are implied. The operation produces a new SSA value of the specified LLVM IR dialect type. The type of that value must correspond to the attribute type converted to LLVM IR.

Examples:

// Integer constant, internal i32 is mandatory
%0 = llvm.mlir.constant(42 : i32) : !llvm.i32

// It's okay to omit i64.
%1 = llvm.mlir.constant(42) : !llvm.i64

// Floating point constant.
%2 = llvm.mlir.constant(42.0 : f32) : !llvm.float

// Splat dense vector constant.
%3 = llvm.mlir.constant(dense<1.0> : vector<4xf32>) : !llvm<"<4 x float>">

llvm.mlir.global 

Since MLIR allows for arbitrary operations to be present at the top level, global variables are defined using the llvm.mlir.global operation. Both global constants and variables can be defined, and the value may also be initialized in both cases.

There are two forms of initialization syntax. Simple constants that can be represented as MLIR attributes can be given in-line:

llvm.mlir.global @variable(32.0 : f32) : !llvm.float

This initialization and type syntax is similar to llvm.mlir.constant and may use two types: one for MLIR attribute and another for the LLVM value. These types must be compatible.

More complex constants that cannot be represented as MLIR attributes can be given in an initializer region:

// This global is initialized with the equivalent of:
//   i32* getelementptr (i32* @g2, i32 2)
llvm.mlir.global constant @int_gep() : !llvm<"i32*"> {
  %0 = llvm.mlir.addressof @g2 : !llvm<"i32*">
  %1 = llvm.mlir.constant(2 : i32) : !llvm.i32
  %2 = llvm.getelementptr %0[%1] : (!llvm<"i32*">, !llvm.i32) -> !llvm<"i32*">
  // The initializer region must end with `llvm.return`.
  llvm.return %2 : !llvm<"i32*">
}

Only one of the initializer attribute or initializer region may be provided.

llvm.mlir.global must appear at top-level of the enclosing module. It uses an @-identifier for its value, which will be uniqued by the module with respect to other @-identifiers in it.

Examples:

// Global values use @-identifiers.
llvm.mlir.global constant @cst(42 : i32) : !llvm.i32

// Non-constant values must also be initialized.
llvm.mlir.global @variable(32.0 : f32) : !llvm.float

// Strings are expected to be of wrapped LLVM i8 array type and do not
// automatically include the trailing zero.
llvm.mlir.global @string("abc") : !llvm<"[3 x i8]">

// For strings globals, the trailing type may be omitted.
llvm.mlir.global constant @no_trailing_type("foo bar")

// A complex initializer is constructed with an initializer region.
llvm.mlir.global constant @int_gep() : !llvm<"i32*"> {
  %0 = llvm.mlir.addressof @g2 : !llvm<"i32*">
  %1 = llvm.mlir.constant(2 : i32) : !llvm.i32
  %2 = llvm.getelementptr %0[%1] : (!llvm<"i32*">, !llvm.i32) -> !llvm<"i32*">
  llvm.return %2 : !llvm<"i32*">
}

Similarly to functions, globals have a linkage attribute. In the custom syntax, this attribute is placed between llvm.mlir.global and the optional constant keyword. If the attribute is omitted, external linkage is assumed by default.

Examples:

// A constant with internal linkage will not participate in linking.
llvm.mlir.global internal constant @cst(42 : i32) : !llvm.i32

// By default, "external" linkage is assumed and the global participates in
// symbol resolution at link-time.
llvm.mlir.global @glob(0 : f32) : !llvm.float

llvm.mlir.null 

Unlike LLVM IR, MLIR does not have first-class null pointers. They must be explicitly created as SSA values using llvm.mlir.null. This operation has no operands or attributes, and returns a null value of a wrapped LLVM IR pointer type.

Examples:

// Null pointer to i8 value.
%0 = llvm.mlir.null : !llvm<"i8*">

// Null pointer to a function with signature void() value.
%1 = llvm.mlir.null : !llvm<"void()*">

llvm.mlir.undef 

Unlike LLVM IR, MLIR does not have first-class undefined values. Such values must be created as SSA values using llvm.mlir.undef. This operation has no operands or attributes. It creates an undefined value of the specified LLVM IR dialect type wrapping an LLVM IR structure type.

Example:

// Create a structure with a 32-bit integer followed by a float.
%0 = llvm.mlir.undef : !llvm<"{i32, float}">