[Mlir-commits] [mlir] [mlir] Remove redundant DCE worklist visited set (PR #195662)

[Mehdi Amini]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/195662

The eliminateTriviallyDeadOps worklist only enqueues operations after checking that they are trivially dead. Dropping an operand before testing the defining operation means a propagated enqueue happens only when that defining operation has no remaining users.

During the simplification in #194041 I didn't simplify it far enough to actually entirely remove the visited set, even though it isn't useful to the algorithm right now.

Assisted-by: Codex

>From c54b6e65727410c002aea5f4e9f8c4efc1ea7db9 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Mon, 4 May 2026 07:49:29 -0700
Subject: [PATCH] [mlir] Remove redundant DCE worklist visited set

The eliminateTriviallyDeadOps worklist only enqueues operations after checking
that they are trivially dead. Dropping an operand before testing the defining
operation means a propagated enqueue happens only when that defining operation
has no remaining users.

The visited set was only populated by the initial seed path, so it did not
actually provide deduplication for propagated defining operations. Remove the
set and the stale skip branch to keep the worklist contract explicit.

Assisted-by: Codex
---
 mlir/lib/Transforms/Utils/RegionUtils.cpp | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/mlir/lib/Transforms/Utils/RegionUtils.cpp b/mlir/lib/Transforms/Utils/RegionUtils.cpp
index cee48b0b6b126..ae3ea83758dab 100644
--- a/mlir/lib/Transforms/Utils/RegionUtils.cpp
+++ b/mlir/lib/Transforms/Utils/RegionUtils.cpp
@@ -564,8 +564,8 @@ bool mlir::eliminateTriviallyDeadOps(RewriterBase &rewriter, Region &region,
 
   // Step 2: worklist over ops in this region only.
   //
-  // Worklist invariant: an op is pushed *only* once we have verified it is
-  // trivially dead. No speculative enqueues — every op on the worklist will
+  // Worklist invariant: an op is pushed only once we have verified it is
+  // trivially dead. No speculative enqueues: every op on the worklist will
   // be erased when popped. Two things enforce this:
   //   - the initial seed below calls isOpTriviallyDead before enqueueing,
   //   - the propagation inside the loop drops the erasing op's use of
@@ -573,13 +573,11 @@ bool mlir::eliminateTriviallyDeadOps(RewriterBase &rewriter, Region &region,
   //     sees the post-erase use count and only enqueues when actually dead.
   // Deadness is monotonic within this pass (we never add users, only remove
   // them), so an op that was dead at enqueue time is still dead at pop time.
-  // The `visited` set is just for dedup; no re-check is needed on pop.
   SmallVector<Operation *> worklist;
-  DenseSet<Operation *> visited;
 
   LDBG(2) << "Stage 2: Seeding trivially dead operation worklist";
   for (Operation &op : region.getOps()) {
-    if (isOpTriviallyDead(&op) && visited.insert(&op).second) {
+    if (isOpTriviallyDead(&op)) {
       LDBG(2) << "Seeded worklist with operation: "
               << OpWithFlags(&op, OpPrintingFlags().skipRegions());
       worklist.push_back(&op);
@@ -616,11 +614,6 @@ bool mlir::eliminateTriviallyDeadOps(RewriterBase &rewriter, Region &region,
                   << ": defining operation is outside the current region";
           continue;
         }
-        if (visited.count(defOp)) {
-          LDBG(4) << "Skipping operand #" << opOperand.getOperandNumber()
-                  << ": defining operation was already visited";
-          continue;
-        }
         LDBG(4) << "Dropping operand #" << opOperand.getOperandNumber()
                 << " from defining operation: "
                 << OpWithFlags(defOp, OpPrintingFlags().skipRegions());