MLIR

Multi-Level IR Compiler Framework

Introduction and Usage Guide to MLIR's Diagnostics Infrastructure

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

See MLIR specification 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.

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 that can be modified further.

InFlightDiagnostic emit(Location loc, DiagnosticSeverity severity);

Using the DiagnosticEngine, though, is generally not the preferred way to emit diagnostics in MLIR. operation provides utility methods for emitting diagnostics:

// `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
  • 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:

// 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.

// 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 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.

{
  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 are listed below:

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:

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"() : () -> ()
  ^

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:

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 . 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.

// 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.

$ 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:

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:

// 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.

$ 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 , this handler can be added to any tool via the following:

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:

MLIRContext *context = ...;
ParallelDiagnosticHandler handler(context);

// Process a list of operations in parallel.
std::vector<Operation *> opsToProcess = ...;
llvm::for_each_n(llvm::parallel::par, 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();
});