[MLIR] Implement remark emitting policies in MLIR

This update introduces two new remark emitting policies:
1. `RemarkEmittingPolicyAll`, which emits all remarks,
2. `RemarkEmittingPolicyFinal`, which only emits final remarks after processing.

The `RemarkEngine` is modified to support these policies, allowing for more flexible remark handling based on user configuration.

PR also adds flag to `mlir-opt`
```
  --remark-policy=<value>                                    - Specify the policy for remark output.
    =all                                                     -   Print all remarks
    =final                                                   -   Print final remarks
```

Author: grypp

mlir/include/mlir/IR/Remarks.h

@@ -18,7 +18,6 @@
 #include "llvm/Remarks/Remark.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Regex.h"
-#include <optional>
 
 #include "mlir/IR/Diagnostics.h"
 #include "mlir/IR/MLIRContext.h"
@@ -144,7 +143,7 @@ class Remark {
 
   llvm::StringRef getCategoryName() const { return categoryName; }
 
-  llvm::StringRef getFullCategoryName() const {
+  llvm::StringRef getCombinedCategoryName() const {
     if (categoryName.empty() && subCategoryName.empty())
       return {};
     if (subCategoryName.empty())
@@ -318,7 +317,7 @@ class InFlightRemark {
 };
 
 //===----------------------------------------------------------------------===//
-// MLIR Remark Streamer
+// Pluggable Remark Utilities
 //===----------------------------------------------------------------------===//
 
 /// Base class for MLIR remark streamers that is used to stream
@@ -338,6 +337,26 @@ class MLIRRemarkStreamerBase {
   virtual void finalize() {} // optional
 };
 
+using ReportFn = llvm::unique_function<void(const Remark &)>;
+
+/// Base class for MLIR remark emitting policies that is used to emit
+/// optimization remarks to the underlying remark streamer. The derived classes
+/// should implement the `reportRemark` method to provide the actual emitting
+/// implementation.
+class RemarkEmittingPolicyBase {
+protected:
+  ReportFn reportImpl;
+
+public:
+  RemarkEmittingPolicyBase() = default;
+  virtual ~RemarkEmittingPolicyBase() = default;
+
+  void initialize(ReportFn fn) { reportImpl = std::move(fn); }
+
+  virtual void reportRemark(const Remark &remark) = 0;
+  virtual void finalize() = 0;
+};
+
 //===----------------------------------------------------------------------===//
 // Remark Engine (MLIR Context will own this class)
 //===----------------------------------------------------------------------===//
@@ -355,6 +374,8 @@ class RemarkEngine {
   std::optional<llvm::Regex> failedFilter;
   /// The MLIR remark streamer that will be used to emit the remarks.
   std::unique_ptr<MLIRRemarkStreamerBase> remarkStreamer;
+  /// The MLIR remark policy that will be used to emit the remarks.
+  std::unique_ptr<RemarkEmittingPolicyBase> remarkEmittingPolicy;
   /// When is enabled, engine also prints remarks as mlir::emitRemarks.
   bool printAsEmitRemarks = false;
 
@@ -392,6 +413,8 @@ class RemarkEngine {
   InFlightRemark emitIfEnabled(Location loc, RemarkOpts opts,
                                bool (RemarkEngine::*isEnabled)(StringRef)
                                    const);
+  /// Report a remark.
+  void reportImpl(const Remark &remark);
 
 public:
   /// Default constructor is deleted, use the other constructor.
@@ -406,9 +429,15 @@ class RemarkEngine {
   /// main remark streamer.
   ~RemarkEngine();
 
+  /// Shutdown the remark engine. This will finalize the remark engine and
+  /// close the output file.
+  void shutdown();
+
   /// Setup the remark engine with the given output path and format.
-  LogicalResult initialize(std::unique_ptr<MLIRRemarkStreamerBase> streamer,
-                           std::string *errMsg);
+  LogicalResult
+  initialize(std::unique_ptr<MLIRRemarkStreamerBase> streamer,
+             std::unique_ptr<RemarkEmittingPolicyBase> remarkEmittingPolicy,
+             std::string *errMsg);
 
   /// Report a remark.
   void report(const Remark &&remark);
@@ -446,6 +475,46 @@ inline InFlightRemark withEngine(Fn fn, Location loc, Args &&...args) {
 
 namespace mlir::remark {
 
+//===----------------------------------------------------------------------===//
+// Remark Emitting Policies
+//===----------------------------------------------------------------------===//
+
+/// Policy that emits all remarks.
+class RemarkEmittingPolicyAll : public detail::RemarkEmittingPolicyBase {
+public:
+  RemarkEmittingPolicyAll();
+
+  void reportRemark(const detail::Remark &remark) override {
+    assert(reportImpl && "reportImpl is not set");
+    reportImpl(remark);
+  }
+  void finalize() override {}
+};
+
+/// Policy that emits final remarks.
+class RemarkEmittingPolicyFinal : public detail::RemarkEmittingPolicyBase {
+private:
+  /// user can intercept them for custom processing via a registered callback,
+  /// otherwise they will be reported on engine destruction.
+  llvm::DenseSet<detail::Remark> postponedRemarks;
+
+public:
+  RemarkEmittingPolicyFinal();
+
+  void reportRemark(const detail::Remark &remark) override {
+    postponedRemarks.erase(remark);
+    postponedRemarks.insert(remark);
+  }
+
+  void finalize() override {
+    assert(reportImpl && "reportImpl is not set");
+    for (auto &remark : postponedRemarks) {
+      if (reportImpl)
+        reportImpl(remark);
+    }
+  }
+};
+
 /// Create a Reason with llvm::formatv formatting.
 template <class... Ts>
 inline detail::LazyTextBuild reason(const char *fmt, Ts &&...ts) {
@@ -505,16 +574,72 @@ inline detail::InFlightRemark analysis(Location loc, RemarkOpts opts) {
 
 /// Setup remarks for the context. This function will enable the remark engine
 /// and set the streamer to be used for optimization remarks. The remark
-/// categories are used to filter the remarks that will be emitted by the remark
-/// engine. If a category is not specified, it will not be emitted. If
+/// categories are used to filter the remarks that will be emitted by the
+/// remark engine. If a category is not specified, it will not be emitted. If
 /// `printAsEmitRemarks` is true, the remarks will be printed as
 /// mlir::emitRemarks. 'streamer' must inherit from MLIRRemarkStreamerBase and
 /// will be used to stream the remarks.
 LogicalResult enableOptimizationRemarks(
     MLIRContext &ctx,
     std::unique_ptr<remark::detail::MLIRRemarkStreamerBase> streamer,
+    std::unique_ptr<remark::detail::RemarkEmittingPolicyBase>
+        remarkEmittingPolicy,
     const remark::RemarkCategories &cats, bool printAsEmitRemarks = false);
 
 } // namespace mlir::remark
 
+// DenseMapInfo specialization for Remark
+namespace llvm {
+template <>
+struct DenseMapInfo<mlir::remark::detail::Remark> {
+  static constexpr StringRef kEmptyKey = "<EMPTY_KEY>";
+  static constexpr StringRef kTombstoneKey = "<TOMBSTONE_KEY>";
+
+  /// Helper to provide a static dummy context for sentinel keys.
+  static mlir::MLIRContext *getStaticDummyContext() {
+    static mlir::MLIRContext dummyContext;
+    return &dummyContext;
+  }
+
+  /// Create an empty remark
+  static inline mlir::remark::detail::Remark getEmptyKey() {
+    return mlir::remark::detail::Remark(
+        mlir::remark::RemarkKind::RemarkUnknown, mlir::DiagnosticSeverity::Note,
+        mlir::UnknownLoc::get(getStaticDummyContext()),
+        mlir::remark::RemarkOpts::name(kEmptyKey));
+  }
+
+  /// Create a dead remark
+  static inline mlir::remark::detail::Remark getTombstoneKey() {
+    return mlir::remark::detail::Remark(
+        mlir::remark::RemarkKind::RemarkUnknown, mlir::DiagnosticSeverity::Note,
+        mlir::UnknownLoc::get(getStaticDummyContext()),
+        mlir::remark::RemarkOpts::name(kTombstoneKey));
+  }
+
+  /// Compute the hash value of the remark
+  static unsigned getHashValue(const mlir::remark::detail::Remark &remark) {
+    return llvm::hash_combine(
+        remark.getLocation().getAsOpaquePointer(),
+        llvm::hash_value(remark.getRemarkName()),
+        llvm::hash_value(remark.getCombinedCategoryName()));
+  }
+
+  static bool isEqual(const mlir::remark::detail::Remark &lhs,
+                      const mlir::remark::detail::Remark &rhs) {
+    // Check for empty/tombstone keys first
+    if (lhs.getRemarkName() == kEmptyKey ||
+        lhs.getRemarkName() == kTombstoneKey ||
+        rhs.getRemarkName() == kEmptyKey ||
+        rhs.getRemarkName() == kTombstoneKey) {
+      return lhs.getRemarkName() == rhs.getRemarkName();
+    }
+
+    // For regular remarks, compare key identifying fields
+    return lhs.getLocation() == rhs.getLocation() &&
+           lhs.getRemarkName() == rhs.getRemarkName() &&
+           lhs.getCombinedCategoryName() == rhs.getCombinedCategoryName();
+  }
+};
+} // namespace llvm
 #endif // MLIR_IR_REMARKS_H

mlir/include/mlir/Remark/RemarkStreamer.h

@@ -45,6 +45,7 @@ namespace mlir::remark {
 /// mlir::emitRemarks.
 LogicalResult enableOptimizationRemarksWithLLVMStreamer(
     MLIRContext &ctx, StringRef filePath, llvm::remarks::Format fmt,
+    std::unique_ptr<detail::RemarkEmittingPolicyBase> remarkEmittingPolicy,
     const RemarkCategories &cat, bool printAsEmitRemarks = false);
 
 } // namespace mlir::remark

mlir/include/mlir/Tools/mlir-opt/MlirOptMain.h

@@ -44,6 +44,11 @@ enum class RemarkFormat {
   REMARK_FORMAT_BITSTREAM,
 };
 
+enum class RemarkPolicy {
+  REMARK_POLICY_ALL,
+  REMARK_POLICY_FINAL,
+};
+
 /// Configuration options for the mlir-opt tool.
 /// This is intended to help building tools like mlir-opt by collecting the
 /// supported options.
@@ -242,6 +247,8 @@ class MlirOptMainConfig {
 
   /// Set the reproducer output filename
   RemarkFormat getRemarkFormat() const { return remarkFormatFlag; }
+  /// Set the remark policy to use.
+  RemarkPolicy getRemarkPolicy() const { return remarkPolicyFlag; }
   /// Set the remark format to use.
   std::string getRemarksAllFilter() const { return remarksAllFilterFlag; }
   /// Set the remark output file.
@@ -265,6 +272,8 @@ class MlirOptMainConfig {
 
   /// Remark format
   RemarkFormat remarkFormatFlag = RemarkFormat::REMARK_FORMAT_STDOUT;
+  /// Remark policy
+  RemarkPolicy remarkPolicyFlag = RemarkPolicy::REMARK_POLICY_ALL;
   /// Remark file to output to
   std::string remarksOutputFileFlag = "";
   /// Remark filters

mlir/lib/IR/MLIRContext.cpp

@@ -120,6 +120,11 @@ namespace mlir {
 /// This class is completely private to this file, so everything is public.
 class MLIRContextImpl {
 public:
+  //===--------------------------------------------------------------------===//
+  // Remark
+  //===--------------------------------------------------------------------===//
+  std::unique_ptr<remark::detail::RemarkEngine> remarkEngine;
+
   //===--------------------------------------------------------------------===//
   // Debugging
   //===--------------------------------------------------------------------===//
@@ -134,11 +139,6 @@ class MLIRContextImpl {
   //===--------------------------------------------------------------------===//
   DiagnosticEngine diagEngine;
 
-  //===--------------------------------------------------------------------===//
-  // Remark
-  //===--------------------------------------------------------------------===//
-  std::unique_ptr<remark::detail::RemarkEngine> remarkEngine;
-
   //===--------------------------------------------------------------------===//
   // Options
   //===--------------------------------------------------------------------===//
@@ -357,7 +357,10 @@ MLIRContext::MLIRContext(const DialectRegistry &registry, Threading setting)
   impl->affineUniquer.registerParametricStorageType<IntegerSetStorage>();
 }
 
-MLIRContext::~MLIRContext() = default;
+MLIRContext::~MLIRContext() {
+  // finalize remark engine before destroying anything else.
+  impl->remarkEngine.reset();
+}
 
 /// Copy the specified array of elements into memory managed by the provided
 /// bump pointer allocator.  This assumes the elements are all PODs.