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Markdown
453 lines
22 KiB
Markdown
# Dialect Conversion
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This document describes a framework in MLIR in which to perform operation
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conversions between, and within dialects. This framework allows for transforming
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illegal operations to those supported by a provided conversion target, via a set
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of pattern-based operation rewriting patterns.
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[TOC]
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The dialect conversion framework consists of the following components:
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* A [Conversion Target](#conversion-target)
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* A set of [Rewrite Patterns](#rewrite-pattern-specification)
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* A [Type Converter](#type-conversion) (Optional)
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## Modes of Conversion
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When applying a conversion to a set of operations, there are several different
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conversion modes that may be selected from:
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* Partial Conversion
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- A partial conversion will legalize as many operations to the target as
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possible, but will allow pre-existing operations that were not
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explicitly marked as "illegal" to remain unconverted. This allows for
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partially lowering parts of the input in the presence of unknown
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operations.
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- A partial conversion can be applied via `applyPartialConversion`.
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* Full Conversion
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- A full conversion legalizes all input operations, and is only successful
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if all operations are properly legalized to the given conversion target.
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This ensures that only known operations will exist after the conversion
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process.
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- A full conversion can be applied via `applyFullConversion`.
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* Analysis Conversion
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- An analysis conversion will analyze which operations are legalizable to
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the given conversion target if a conversion were to be applied. This is
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done by performing a 'partial' conversion and recording which operations
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would have been successfully converted if successful. Note that no
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rewrites, or transformations, are actually applied to the input
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operations.
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- An analysis conversion can be applied via `applyAnalysisConversion`.
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In all cases, the framework walks the operations in preorder, examining an op
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before the ops in any regions it has.
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## Conversion Target
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The conversion target is a formal definition of what is considered to be legal
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during the conversion process. The final operations generated by the conversion
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framework must be marked as legal on the `ConversionTarget` for the rewrite to
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be a success. Depending on the conversion mode, existing operations need not
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always be legal. Operations and dialects may be marked with any of the provided
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legality actions below:
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* Legal
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- This action signals that every instance of a given operation is legal,
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i.e. any combination of attributes, operands, types, etc. are valid.
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* Dynamic
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- This action signals that only some instances of a given operation are
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legal. This allows for defining fine-tune constraints, e.g. saying that
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`arith.addi` is only legal when operating on 32-bit integers.
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* Illegal
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- This action signals that no instance of a given operation is legal.
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Operations marked as "illegal" must always be converted for the
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conversion to be successful. This action also allows for selectively
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marking specific operations as illegal in an otherwise legal dialect.
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Operations and dialects that are neither explicitly marked legal nor illegal are
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separate from the above ("unknown" operations) and are treated differently, for
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example, for the purposes of partial conversion as mentioned above.
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An example conversion target is shown below:
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```c++
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struct MyTarget : public ConversionTarget {
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MyTarget(MLIRContext &ctx) : ConversionTarget(ctx) {
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//--------------------------------------------------------------------------
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// Marking an operation as Legal:
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/// Mark all operations within the LLVM dialect are legal.
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addLegalDialect<LLVMDialect>();
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/// Mark `arith.constant` op is always legal on this target.
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addLegalOp<arith::ConstantOp>();
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//--------------------------------------------------------------------------
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// Marking an operation as dynamically legal.
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/// Mark all operations within Affine dialect have dynamic legality
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/// constraints.
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addDynamicallyLegalDialect<AffineDialect>([](Operation *op) { ... });
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/// Mark `std.return` as dynamically legal, but provide a specific legality
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/// callback.
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addDynamicallyLegalOp<ReturnOp>([](ReturnOp op) { ... });
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/// Treat unknown operations, i.e. those without a legalization action
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/// directly set, as dynamically legal.
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markUnknownOpDynamicallyLegal([](Operation *op) { ... });
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//--------------------------------------------------------------------------
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// Marking an operation as illegal.
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/// All operations within the GPU dialect are illegal.
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addIllegalDialect<GPUDialect>();
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/// Mark `cf.br` and `cf.cond_br` as illegal.
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addIllegalOp<cf::BranchOp, cf::CondBranchOp>();
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}
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/// Implement the default legalization handler to handle operations marked as
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/// dynamically legal that were not provided with an explicit handler.
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bool isDynamicallyLegal(Operation *op) override { ... }
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};
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```
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### Recursive Legality
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In some cases, it may be desirable to mark entire regions as legal. This
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provides an additional granularity of context to the concept of "legal". If an
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operation is marked recursively legal, either statically or dynamically, then
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all of the operations nested within are also considered legal even if they would
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otherwise be considered "illegal". An operation can be marked via
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`markOpRecursivelyLegal<>`:
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```c++
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ConversionTarget &target = ...;
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/// The operation must first be marked as `Legal` or `Dynamic`.
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target.addLegalOp<MyOp>(...);
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target.addDynamicallyLegalOp<MySecondOp>(...);
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/// Mark the operation as always recursively legal.
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target.markOpRecursivelyLegal<MyOp>();
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/// Mark optionally with a callback to allow selective marking.
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target.markOpRecursivelyLegal<MyOp, MySecondOp>([](Operation *op) { ... });
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/// Mark optionally with a callback to allow selective marking.
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target.markOpRecursivelyLegal<MyOp>([](MyOp op) { ... });
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```
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## Rewrite Pattern Specification
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After the conversion target has been defined, a set of legalization patterns
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must be provided to transform illegal operations into legal ones. The patterns
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supplied here have the same structure and restrictions as those described in the
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main [Pattern](PatternRewriter.md) documentation. The patterns provided do not
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need to generate operations that are directly legal on the target. The framework
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will automatically build a graph of conversions to convert non-legal operations
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into a set of legal ones.
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As an example, say you define a target that supports one operation: `foo.add`.
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When providing the following patterns: [`bar.add` -> `baz.add`, `baz.add` ->
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`foo.add`], the framework will automatically detect that it can legalize
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`bar.add` -> `foo.add` even though a direct conversion does not exist. This
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means that you don’t have to define a direct legalization pattern for `bar.add`
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-> `foo.add`.
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### Conversion Patterns
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Along with the general `RewritePattern` classes, the conversion framework
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provides a special type of rewrite pattern that can be used when a pattern
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relies on interacting with constructs specific to the conversion process, the
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`ConversionPattern`. For example, the conversion process does not necessarily
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update operations in-place and instead creates a mapping of events such as
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replacements and erasures, and only applies them when the entire conversion
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process is successful. Certain classes of patterns rely on using the
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updated/remapped operands of an operation, such as when the types of results
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defined by an operation have changed. The general Rewrite Patterns can no longer
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be used in these situations, as the types of the operands of the operation being
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matched will not correspond with those expected by the user. This pattern
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provides, as an additional argument to the `matchAndRewrite` and `rewrite`
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methods, the list of operands that the operation should use after conversion. If
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an operand was the result of a non-converted operation, for example if it was
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already legal, the original operand is used. This means that the operands
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provided always have a 1-1 non-null correspondence with the operands on the
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operation. The original operands of the operation are still intact and may be
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inspected as normal. These patterns also utilize a special `PatternRewriter`,
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`ConversionPatternRewriter`, that provides special hooks for use with the
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conversion infrastructure.
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```c++
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struct MyConversionPattern : public ConversionPattern {
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/// The `matchAndRewrite` hooks on ConversionPatterns take an additional
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/// `operands` parameter, containing the remapped operands of the original
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/// operation.
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virtual LogicalResult
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matchAndRewrite(Operation *op, ArrayRef<Value> operands,
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ConversionPatternRewriter &rewriter) const;
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};
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```
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#### Type Safety
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The types of the remapped operands provided to a conversion pattern must be of a
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type expected by the pattern. The expected types of a pattern are determined by
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a provided [TypeConverter](#type-converter). If no type converter is provided,
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the types of the remapped operands are expected to match the types of the
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original operands. If a type converter is provided, the types of the remapped
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operands are expected to be legal as determined by the converter. If the
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remapped operand types are not of an expected type, and a materialization to the
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expected type could not be performed, the pattern fails application before the
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`matchAndRewrite` hook is invoked. This ensures that patterns do not have to
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explicitly ensure type safety, or sanitize the types of the incoming remapped
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operands. More information on type conversion is detailed in the
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[dedicated section](#type-conversion) below.
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## Type Conversion
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It is sometimes necessary as part of a conversion to convert the set types of
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being operated on. In these cases, a `TypeConverter` object may be defined that
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details how types should be converted when interfacing with a pattern. A
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`TypeConverter` may be used to convert the signatures of block arguments and
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regions, to define the expected inputs types of the pattern, and to reconcile
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type differences in general.
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### Type Converter
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The `TypeConverter` contains several hooks for detailing how to convert types,
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and how to materialize conversions between types in various situations. The two
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main aspects of the `TypeConverter` are conversion and materialization.
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A `conversion` describes how a given illegal source `Type` should be converted
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to N target types. If the source type is already "legal", it should convert to
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itself. Type conversions are specified via the `addConversion` method described
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below.
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A `materialization` describes how a set of values should be converted to a
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single value of a desired type. An important distinction with a `conversion` is
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that a `materialization` can produce IR, whereas a `conversion` cannot. These
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materializations are used by the conversion framework to ensure type safety
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during the conversion process. There are several types of materializations
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depending on the situation.
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* Argument Materialization
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- An argument materialization is used when converting the type of a block
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argument during a [signature conversion](#region-signature-conversion).
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* Source Materialization
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- A source materialization converts from a value with a "legal" target
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type, back to a specific source type. This is used when an operation is
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"legal" during the conversion process, but contains a use of an illegal
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type. This may happen during a conversion where some operations are
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converted to those with different resultant types, but still retain
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users of the original type system.
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- This materialization is used in the following situations:
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* When a block argument has been converted to a different type, but
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the original argument still has users that will remain live after
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the conversion process has finished.
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* When the result type of an operation has been converted to a
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different type, but the original result still has users that will
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remain live after the conversion process is finished.
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* Target Materialization
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- A target materialization converts from a value with an "illegal" source
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type, to a value of a "legal" type. This is used when a pattern expects
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the remapped operands to be of a certain set of types, but the original
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input operands have not been converted. This may happen during a
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conversion where some operations are converted to those with different
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resultant types, but still retain uses of the original type system.
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- This materialization is used in the following situations:
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* When the remapped operands of a
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[conversion pattern](#conversion-patterns) are not legal for the
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type conversion provided by the pattern.
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If a converted value is used by an operation that isn't converted, it needs a
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conversion back to the `source` type, hence source materialization; if an
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unconverted value is used by an operation that is being converted, it needs
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conversion to the `target` type, hence target materialization.
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As noted above, the conversion process guarantees that the type contract of the
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IR is preserved during the conversion. This means that the types of value uses
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will not implicitly change during the conversion process. When the type of a
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value definition, either block argument or operation result, is being changed,
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the users of that definition must also be updated during the conversion process.
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If they aren't, a type conversion must be materialized to ensure that a value of
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the expected type is still present within the IR. If a target materialization is
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required, but cannot be performed, the pattern application fails. If a source
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materialization is required, but cannot be performed, the entire conversion
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process fails.
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Several of the available hooks are detailed below:
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```c++
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class TypeConverter {
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public:
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/// Register a conversion function. A conversion function defines how a given
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/// source type should be converted. A conversion function must be convertible
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/// to any of the following forms(where `T` is a class derived from `Type`:
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/// * Optional<Type>(T)
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/// - This form represents a 1-1 type conversion. It should return nullptr
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/// or `llvm::None` to signify failure. If `llvm::None` is returned, the
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/// converter is allowed to try another conversion function to perform
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/// the conversion.
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/// * Optional<LogicalResult>(T, SmallVectorImpl<Type> &)
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/// - This form represents a 1-N type conversion. It should return
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/// `failure` or `llvm::None` to signify a failed conversion. If the new
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/// set of types is empty, the type is removed and any usages of the
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/// existing value are expected to be removed during conversion. If
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/// `llvm::None` is returned, the converter is allowed to try another
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/// conversion function to perform the conversion.
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/// * Optional<LogicalResult>(T, SmallVectorImpl<Type> &, ArrayRef<Type>)
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/// - This form represents a 1-N type conversion supporting recursive
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/// types. The first two arguments and the return value are the same as
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/// for the regular 1-N form. The third argument is contains is the
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/// "call stack" of the recursive conversion: it contains the list of
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/// types currently being converted, with the current type being the
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/// last one. If it is present more than once in the list, the
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/// conversion concerns a recursive type.
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/// Note: When attempting to convert a type, e.g. via 'convertType', the
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/// mostly recently added conversions will be invoked first.
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template <typename FnT,
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typename T = typename llvm::function_traits<FnT>::template arg_t<0>>
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void addConversion(FnT &&callback) {
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registerConversion(wrapCallback<T>(std::forward<FnT>(callback)));
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}
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/// Register a materialization function, which must be convertible to the
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/// following form:
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/// `Optional<Value> (OpBuilder &, T, ValueRange, Location)`,
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/// where `T` is any subclass of `Type`.
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/// This function is responsible for creating an operation, using the
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/// OpBuilder and Location provided, that "converts" a range of values into a
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/// single value of the given type `T`. It must return a Value of the
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/// converted type on success, an `llvm::None` if it failed but other
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/// materialization can be attempted, and `nullptr` on unrecoverable failure.
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/// It will only be called for (sub)types of `T`.
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///
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/// This method registers a materialization that will be called when
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/// converting an illegal block argument type, to a legal type.
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template <typename FnT,
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typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
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void addArgumentMaterialization(FnT &&callback) {
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argumentMaterializations.emplace_back(
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wrapMaterialization<T>(std::forward<FnT>(callback)));
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}
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/// This method registers a materialization that will be called when
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/// converting a legal type to an illegal source type. This is used when
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/// conversions to an illegal type must persist beyond the main conversion.
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template <typename FnT,
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typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
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void addSourceMaterialization(FnT &&callback) {
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sourceMaterializations.emplace_back(
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wrapMaterialization<T>(std::forward<FnT>(callback)));
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}
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/// This method registers a materialization that will be called when
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/// converting type from an illegal, or source, type to a legal type.
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template <typename FnT,
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typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
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void addTargetMaterialization(FnT &&callback) {
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targetMaterializations.emplace_back(
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wrapMaterialization<T>(std::forward<FnT>(callback)));
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}
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};
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```
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### Region Signature Conversion
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From the perspective of type conversion, the types of block arguments are a bit
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special. Throughout the conversion process, blocks may move between regions of
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different operations. Given this, the conversion of the types for blocks must be
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done explicitly via a conversion pattern. To convert the types of block
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arguments within a Region, a custom hook on the `ConversionPatternRewriter` must
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be invoked; `convertRegionTypes`. This hook uses a provided type converter to
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apply type conversions to all blocks within a given region, and all blocks that
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move into that region. As noted above, the conversions performed by this method
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use the argument materialization hook on the `TypeConverter`. This hook also
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takes an optional `TypeConverter::SignatureConversion` parameter that applies a
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custom conversion to the entry block of the region. The types of the entry block
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arguments are often tied semantically to details on the operation, e.g. FuncOp,
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AffineForOp, etc. To convert the signature of just the region entry block, and
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not any other blocks within the region, the `applySignatureConversion` hook may
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be used instead. A signature conversion, `TypeConverter::SignatureConversion`,
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can be built programmatically:
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```c++
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class SignatureConversion {
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public:
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/// Remap an input of the original signature with a new set of types. The
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/// new types are appended to the new signature conversion.
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void addInputs(unsigned origInputNo, ArrayRef<Type> types);
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/// Append new input types to the signature conversion, this should only be
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/// used if the new types are not intended to remap an existing input.
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void addInputs(ArrayRef<Type> types);
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/// Remap an input of the original signature with a range of types in the
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/// new signature.
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void remapInput(unsigned origInputNo, unsigned newInputNo,
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unsigned newInputCount = 1);
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/// Remap an input of the original signature to another `replacement`
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/// value. This drops the original argument.
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void remapInput(unsigned origInputNo, Value replacement);
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};
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```
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The `TypeConverter` provides several default utilities for signature conversion
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and legality checking:
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`convertSignatureArgs`/`convertBlockSignature`/`isLegal(Region *|Type)`.
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## Debugging
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To debug the execution of the dialect conversion framework,
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`-debug-only=dialect-conversion` may be used. This command line flag activates
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LLVM's debug logging infrastructure solely for the conversion framework. The
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output is formatted as a tree structure, mirroring the structure of the
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conversion process. This output contains all of the actions performed by the
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rewriter, how generated operations get legalized, and why they fail.
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Example output is shown below:
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```
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//===-------------------------------------------===//
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Legalizing operation : 'std.return'(0x608000002e20) {
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"std.return"() : () -> ()
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* Fold {
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} -> FAILURE : unable to fold
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* Pattern : 'std.return -> ()' {
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** Insert : 'spv.Return'(0x6070000453e0)
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** Replace : 'std.return'(0x608000002e20)
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//===-------------------------------------------===//
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Legalizing operation : 'spv.Return'(0x6070000453e0) {
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"spv.Return"() : () -> ()
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} -> SUCCESS : operation marked legal by the target
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//===-------------------------------------------===//
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} -> SUCCESS : pattern applied successfully
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} -> SUCCESS
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//===-------------------------------------------===//
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```
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This output is describing the legalization of an `std.return` operation. We
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first try to legalize by folding the operation, but that is unsuccessful for
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`std.return`. From there, a pattern is applied that replaces the `std.return`
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with a `spv.Return`. The newly generated `spv.Return` is then processed for
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legalization, but is found to already legal as per the target.
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