[Mlir-commits] [mlir] [mlir][Transforms] Add dead code elimination pass (PR #106258)

[Mehdi Amini]

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@@ -0,0 +1,75 @@
+//===- DeadCodeElimination.cpp - Dead Code Elimination --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/IR/Iterators.h"
+#include "mlir/IR/Operation.h"
+#include "mlir/Interfaces/SideEffectInterfaces.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/Passes.h"
+
+namespace mlir {
+#define GEN_PASS_DEF_DEADCODEELIMINATION
+#include "mlir/Transforms/Passes.h.inc"
+} // namespace mlir
+
+using namespace mlir;
+
+namespace {
+struct DeadCodeElimination
+    : public impl::DeadCodeEliminationBase<DeadCodeElimination> {
+  void runOnOperation() override;
+};
+} // namespace
+
+void DeadCodeElimination::runOnOperation() {
+  Operation *topLevel = getOperation();
+
+  // Visit operations in reverse dominance order. This visits all users before
+  // their definitions. (Also takes into account unstructured control flow
+  // between blocks.)
+  topLevel->walk<WalkOrder::PostOrder,
+                 ReverseDominanceIterator</*NoGraphRegions=*/false>>(
+      [&](Operation *op) {
+        // Do not remove the top-level op.
+        if (op == topLevel)
+          return WalkResult::advance();
+
+        // Do not remove ops from regions that may be graph regions.
+        if (mayBeGraphRegion(*op->getParentRegion()))
+          return WalkResult::advance();
+
+        // Remove dead ops.
+        if (isOpTriviallyDead(op)) {
+          op->erase();
+          return WalkResult::skip();
+        }
+
+        return WalkResult::advance();
+      });
+
+  // ReverseDominanceIterator does not visit unreachable blocks. Erase those in
+  // a second walk. First collect all reachable blocks.
+  // TODO: Extend walker API to provide a callback for both ops and blocks, so
+  // that reachable blocks can be collected in the same walk.
+  DenseSet<Block *> reachableBlocks;
+  topLevel->walk<WalkOrder::PostOrder,
+                 ForwardDominanceIterator</*NoGraphRegions=*/false>>(
+      [&](Block *block) { reachableBlocks.insert(block); });
+  // Erase all blocks that were not visited. These are unreachable and thus
+  // dead.
+  topLevel->walk<WalkOrder::PostOrder>([&](Block *block) {
+    if (!reachableBlocks.contains(block)) {
+      block->dropAllDefinedValueUses();
+      block->erase();
+    }
+  });
+}
+
+std::unique_ptr<Pass> mlir::createDeadCodeEliminationPass() {
----------------
joker-eph wrote:

This should be auto-generated if you remove the constructor field from the .td file.

https://github.com/llvm/llvm-project/pull/106258