From d8007008a80b4128fdd0a09b7b533c5c7b3eb8ab Mon Sep 17 00:00:00 2001
From: Simon Hofmann <simon.t.hofmann@tum.de>
Date: Tue, 1 Sep 2026 03:11:19 +0200
Subject: [PATCH] =?UTF-8?q?=F0=9F=90=9B=20Reject=20nonlinear=20Hadamard=20?=
 =?UTF-8?q?lifting?=
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Validate quantum SSA linearity before applying greedy lifting patterns, leaving invalid input untouched.

Assisted-by: GPT-5.6 via Codex
---
 .../Optimizations/HadamardLifting.cpp         |  5 ++
 .../test_qco_hadamard_lifting.cpp             | 53 +++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/mlir/lib/Dialect/QCO/Transforms/Optimizations/HadamardLifting.cpp b/mlir/lib/Dialect/QCO/Transforms/Optimizations/HadamardLifting.cpp
index 49ac6f7a9b..0a24b7c621 100644
--- a/mlir/lib/Dialect/QCO/Transforms/Optimizations/HadamardLifting.cpp
+++ b/mlir/lib/Dialect/QCO/Transforms/Optimizations/HadamardLifting.cpp
@@ -12,6 +12,7 @@
 #include "mlir/Dialect/MQT/Utils/Modifiers.h"
 #include "mlir/Dialect/QCO/IR/QCOInterfaces.h"
 #include "mlir/Dialect/QCO/IR/QCOOps.h"
+#include "mlir/Dialect/QCO/QCOUtils.h"
 #include "mlir/Dialect/QCO/Transforms/Passes.h"
 
 #include <llvm/ADT/STLExtras.h>
@@ -228,6 +229,10 @@ struct HadamardLifting final : impl::HadamardLiftingBase<HadamardLifting> {
   void runOnOperation() override {
     auto op = getOperation();
     auto* ctx = &getContext();
+    if (failed(qco::verifyLinearity(op))) {
+      signalPassFailure();
+      return;
+    }
 
     // Define the set of patterns to use.
     RewritePatternSet patterns(ctx);
diff --git a/mlir/unittests/Dialect/QCO/Transforms/Optimizations/test_qco_hadamard_lifting.cpp b/mlir/unittests/Dialect/QCO/Transforms/Optimizations/test_qco_hadamard_lifting.cpp
index 56e38eff1c..214cf35ce8 100644
--- a/mlir/unittests/Dialect/QCO/Transforms/Optimizations/test_qco_hadamard_lifting.cpp
+++ b/mlir/unittests/Dialect/QCO/Transforms/Optimizations/test_qco_hadamard_lifting.cpp
@@ -20,8 +20,11 @@
 #include <mlir/Dialect/MemRef/IR/MemRef.h>
 #include <mlir/IR/BuiltinOps.h>
 #include <mlir/IR/DialectRegistry.h>
+#include <mlir/IR/OperationSupport.h>
 #include <mlir/IR/OwningOpRef.h>
 #include <mlir/IR/Value.h>
+#include <mlir/IR/Verifier.h>
+#include <mlir/Parser/Parser.h>
 #include <mlir/Pass/PassManager.h>
 #include <mlir/Support/LLVM.h>
 #include <mlir/Support/LogicalResult.h>
@@ -81,6 +84,56 @@ class QCOHadamardLiftingTest : public testing::Test {
 
 } // namespace
 
+TEST_F(QCOHadamardLiftingTest, HandlesUnusedPauliOutput) {
+  auto input = parseSourceString<ModuleOp>(R"mlir(
+    module {
+      func.func @main() {
+        %q = qco.static 0 : !qco.qubit
+        %unused = qco.x %q : !qco.qubit -> !qco.qubit
+        return
+      }
+    }
+  )mlir",
+                                           &context);
+  ASSERT_TRUE(input);
+  ASSERT_TRUE(succeeded(verify(*input)));
+  OwningOpRef<ModuleOp> original(input->clone());
+
+  EXPECT_TRUE(failed(runHadamardLiftingPass(*input)));
+  EXPECT_TRUE(OperationEquivalence::isEquivalentTo(
+      input->getOperation(), original->getOperation(),
+      OperationEquivalence::Flags::None));
+}
+
+TEST_F(QCOHadamardLiftingTest, HandlesUnusedCnotControlOutput) {
+  auto input = parseSourceString<ModuleOp>(R"mlir(
+    module {
+      func.func @main() {
+        %control = qco.static 0 : !qco.qubit
+        %target = qco.static 1 : !qco.qubit
+        %unused, %target_out = qco.ctrl(%control) targets(%arg = %target) {
+          %body = qco.x %arg : !qco.qubit -> !qco.qubit
+          qco.yield %body : !qco.qubit
+        } : ({!qco.qubit}, {!qco.qubit})
+          -> ({!qco.qubit}, {!qco.qubit})
+        %hadamard = qco.h %target_out : !qco.qubit -> !qco.qubit
+        %measured, %result = qco.measure %hadamard : !qco.qubit
+        qco.sink %measured : !qco.qubit
+        return
+      }
+    }
+  )mlir",
+                                           &context);
+  ASSERT_TRUE(input);
+  ASSERT_TRUE(succeeded(verify(*input)));
+  OwningOpRef<ModuleOp> original(input->clone());
+
+  EXPECT_TRUE(failed(runHadamardLiftingPass(*input)));
+  EXPECT_TRUE(OperationEquivalence::isEquivalentTo(
+      input->getOperation(), original->getOperation(),
+      OperationEquivalence::Flags::None));
+}
+
 // ##################################################
 // # Raise Hadamard over uncontrolled Pauli gate Tests
 // ##################################################