This document describes a framework in MLIR in which to perform operation conversions between, and within dialects. This framework allows for transforming illegal operations to those supported by a provided conversion target, via a set of pattern-based operation rewriting patterns.
To utilize the framework, a few things must be provided:
When applying a conversion to a set of operations, there are several conversion modes that can be selected from:
Partial Conversion
illegal
to remain unconverted. This allows for partially lowering parts of the module in the presence of unknown operations.applyPartialConversion
.Full Conversion
applyFullConversion
.Analysis Conversion
applyAnalysisConversion
.The conversion target is the formal definition of what is considered to be legal during the conversion process. The final operations generated by the conversion framework must be marked as legal on the ConversionTarget
for the rewrite to be a success. Existing operations need not always be legal, though; see the different conversion modes for why. Operations and dialects may be marked with any of the provided legality actions below:
Legal
Dynamic
addi
is only legal when operating on 32-bit integers.isDynamicallyLegal
hook provided by ConversionTarget
.Illegal
illegal
must always be converted for the conversion to be successful. This action also allows for selectively marking specific operations as illegal in an otherwise legal dialect.An example conversion target is shown below:
struct MyTarget : public ConversionTarget { MyTarget(MLIRContext &ctx) : ConversionTarget(ctx) { //-------------------------------------------------------------------------- // Marking an operation as Legal: /// Mark all operations within the LLVM dialect are legal. addLegalDialects<LLVMDialect>(); /// Mark `std.constant` op is always legal on this target. addLegalOps<ConstantOp>(); //-------------------------------------------------------------------------- // Marking an operation as dynamically legal. /// Mark all operations within Affine dialect have dynamic legality /// constraints. addDynamicallyLegalDialects<AffineDialect>(); /// Mark `std.return` as dynamically legal. addDynamicallyLegalOp<ReturnOp>(); /// Mark `std.return` as dynamically legal, but provide a specific legality /// callback. addDynamicallyLegalOp<ReturnOp>([](ReturnOp op) { ... }); /// Treat unknown operations, i.e. those without a legalization action /// directly set, as dynamically legal. markUnknownOpDynamicallyLegal(); markUnknownOpDynamicallyLegal([](Operation *op) { ... }); //-------------------------------------------------------------------------- // Marking an operation as illegal. /// All operations within the GPU dialect are illegal. addIllegalDialect<GPUDialect>(); /// Mark `std.br` and `std.cond_br` as illegal. addIllegalOp<BranchOp, CondBranchOp>(); } /// Implement the default legalization handler to handle operations marked as /// dynamically legal that were not provided with an explicit handler. bool isDynamicallyLegal(Operation *op) override { ... } };
In some cases, it may be desirable to mark entire regions of operations as legal. This provides an additional granularity of context to the concept of “legal”. The ConversionTarget
supports marking operations, that were previously added as Legal
or Dynamic
, as recursively
legal. Recursive legality means that if an operation instance is legal, either statically or dynamically, all of the operations nested within are also considered legal. An operation can be marked via markOpRecursivelyLegal<>
:
ConversionTarget &target = ...; /// The operation must first be marked as `Legal` or `Dynamic`. target.addLegalOp<MyOp>(...); target.addDynamicallyLegalOp<MySecondOp>(...); /// Mark the operation as always recursively legal. target.markOpRecursivelyLegal<MyOp>(); /// Mark optionally with a callback to allow selective marking. target.markOpRecursivelyLegal<MyOp, MySecondOp>([](Operation *op) { ... }); /// Mark optionally with a callback to allow selective marking. target.markOpRecursivelyLegal<MyOp>([](MyOp op) { ... });
After the conversion target has been defined, a set of legalization patterns must be provided to transform illegal operations into legal ones. The patterns supplied here, that do not require type changes, are the same as those described in the quickstart rewrites guide, but have a few additional restrictions. The patterns provided do not need to generate operations that are directly legal on the target. The framework will automatically build a graph of conversions to convert non-legal operations into a set of legal ones.
As an example, say you define a target that supports one operation: foo.add
. When providing the following patterns: [bar.add
-> baz.add
, baz.add
-> foo.add
], the framework will automatically detect that it can legalize bar.add
-> foo.add
even though a direct conversion does not exist. This means that you don’t have to define a direct legalization pattern for bar.add
-> foo.add
.
The framework processes operations in topological order, trying to legalize them individually. As such, patterns used in the conversion framework have a few additional restrictions:
It is sometimes necessary as part of a conversion to convert the set types of being operated on. In these cases, a TypeConverter
object may be defined that details how types should be converted. The TypeConverter
is used by patterns and by the general conversion infrastructure to convert the signatures of blocks and regions.
As stated above, the TypeConverter
contains several hooks for detailing how to convert types. Several of these hooks are detailed below:
class TypeConverter { public: /// Register a conversion function. A conversion function must be convertible /// to any of the following forms(where `T` is a class derived from `Type`: /// * Optional<Type>(T) /// - This form represents a 1-1 type conversion. It should return nullptr /// or `llvm::None` to signify failure. If `llvm::None` is returned, the /// converter is allowed to try another conversion function to perform /// the conversion. /// * Optional<LogicalResult>(T, SmallVectorImpl<Type> &) /// - This form represents a 1-N type conversion. It should return /// `failure` or `llvm::None` to signify a failed conversion. If the new /// set of types is empty, the type is removed and any usages of the /// existing value are expected to be removed during conversion. If /// `llvm::None` is returned, the converter is allowed to try another /// conversion function to perform the conversion. /// /// When attempting to convert a type, e.g. via `convertType`, the /// `TypeConverter` will invoke each of the converters starting with the one /// most recently registered. template <typename ConversionFnT> void addConversion(ConversionFnT &&callback); /// Register a materialization function, which must be convertibe to the /// following form /// `Optional<Value>(PatternRewriter &, T, ValueRange, Location)`, /// where `T` is any subclass of `Type`. This function is responsible for /// creating an operation, using the PatternRewriter and Location provided, /// that "casts" a range of values into a single value of the given type `T`. /// It must return a Value of the converted type on success, an `llvm::None` /// if it failed but other materialization can be attempted, and `nullptr` on /// unrecoverable failure. It will only be called for (sub)types of `T`. /// Materialization functions must be provided when a type conversion /// results in more than one type, or if a type conversion may persist after /// the conversion has finished. template <typename FnT> void addMaterialization(FnT &&callback); };
When type conversion comes into play, the general Rewrite Patterns can no longer be used. This is due to the fact that the operands of the operation being matched will not correspond with the operands of the correct type as determined by TypeConverter
. The operation rewrites on type boundaries must thus use a special pattern, the ConversionPattern
. This pattern provides, as an additional argument to the matchAndRewrite
and rewrite
methods, the set of remapped operands corresponding to the desired type. These patterns also utilize a special PatternRewriter
, ConversionPatternRewriter
, that provides special hooks for use with the conversion infrastructure.
struct MyConversionPattern : public ConversionPattern { /// The `matchAndRewrite` hooks on ConversionPatterns take an additional /// `operands` parameter, containing the remapped operands of the original /// operation. virtual LogicalResult matchAndRewrite(Operation *op, ArrayRef<Value> operands, ConversionPatternRewriter &rewriter) const; };
These patterns have the same restrictions as the basic rewrite patterns used in dialect conversion.
From the perspective of type conversion, the types of block arguments are a bit special. Throughout the conversion process, blocks may move between regions of different operations. Given this, the conversion of the types for blocks must be done explicitly via a conversion pattern. To convert the types of block arguments within a Region, a custom hook on the ConversionPatternRewriter
must be invoked; convertRegionTypes
. This hook uses a provided type converter to apply type conversions to all blocks within the region, and all blocks that move into that region. This hook also takes an optional TypeConverter::SignatureConversion
parameter that applies a custom conversion to the entry block of the region. The types of the entry block arguments are often tied semantically to details on the operation, e.g. FuncOp, AffineForOp, etc. To convert the signature of just the region entry block, and not any other blocks within the region, the applySignatureConversion
hook may be used instead. A signature conversion, TypeConverter::SignatureConversion
, can be built programmatically:
class SignatureConversion { public: /// Remap an input of the original signature with a new set of types. The /// new types are appended to the new signature conversion. void addInputs(unsigned origInputNo, ArrayRef<Type> types); /// Append new input types to the signature conversion, this should only be /// used if the new types are not intended to remap an existing input. void addInputs(ArrayRef<Type> types); /// Remap an input of the original signature with a range of types in the /// new signature. void remapInput(unsigned origInputNo, unsigned newInputNo, unsigned newInputCount = 1); /// Remap an input of the original signature to another `replacement` /// value. This drops the original argument. void remapInput(unsigned origInputNo, Value replacement); };
The TypeConverter
provides several default utilities for signature conversion and legality checking: convertSignatureArgs
/convertBlockSignature
/isLegal(Region *|Type)
.