[all-commits] [llvm/llvm-project] 88a1f6: [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: 88a1f6bff2329279a08c2304e21c38a4eedb3f6a
      https://github.com/llvm/llvm-project/commit/88a1f6bff2329279a08c2304e21c38a4eedb3f6a
  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 and compare final successors after each
rewrite. Preserve the cache for successor-equivalent changes, added edges,
and block merges that keep surviving blocks reachable. Recompute for other
CFG changes, including entry changes and cross-region block moves.

Keep cache updates separate from reachability queries, returning failure
when a full traversal is required. Collect each successor set once for
validation and retain compact vector snapshots in the cache. Entry-block
queries return before cache lookup or CFG traversal.

Cover inserted and redirected blocks, blocks connected before rewrite
completion, nested regions, block moves and merges, and explicit op lists.
Use observer diagnostics to check whether affected operations are processed
within a single greedy iteration. Document that skipped operations are not
automatically retried if a later rewrite makes their block reachable, and
test that behavior in each strictness mode.

Before the bookkeeping simplifications, 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.

Before those simplifications, 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.

Assisted-by: Codex



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