Add a clear() method on the PassManager (NFC)

This allows to clear an OpPassManager and populated it again with a new
pipeline, while preserving all the other options (including instrumentations).

Differential Revision: https://reviews.llvm.org/D112393

mlir/include/mlir/Pass/PassManager.h

@@ -85,6 +85,9 @@ public:
   /// operation type, it must be the same type as this pass manager.
   void addPass(std::unique_ptr<Pass> pass);
 
+  /// Clear the pipeline, but not the other options set on this OpPassManager.
+  void clear();
+
   /// Add the given pass to a nested pass manager for the given operation kind
   /// `OpT`.
   template <typename OpT> void addNestedPass(std::unique_ptr<Pass> pass) {

mlir/lib/Pass/Pass.cpp

@@ -98,6 +98,10 @@ struct OpPassManagerImpl {
   /// operation type, it must be the same type as this pass manager.
   void addPass(std::unique_ptr<Pass> pass);
 
+  /// Clear the list of passes in this pass manager, other options are
+  /// preserved.
+  void clear();
+
   /// Coalesce adjacent AdaptorPasses into one large adaptor. This runs
   /// recursively through the pipeline graph.
   void coalesceAdjacentAdaptorPasses();
@@ -167,6 +171,8 @@ void OpPassManagerImpl::addPass(std::unique_ptr<Pass> pass) {
   passes.emplace_back(std::move(pass));
 }
 
+void OpPassManagerImpl::clear() { passes.clear(); }
+
 void OpPassManagerImpl::coalesceAdjacentAdaptorPasses() {
   // Bail out early if there are no adaptor passes.
   if (llvm::none_of(passes, [](std::unique_ptr<Pass> &pass) {
@@ -261,6 +267,8 @@ void OpPassManager::addPass(std::unique_ptr<Pass> pass) {
   impl->addPass(std::move(pass));
 }
 
+void OpPassManager::clear() { impl->clear(); }
+
 /// Returns the number of passes held by this manager.
 size_t OpPassManager::size() const { return impl->passes.size(); }
 

mlir/unittests/Pass/PassManagerTest.cpp

@@ -118,6 +118,11 @@ TEST(PassManagerTest, InvalidPass) {
       diagnostic->str(),
       "'invalid_op' op trying to schedule a pass on an unregistered operation");
 
+  // Check that clearing the pass manager effectively removed the pass.
+  pm.clear();
+  result = pm.run(module.get());
+  EXPECT_TRUE(succeeded(result));
+
   // Check that adding the pass at the top-level triggers a fatal error.
   ASSERT_DEATH(pm.addPass(std::make_unique<InvalidPass>()), "");
 }