[all-commits] [llvm/llvm-project] d432bb: [mlir] Avoid rewriting unreachable blocks in the g...

[Vijay Kandiah via All-commits]

  Branch: refs/heads/users/mamini/greedy-rewrite-reachability
  Home:   https://github.com/llvm/llvm-project
  Commit: d432bbe14bc40f4b6cb3d39b62981168643655ab
      https://github.com/llvm/llvm-project/commit/d432bbe14bc40f4b6cb3d39b62981168643655ab
  Author: Vijay Kandiah <vkandiah at nvidia.com>
  Date:   2026-09-25 (Fri, 25 Sep 2026)

  Changed paths:
    M mlir/include/mlir/Transforms/GreedyPatternRewriteDriver.h
    M mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp
    M mlir/test/Transforms/canonicalize-dce.mlir
    A mlir/test/Transforms/greedy-cfg-reachability.mlir
    M mlir/test/Transforms/test-strict-pattern-driver.mlir
    M mlir/test/lib/Dialect/Test/TestPatterns.cpp

  Log Message:
  -----------
  [mlir] Avoid rewriting unreachable blocks in the greedy driver

A rewrite can disconnect a block after the iteration's initial CFG sweep.
Track reachability through rewriter notifications and skip worklist operations
in unreachable blocks until the next iteration removes them.

Cache reachability per region. Compare the final successors of changed blocks
after each rewrite, preserving the cache for successor-equivalent changes and
block merges that keep surviving blocks reachable. Recompute for other CFG
changes. This avoids repeated full CFG scans during branch canonicalization.

Single-block regions return before cache lookup or CFG traversal. In the same
Release build, canonicalizing 32,000 sibling single-block regions took 22.8 ms
versus 21.9 ms with the parent driver. With 16,000 regions containing folds,
the times were 35.7 ms and 34.3 ms, respectively. Nesting to 256 levels
showed no measurable difference.

On a synthetic CFG chain whose conditional branches have identical
successors, median CanonicalizerPass times with region simplification
disabled were 0.3, 0.7, 1.3, and 2.5 ms in the parent commit, versus
0.6, 1.2, 2.5, and 5.0 ms with this commit, for 500, 1,000, 2,000,
and 4,000 blocks respectively. Both used the same Release build.

Cover inserted and redirected blocks, blocks connected before rewrite
completion, nested regions, and explicit op-list behavior.

Assisted-by: Codex



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