[MLIR][Remark] Add support for dropping remarks

[grypp]

This PR adds support for dropping remarks emitted by earlier passes. It is a common use case in optimizing compilers where many passes are involved.

For example, consider the following pipeline. In that case, pass-late can drop the failed unrolling remark because it is irrelevant, and emit new remark.

    pass-early : failed unrolling
    ... other passes ...
    pass-late  : succeeded unrolling

To be droppable, a remark must be emitted as a postponed remark. Non-postponed remarks are emitted immediately and cannot be dropped later.

Requires #157434

[Copilot]

Pull Request Overview

This PR adds support for dropping remarks in MLIR's remark system, allowing later passes to remove remarks emitted by earlier passes. This is particularly useful in optimization pipelines where multiple passes operate on the same code and later passes may want to override earlier remarks.

Key changes include:

• Addition of postponed remarks that are deferred until pipeline completion
• API for dropping specific postponed remarks based on location and metadata
• Hash-based storage system for efficient remark lookup and removal

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
mlir/include/mlir/IR/Remarks.h	Adds postponed flag to RemarkOpts, DenseSet storage for postponed remarks, and hash specialization for Remark comparison
mlir/lib/IR/Remarks.cpp	Implements postponed remark storage, emission deferral, and cleanup logic
mlir/lib/IR/MLIRContext.cpp	Ensures remark engine finalization before context destruction
mlir/unittests/IR/RemarkTest.cpp	Adds comprehensive tests for postponed remark functionality and drop behavior

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions cpp,h -- mlir/include/mlir/IR/Remarks.h mlir/lib/IR/MLIRContext.cpp mlir/lib/IR/Remarks.cpp mlir/unittests/IR/RemarkTest.cpp

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View the diff from clang-format here.

diff --git a/mlir/include/mlir/IR/Remarks.h b/mlir/include/mlir/IR/Remarks.h
index c74070122..0c16e741b 100644
--- a/mlir/include/mlir/IR/Remarks.h
+++ b/mlir/include/mlir/IR/Remarks.h
@@ -453,7 +453,9 @@ public:
   InFlightRemark emitOptimizationRemarkAnalysis(Location loc, RemarkOpts opts);
 
   /// Get the postponed remarks.
-  const DenseSet<Remark> &getPostponedRemarks() const { return postponedRemarks; }
+  const DenseSet<Remark> &getPostponedRemarks() const {
+    return postponedRemarks;
+  }
 
   /// Clear the postponed remarks.
   void clearPostponedRemarks() { postponedRemarks.clear(); }