[Mlir-commits] [mlir] [mlir][Transforms][NFC] `remove-dead-values`: Erase ops at the end (PR #174208)

[Matthias Springer]

https://github.com/matthias-springer created https://github.com/llvm/llvm-project/pull/174208

`remove-dead-values` performs various cleanups:
1. Erasing block arguments
2. Erasing successor operands
3. Erasing operations
4. Erasing function arguments / results
5. Erasing operands
6. Erasing results

This commit moves Step 3 (erasing operations) to the end. While that does not fix any bugs by itself, it is potentially safer. If an operation is erased, we must be careful that the operation is not accessed in the following steps. That can no longer happen if IR is erased only in the final step and not before.

This commit is prefetching a change from #173505 (to keep that PR shorter). With #173505, it will become necessary to erase IR in the final step.


>From 29768f53ce060c32ef084d889ff8f7cb129ce2e6 Mon Sep 17 00:00:00 2001
From: Matthias Springer <me at m-sp.org>
Date: Fri, 2 Jan 2026 13:07:09 +0000
Subject: [PATCH] [mlir][Transforms][NFC] `remove-dead-values`: Erase ops at
 the end

---
 mlir/lib/Transforms/RemoveDeadValues.cpp | 37 ++++++++++++------------
 1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/mlir/lib/Transforms/RemoveDeadValues.cpp b/mlir/lib/Transforms/RemoveDeadValues.cpp
index 07911c6111043..fc2c2acf8afd3 100644
--- a/mlir/lib/Transforms/RemoveDeadValues.cpp
+++ b/mlir/lib/Transforms/RemoveDeadValues.cpp
@@ -805,22 +805,7 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
     }
   }
 
-  // 3. Operations
-  LDBG() << "Cleaning up " << list.operations.size() << " operations";
-  for (Operation *op : list.operations) {
-    LDBG() << "Erasing operation: "
-           << OpWithFlags(op,
-                          OpPrintingFlags().skipRegions().printGenericOpForm());
-    if (op->hasTrait<OpTrait::IsTerminator>()) {
-      // When erasing a terminator, insert an unreachable op in its place.
-      OpBuilder b(op);
-      ub::UnreachableOp::create(b, op->getLoc());
-    }
-    op->dropAllUses();
-    op->erase();
-  }
-
-  // 4. Functions
+  // 3. Functions
   LDBG() << "Cleaning up " << list.functions.size() << " functions";
   // Record which function arguments were erased so we can shrink call-site
   // argument segments for CallOpInterface operations (e.g. ops using
@@ -851,7 +836,7 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
     (void)f.funcOp.eraseResults(f.nonLiveRets);
   }
 
-  // 5. Operands
+  // 4. Operands
   LDBG() << "Cleaning up " << list.operands.size() << " operand lists";
   for (OperandsToCleanup &o : list.operands) {
     // Handle call-specific cleanup only when we have a cached callee reference.
@@ -900,7 +885,7 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
     }
   }
 
-  // 6. Results
+  // 5. Results
   LDBG() << "Cleaning up " << list.results.size() << " result lists";
   for (auto &r : list.results) {
     LDBG_OS([&](raw_ostream &os) {
@@ -912,6 +897,22 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
     });
     dropUsesAndEraseResults(r.op, r.nonLive);
   }
+
+  // 6. Operations
+  LDBG() << "Cleaning up " << list.operations.size() << " operations";
+  for (Operation *op : list.operations) {
+    LDBG() << "Erasing operation: "
+           << OpWithFlags(op,
+                          OpPrintingFlags().skipRegions().printGenericOpForm());
+    if (op->hasTrait<OpTrait::IsTerminator>()) {
+      // When erasing a terminator, insert an unreachable op in its place.
+      OpBuilder b(op);
+      ub::UnreachableOp::create(b, op->getLoc());
+    }
+    op->dropAllUses();
+    op->erase();
+  }
+
   LDBG() << "Finished cleanup of dead values";
 }