9.6 KiB
LLVM IR 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.
[TOC]
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 is
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 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
Memory-related operations
<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 pointee 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.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.constant(1) : !llvm.i64
%1 = llvm.getelementptr %0[%c1] : (!llvm<"float*">, !llvm.i64)
-> !llvm<"float*">
// Store a constant into this element.
%cf = llvm.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 possbile
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 comparisonsle
- signed less-than-or-equal comparisonsgt
- signed greater-than comparisonsge
- signed greater-than-or-equal comparisonult
- unsigned less-than comparisonule
- unsigned less-than-or-equal comparisonugt
- unsigned greater-than comparisonuge
- unsigned greater-than-or-equal comparison
Bitwise reinterpretation: bitcast <value>
.
Selection: select <condition>, <lhs>, <rhs>
.
Pseudo-operations
These operations do not have LLVM IR counterparts but are necessary to map LLVM IR into MLIR.
llvm.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.constant
creates such values for scalars and vectors. It has a mandatory
value
attribute, which may be an integer, floating point attribute; splat,
dense or sparse attribute containing integers or floats. The type of the
attribute is one the corresponding MLIR standard 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.constant(42 : i32) : !llvm.i32
// It's okay to omit i64.
%1 = llvm.constant(42) : !llvm.i64
// Floating point constant.
%2 = llvm.constant(42.0 : f32) : !llvm.float
// Splat vector constant,.
%3 = llvm.constant(splat<vector<4xf32>, 1.0>) : !llvm<"<4 x float>">
llvm.undef
Unlike LLVM IR, MLIR does not have first-class undefined values. Such values
must be created as SSA values using llvm.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.undef : !llvm<"{i32, float}">