mlir/docs/Diagnostics.md - rust-lang/llvm-project - Git at Google

 # Diagnostic Infrastructure

 [TOC]

 This document presents an introduction to using and interfacing with MLIR's
 diagnostics infrastructure.

 See [MLIR specification](LangRef.md) for more information about MLIR, the
 structure of the IR, operations, etc.

 ## Source Locations

 Source location information is extremely important for any compiler, because it
 provides a baseline for debuggability and error-reporting. MLIR provides several
 different location types depending on the situational need.

 ### CallSite Location

 ```
 callsite-location ::= 'callsite' '(' location 'at' location ')'
 ```

 An instance of this location allows for representing a directed stack of
 location usages. This connects a location of a `callee` with the location of a
 `caller`.

 ### FileLineCol Location

 ```
 filelinecol-location ::= string-literal ':' integer-literal ':' integer-literal
 ```

 An instance of this location represents a tuple of file, line number, and column
 number. This is similar to the type of location that you get from most source
 languages.

 ### Fused Location

 ```
 fused-location ::= `fused` fusion-metadata? '[' location (location ',')* ']'
 fusion-metadata ::= '<' attribute-value '>'
 ```

 An instance of a `fused` location represents a grouping of several other source
 locations, with optional metadata that describes the context of the fusion.
 There are many places within a compiler in which several constructs may be fused
 together, e.g. pattern rewriting, that normally result partial or even total
 loss of location information. With `fused` locations, this is a non-issue.

 ### Name Location

 ```
 name-location ::= string-literal ('(' location ')')?
 ```

 An instance of this location allows for attaching a name to a child location.
 This can be useful for representing the locations of variable, or node,
 definitions.

 ### Opaque Location

 An instance of this location essentially contains a pointer to some data
 structure that is external to MLIR and an optional location that can be used if
 the first one is not suitable. Since it contains an external structure, only the
 optional location is used during serialization.

 ### Unknown Location

 ```
 unknown-location ::= `unknown`
 ```

 Source location information is an extremely integral part of the MLIR
 infrastructure. As such, location information is always present in the IR, and
 must explicitly be set to unknown. Thus an instance of the `unknown` location,
 represents an unspecified source location.

 ## Diagnostic Engine

 The `DiagnosticEngine` acts as the main interface for diagnostics in MLIR. It
 manages the registration of diagnostic handlers, as well as the core API for
 diagnostic emission. Handlers generally take the form of
 `LogicalResult(Diagnostic &)`. If the result is `success`, it signals that the
 diagnostic has been fully processed and consumed. If `failure`, it signals that
 the diagnostic should be propagated to any previously registered handlers. It
 can be interfaced with via an `MLIRContext` instance.

 ```c++
 DiagnosticEngine engine = ctx->getDiagEngine();

 /// Handle the reported diagnostic.
 // Return success to signal that the diagnostic has either been fully processed,
 // or failure if the diagnostic should be propagated to the previous handlers.
 DiagnosticEngine::HandlerID id = engine.registerHandler(
     [](Diagnostic &diag) -> LogicalResult {
   bool should_propagate_diagnostic = ...;
   return failure(should_propagate_diagnostic);
 });


 // We can also elide the return value completely, in which the engine assumes
 // that all diagnostics are consumed(i.e. a success() result).
 DiagnosticEngine::HandlerID id = engine.registerHandler([](Diagnostic &diag) {
   return;
 });

 // Unregister this handler when we are done.
 engine.eraseHandler(id);
 ```

 ### Constructing a Diagnostic

 As stated above, the `DiagnosticEngine` holds the core API for diagnostic
 emission. A new diagnostic can be emitted with the engine via `emit`. This
 method returns an [InFlightDiagnostic](#inflight-diagnostic) that can be
 modified further.

 ```c++
 InFlightDiagnostic emit(Location loc, DiagnosticSeverity severity);
 ```

 Using the `DiagnosticEngine`, though, is generally not the preferred way to emit
 diagnostics in MLIR. [`operation`](LangRef.md#operations) provides utility
 methods for emitting diagnostics:

 ```c++
 // `emit` methods available in the mlir namespace.
 InFlightDiagnostic emitError/Remark/Warning(Location);

 // These methods use the location attached to the operation.
 InFlightDiagnostic Operation::emitError/Remark/Warning();

 // This method creates a diagnostic prefixed with "'op-name' op ".
 InFlightDiagnostic Operation::emitOpError();
 ```

 ## Diagnostic

 A `Diagnostic` in MLIR contains all of the necessary information for reporting a
 message to the user. A `Diagnostic` essentially boils down to three main
 components:

 *   [Source Location](#source-locations)
 *   Severity Level
     -   Error, Note, Remark, Warning
 *   Diagnostic Arguments
     -   The diagnostic arguments are used when constructing the output message.

 ### Appending arguments

 One a diagnostic has been constructed, the user can start composing it. The
 output message of a diagnostic is composed of a set of diagnostic arguments that
 have been attached to it. New arguments can be attached to a diagnostic in a few
 different ways:

 ```c++
 // A few interesting things to use when composing a diagnostic.
 Attribute fooAttr;
 Type fooType;
 SmallVector<int> fooInts;

 // Diagnostics can be composed via the streaming operators.
 op->emitError() << "Compose an interesting error: " << fooAttr << ", " << fooType
                 << ", (" << fooInts << ')';

 // This could generate something like (FuncAttr:@foo, IntegerType:i32, {0,1,2}):
 "Compose an interesting error: @foo, i32, (0, 1, 2)"
 ```

 ### Attaching notes

 Unlike many other compiler frameworks, notes in MLIR cannot be emitted directly.
 They must be explicitly attached to another diagnostic non-note diagnostic. When
 emitting a diagnostic, notes can be directly attached via `attachNote`. When
 attaching a note, if the user does not provide an explicit source location the
 note will inherit the location of the parent diagnostic.

 ```c++
 // Emit a note with an explicit source location.
 op->emitError("...").attachNote(noteLoc) << "...";

 // Emit a note that inherits the parent location.
 op->emitError("...").attachNote() << "...";
 ```

 ## InFlight Diagnostic

 Now that [Diagnostics](#diagnostic) have been explained, we introduce the
 `InFlightDiagnostic`, an RAII wrapper around a diagnostic that is set to be
 reported. This allows for modifying a diagnostic while it is still in flight. If
 it is not reported directly by the user it will automatically report when
 destroyed.

 ```c++
 {
   InFlightDiagnostic diag = op->emitError() << "...";
 }  // The diagnostic is automatically reported here.
 ```

 ## Diagnostic Configuration Options

 Several options are provided to help control and enhance the behavior of
 diagnostics. These options can be configured via the MLIRContext, and registered
 to the command line with the `registerMLIRContextCLOptions` method. These
 options are listed below:

 ### Print Operation On Diagnostic

 Command Line Flag: `-mlir-print-op-on-diagnostic`

 When a diagnostic is emitted on an operation, via `Operation::emitError/...`,
 the textual form of that operation is printed and attached as a note to the
 diagnostic. This option is useful for understanding the current form of an
 operation that may be invalid, especially when debugging verifier failures. An
 example output is shown below:

 ```shell
 test.mlir:3:3: error: 'module_terminator' op expects parent op 'module'
   "module_terminator"() : () -> ()
   ^
 test.mlir:3:3: note: see current operation: "module_terminator"() : () -> ()
   "module_terminator"() : () -> ()
   ^
 ```

 ### Print StackTrace On Diagnostic

 Command Line Flag: `-mlir-print-stacktrace-on-diagnostic`

 When a diagnostic is emitted, attach the current stack trace as a note to the
 diagnostic. This option is useful for understanding which part of the compiler
 generated certain diagnostics. An example output is shown below:

 ```shell
 test.mlir:3:3: error: 'module_terminator' op expects parent op 'module'
   "module_terminator"() : () -> ()
   ^
 test.mlir:3:3: note: diagnostic emitted with trace:
  #0 0x000055dd40543805 llvm::sys::PrintStackTrace(llvm::raw_ostream&) llvm/lib/Support/Unix/Signals.inc:553:11
  #1 0x000055dd3f8ac162 emitDiag(mlir::Location, mlir::DiagnosticSeverity, llvm::Twine const&) /lib/IR/Diagnostics.cpp:292:7
  #2 0x000055dd3f8abe8e mlir::emitError(mlir::Location, llvm::Twine const&) /lib/IR/Diagnostics.cpp:304:10
  #3 0x000055dd3f998e87 mlir::Operation::emitError(llvm::Twine const&) /lib/IR/Operation.cpp:324:29
  #4 0x000055dd3f99d21c mlir::Operation::emitOpError(llvm::Twine const&) /lib/IR/Operation.cpp:652:10
  #5 0x000055dd3f96b01c mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl<mlir::ModuleTerminatorOp>::verifyTrait(mlir::Operation*) /mlir/IR/OpDefinition.h:897:18
  #6 0x000055dd3f96ab38 mlir::Op<mlir::ModuleTerminatorOp, mlir::OpTrait::ZeroOperands, mlir::OpTrait::ZeroResult, mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl, mlir::OpTrait::IsTerminator>::BaseVerifier<mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl<mlir::ModuleTerminatorOp>, mlir::OpTrait::IsTerminator<mlir::ModuleTerminatorOp> >::verifyTrait(mlir::Operation*) /mlir/IR/OpDefinition.h:1052:29
  #  ...
   "module_terminator"() : () -> ()
   ^
 ```

 ## Common Diagnostic Handlers

 To interface with the diagnostics infrastructure, users will need to register a
 diagnostic handler with the [`DiagnosticEngine`](#diagnostic-engine).
 Recognizing the many users will want the same handler functionality, MLIR
 provides several common diagnostic handlers for immediate use.

 ### Scoped Diagnostic Handler

 This diagnostic handler is a simple RAII class that registers and unregisters a
 given diagnostic handler. This class can be either be used directly, or in
 conjunction with a derived diagnostic handler.

 ```c++
 // Construct the handler directly.
 MLIRContext context;
 ScopedDiagnosticHandler scopedHandler(&context, [](Diagnostic &diag) {
   ...
 });

 // Use this handler in conjunction with another.
 class MyDerivedHandler : public ScopedDiagnosticHandler {
   MyDerivedHandler(MLIRContext *ctx) : ScopedDiagnosticHandler(ctx) {
     // Set the handler that should be RAII managed.
     setHandler([&](Diagnostic diag) {
       ...
     });
   }
 };
 ```

 ### SourceMgr Diagnostic Handler

 This diagnostic handler is a wrapper around an llvm::SourceMgr instance. It
 provides support for displaying diagnostic messages inline with a line of a
 respective source file. This handler will also automatically load newly seen
 source files into the SourceMgr when attempting to display the source line of a
 diagnostic. Example usage of this handler can be seen in the `mlir-opt` tool.

 ```shell
 $ mlir-opt foo.mlir

 /tmp/test.mlir:6:24: error: expected non-function type
 func @foo() -> (index, ind) {
                        ^
 ```

 To use this handler in your tool, add the following:

 ```c++
 SourceMgr sourceMgr;
 MLIRContext context;
 SourceMgrDiagnosticHandler sourceMgrHandler(sourceMgr, &context);
 ```

 ### SourceMgr Diagnostic Verifier Handler

 This handler is a wrapper around a llvm::SourceMgr that is used to verify that
 certain diagnostics have been emitted to the context. To use this handler,
 annotate your source file with expected diagnostics in the form of:

 *   `expected-(error|note|remark|warning) {{ message }}`

 A few examples are shown below:

 ```mlir
 // Expect an error on the same line.
 func @bad_branch() {
   br ^missing  // expected-error {{reference to an undefined block}}
 }

 // Expect an error on an adjacent line.
 func @foo(%a : f32) {
   // expected-error@+1 {{unknown comparison predicate "foo"}}
   %result = cmpf "foo", %a, %a : f32
   return
 }

 // Expect an error on the next line that does not contain a designator.
 // expected-remark@below {{remark on function below}}
 // expected-remark@below {{another remark on function below}}
 func @bar(%a : f32)

 // Expect an error on the previous line that does not contain a designator.
 func @baz(%a : f32)
 // expected-remark@above {{remark on function above}}
 // expected-remark@above {{another remark on function above}}

 ```

 The handler will report an error if any unexpected diagnostics were seen, or if
 any expected diagnostics weren't.

 ```shell
 $ mlir-opt foo.mlir

 /tmp/test.mlir:6:24: error: unexpected error: expected non-function type
 func @foo() -> (index, ind) {
                        ^

 /tmp/test.mlir:15:4: error: expected remark "expected some remark" was not produced
 // expected-remark {{expected some remark}}
    ^~~~~~~~~~~~~~~~~~~~~~~~~~
 ```

 Similarly to the [SourceMgr Diagnostic Handler](#sourcemgr-diagnostic-handler),
 this handler can be added to any tool via the following:

 ```c++
 SourceMgr sourceMgr;
 MLIRContext context;
 SourceMgrDiagnosticVerifierHandler sourceMgrHandler(sourceMgr, &context);
 ```

 ### Parallel Diagnostic Handler

 MLIR is designed from the ground up to be multi-threaded. One important to thing
 to keep in mind when multi-threading is determinism. This means that the
 behavior seen when operating on multiple threads is the same as when operating
 on a single thread. For diagnostics, this means that the ordering of the
 diagnostics is the same regardless of the amount of threads being operated on.
 The ParallelDiagnosticHandler is introduced to solve this problem.

 After creating a handler of this type, the only remaining step is to ensure that
 each thread that will be emitting diagnostics to the handler sets a respective
 'orderID'. The orderID corresponds to the order in which diagnostics would be
 emitted when executing synchronously. For example, if we were processing a list
 of operations [a, b, c] on a single-thread. Diagnostics emitted while processing
 operation 'a' would be emitted before those for 'b' or 'c'. This corresponds 1-1
 with the 'orderID'. The thread that is processing 'a' should set the orderID to
 '0'; the thread processing 'b' should set it to '1'; and so on and so forth.
 This provides a way for the handler to deterministically order the diagnostics
 that it receives given the thread that it is receiving on.

 A simple example is shown below:

 ```c++
 MLIRContext *context = ...;
 ParallelDiagnosticHandler handler(context);

 // Process a list of operations in parallel.
 std::vector<Operation *> opsToProcess = ...;
 llvm::parallelForEachN(0, opsToProcess.size(), [&](size_t i) {
   // Notify the handler that we are processing the i'th operation.
   handler.setOrderIDForThread(i);
   auto *op = opsToProcess[i];
   ...

   // Notify the handler that we are finished processing diagnostics on this
   // thread.
   handler.eraseOrderIDForThread();
 });
 ```
	# Diagnostic Infrastructure

	[TOC]

	This document presents an introduction to using and interfacing with MLIR's
	diagnostics infrastructure.

	See [MLIR specification](LangRef.md) for more information about MLIR, the
	structure of the IR, operations, etc.

	## Source Locations

	Source location information is extremely important for any compiler, because it
	provides a baseline for debuggability and error-reporting. MLIR provides several
	different location types depending on the situational need.

	### CallSite Location

	```
	callsite-location ::= 'callsite' '(' location 'at' location ')'
	```

	An instance of this location allows for representing a directed stack of
	location usages. This connects a location of a `callee` with the location of a
	`caller`.

	### FileLineCol Location

	```
	filelinecol-location ::= string-literal ':' integer-literal ':' integer-literal
	```

	An instance of this location represents a tuple of file, line number, and column
	number. This is similar to the type of location that you get from most source
	languages.

	### Fused Location

	```
	fused-location ::= `fused` fusion-metadata? '[' location (location ',')* ']'
	fusion-metadata ::= '<' attribute-value '>'
	```

	An instance of a `fused` location represents a grouping of several other source
	locations, with optional metadata that describes the context of the fusion.
	There are many places within a compiler in which several constructs may be fused
	together, e.g. pattern rewriting, that normally result partial or even total
	loss of location information. With `fused` locations, this is a non-issue.

	### Name Location

	```
	name-location ::= string-literal ('(' location ')')?
	```

	An instance of this location allows for attaching a name to a child location.
	This can be useful for representing the locations of variable, or node,
	definitions.

	### Opaque Location

	An instance of this location essentially contains a pointer to some data
	structure that is external to MLIR and an optional location that can be used if
	the first one is not suitable. Since it contains an external structure, only the
	optional location is used during serialization.

	### Unknown Location

	```
	unknown-location ::= `unknown`
	```

	Source location information is an extremely integral part of the MLIR
	infrastructure. As such, location information is always present in the IR, and
	must explicitly be set to unknown. Thus an instance of the `unknown` location,
	represents an unspecified source location.

	## Diagnostic Engine

	The `DiagnosticEngine` acts as the main interface for diagnostics in MLIR. It
	manages the registration of diagnostic handlers, as well as the core API for
	diagnostic emission. Handlers generally take the form of
	`LogicalResult(Diagnostic &)`. If the result is `success`, it signals that the
	diagnostic has been fully processed and consumed. If `failure`, it signals that
	the diagnostic should be propagated to any previously registered handlers. It
	can be interfaced with via an `MLIRContext` instance.

	```c++
	DiagnosticEngine engine = ctx->getDiagEngine();

	/// Handle the reported diagnostic.
	// Return success to signal that the diagnostic has either been fully processed,
	// or failure if the diagnostic should be propagated to the previous handlers.
	DiagnosticEngine::HandlerID id = engine.registerHandler(
	[](Diagnostic &diag) -> LogicalResult {
	bool should_propagate_diagnostic = ...;
	return failure(should_propagate_diagnostic);
	});


	// We can also elide the return value completely, in which the engine assumes
	// that all diagnostics are consumed(i.e. a success() result).
	DiagnosticEngine::HandlerID id = engine.registerHandler([](Diagnostic &diag) {
	return;
	});

	// Unregister this handler when we are done.
	engine.eraseHandler(id);
	```

	### Constructing a Diagnostic

	As stated above, the `DiagnosticEngine` holds the core API for diagnostic
	emission. A new diagnostic can be emitted with the engine via `emit`. This
	method returns an [InFlightDiagnostic](#inflight-diagnostic) that can be
	modified further.

	```c++
	InFlightDiagnostic emit(Location loc, DiagnosticSeverity severity);
	```

	Using the `DiagnosticEngine`, though, is generally not the preferred way to emit
	diagnostics in MLIR. [`operation`](LangRef.md#operations) provides utility
	methods for emitting diagnostics:

	```c++
	// `emit` methods available in the mlir namespace.
	InFlightDiagnostic emitError/Remark/Warning(Location);

	// These methods use the location attached to the operation.
	InFlightDiagnostic Operation::emitError/Remark/Warning();

	// This method creates a diagnostic prefixed with "'op-name' op ".
	InFlightDiagnostic Operation::emitOpError();
	```

	## Diagnostic

	A `Diagnostic` in MLIR contains all of the necessary information for reporting a
	message to the user. A `Diagnostic` essentially boils down to three main
	components:

	* [Source Location](#source-locations)
	* Severity Level
	- Error, Note, Remark, Warning
	* Diagnostic Arguments
	- The diagnostic arguments are used when constructing the output message.

	### Appending arguments

	One a diagnostic has been constructed, the user can start composing it. The
	output message of a diagnostic is composed of a set of diagnostic arguments that
	have been attached to it. New arguments can be attached to a diagnostic in a few
	different ways:

	```c++
	// A few interesting things to use when composing a diagnostic.
	Attribute fooAttr;
	Type fooType;
	SmallVector<int> fooInts;

	// Diagnostics can be composed via the streaming operators.
	op->emitError() << "Compose an interesting error: " << fooAttr << ", " << fooType
	<< ", (" << fooInts << ')';

	// This could generate something like (FuncAttr:@foo, IntegerType:i32, {0,1,2}):
	"Compose an interesting error: @foo, i32, (0, 1, 2)"
	```

	### Attaching notes

	Unlike many other compiler frameworks, notes in MLIR cannot be emitted directly.
	They must be explicitly attached to another diagnostic non-note diagnostic. When
	emitting a diagnostic, notes can be directly attached via `attachNote`. When
	attaching a note, if the user does not provide an explicit source location the
	note will inherit the location of the parent diagnostic.

	```c++
	// Emit a note with an explicit source location.
	op->emitError("...").attachNote(noteLoc) << "...";

	// Emit a note that inherits the parent location.
	op->emitError("...").attachNote() << "...";
	```

	## InFlight Diagnostic

	Now that [Diagnostics](#diagnostic) have been explained, we introduce the
	`InFlightDiagnostic`, an RAII wrapper around a diagnostic that is set to be
	reported. This allows for modifying a diagnostic while it is still in flight. If
	it is not reported directly by the user it will automatically report when
	destroyed.

	```c++
	{
	InFlightDiagnostic diag = op->emitError() << "...";
	} // The diagnostic is automatically reported here.
	```

	## Diagnostic Configuration Options

	Several options are provided to help control and enhance the behavior of
	diagnostics. These options can be configured via the MLIRContext, and registered
	to the command line with the `registerMLIRContextCLOptions` method. These
	options are listed below:

	### Print Operation On Diagnostic

	Command Line Flag: `-mlir-print-op-on-diagnostic`

	When a diagnostic is emitted on an operation, via `Operation::emitError/...`,
	the textual form of that operation is printed and attached as a note to the
	diagnostic. This option is useful for understanding the current form of an
	operation that may be invalid, especially when debugging verifier failures. An
	example output is shown below:

	```shell
	test.mlir:3:3: error: 'module_terminator' op expects parent op 'module'
	"module_terminator"() : () -> ()
	^
	test.mlir:3:3: note: see current operation: "module_terminator"() : () -> ()
	"module_terminator"() : () -> ()
	^
	```

	### Print StackTrace On Diagnostic

	Command Line Flag: `-mlir-print-stacktrace-on-diagnostic`

	When a diagnostic is emitted, attach the current stack trace as a note to the
	diagnostic. This option is useful for understanding which part of the compiler
	generated certain diagnostics. An example output is shown below:

	```shell
	test.mlir:3:3: error: 'module_terminator' op expects parent op 'module'
	"module_terminator"() : () -> ()
	^
	test.mlir:3:3: note: diagnostic emitted with trace:
	#0 0x000055dd40543805 llvm::sys::PrintStackTrace(llvm::raw_ostream&) llvm/lib/Support/Unix/Signals.inc:553:11
	#1 0x000055dd3f8ac162 emitDiag(mlir::Location, mlir::DiagnosticSeverity, llvm::Twine const&) /lib/IR/Diagnostics.cpp:292:7
	#2 0x000055dd3f8abe8e mlir::emitError(mlir::Location, llvm::Twine const&) /lib/IR/Diagnostics.cpp:304:10
	#3 0x000055dd3f998e87 mlir::Operation::emitError(llvm::Twine const&) /lib/IR/Operation.cpp:324:29
	#4 0x000055dd3f99d21c mlir::Operation::emitOpError(llvm::Twine const&) /lib/IR/Operation.cpp:652:10
	#5 0x000055dd3f96b01c mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl<mlir::ModuleTerminatorOp>::verifyTrait(mlir::Operation*) /mlir/IR/OpDefinition.h:897:18
	#6 0x000055dd3f96ab38 mlir::Op<mlir::ModuleTerminatorOp, mlir::OpTrait::ZeroOperands, mlir::OpTrait::ZeroResult, mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl, mlir::OpTrait::IsTerminator>::BaseVerifier<mlir::OpTrait::HasParent<mlir::ModuleOp>::Impl<mlir::ModuleTerminatorOp>, mlir::OpTrait::IsTerminator<mlir::ModuleTerminatorOp> >::verifyTrait(mlir::Operation*) /mlir/IR/OpDefinition.h:1052:29
	# ...
	"module_terminator"() : () -> ()
	^
	```

	## Common Diagnostic Handlers

	To interface with the diagnostics infrastructure, users will need to register a
	diagnostic handler with the [`DiagnosticEngine`](#diagnostic-engine).
	Recognizing the many users will want the same handler functionality, MLIR
	provides several common diagnostic handlers for immediate use.

	### Scoped Diagnostic Handler

	This diagnostic handler is a simple RAII class that registers and unregisters a
	given diagnostic handler. This class can be either be used directly, or in
	conjunction with a derived diagnostic handler.

	```c++
	// Construct the handler directly.
	MLIRContext context;
	ScopedDiagnosticHandler scopedHandler(&context, [](Diagnostic &diag) {
	...
	});

	// Use this handler in conjunction with another.
	class MyDerivedHandler : public ScopedDiagnosticHandler {
	MyDerivedHandler(MLIRContext *ctx) : ScopedDiagnosticHandler(ctx) {
	// Set the handler that should be RAII managed.
	setHandler([&](Diagnostic diag) {
	...
	});
	}
	};
	```

	### SourceMgr Diagnostic Handler

	This diagnostic handler is a wrapper around an llvm::SourceMgr instance. It
	provides support for displaying diagnostic messages inline with a line of a
	respective source file. This handler will also automatically load newly seen
	source files into the SourceMgr when attempting to display the source line of a
	diagnostic. Example usage of this handler can be seen in the `mlir-opt` tool.

	```shell
	$ mlir-opt foo.mlir

	/tmp/test.mlir:6:24: error: expected non-function type
	func @foo() -> (index, ind) {
	^
	```

	To use this handler in your tool, add the following:

	```c++
	SourceMgr sourceMgr;
	MLIRContext context;
	SourceMgrDiagnosticHandler sourceMgrHandler(sourceMgr, &context);
	```

	### SourceMgr Diagnostic Verifier Handler

	This handler is a wrapper around a llvm::SourceMgr that is used to verify that
	certain diagnostics have been emitted to the context. To use this handler,
	annotate your source file with expected diagnostics in the form of:

	* `expected-(error\|note\|remark\|warning) {{ message }}`

	A few examples are shown below:

	```mlir
	// Expect an error on the same line.
	func @bad_branch() {
	br ^missing // expected-error {{reference to an undefined block}}
	}

	// Expect an error on an adjacent line.
	func @foo(%a : f32) {
	// expected-error@+1 {{unknown comparison predicate "foo"}}
	%result = cmpf "foo", %a, %a : f32
	return
	}

	// Expect an error on the next line that does not contain a designator.
	// expected-remark@below {{remark on function below}}
	// expected-remark@below {{another remark on function below}}
	func @bar(%a : f32)

	// Expect an error on the previous line that does not contain a designator.
	func @baz(%a : f32)
	// expected-remark@above {{remark on function above}}
	// expected-remark@above {{another remark on function above}}

	```

	The handler will report an error if any unexpected diagnostics were seen, or if
	any expected diagnostics weren't.

	```shell
	$ mlir-opt foo.mlir

	/tmp/test.mlir:6:24: error: unexpected error: expected non-function type
	func @foo() -> (index, ind) {
	^

	/tmp/test.mlir:15:4: error: expected remark "expected some remark" was not produced
	// expected-remark {{expected some remark}}
	^~~~~~~~~~~~~~~~~~~~~~~~~~
	```

	Similarly to the [SourceMgr Diagnostic Handler](#sourcemgr-diagnostic-handler),
	this handler can be added to any tool via the following:

	```c++
	SourceMgr sourceMgr;
	MLIRContext context;
	SourceMgrDiagnosticVerifierHandler sourceMgrHandler(sourceMgr, &context);
	```

	### Parallel Diagnostic Handler

	MLIR is designed from the ground up to be multi-threaded. One important to thing
	to keep in mind when multi-threading is determinism. This means that the
	behavior seen when operating on multiple threads is the same as when operating
	on a single thread. For diagnostics, this means that the ordering of the
	diagnostics is the same regardless of the amount of threads being operated on.
	The ParallelDiagnosticHandler is introduced to solve this problem.

	After creating a handler of this type, the only remaining step is to ensure that
	each thread that will be emitting diagnostics to the handler sets a respective
	'orderID'. The orderID corresponds to the order in which diagnostics would be
	emitted when executing synchronously. For example, if we were processing a list
	of operations [a, b, c] on a single-thread. Diagnostics emitted while processing
	operation 'a' would be emitted before those for 'b' or 'c'. This corresponds 1-1
	with the 'orderID'. The thread that is processing 'a' should set the orderID to
	'0'; the thread processing 'b' should set it to '1'; and so on and so forth.
	This provides a way for the handler to deterministically order the diagnostics
	that it receives given the thread that it is receiving on.

	A simple example is shown below:

	```c++
	MLIRContext *context = ...;
	ParallelDiagnosticHandler handler(context);

	// Process a list of operations in parallel.
	std::vector<Operation *> opsToProcess = ...;
	llvm::parallelForEachN(0, opsToProcess.size(), [&](size_t i) {
	// Notify the handler that we are processing the i'th operation.
	handler.setOrderIDForThread(i);
	auto *op = opsToProcess[i];
	...

	// Notify the handler that we are finished processing diagnostics on this
	// thread.
	handler.eraseOrderIDForThread();
	});
	```