[Dominators] Refine the logic of recalculate() in the DomTreeUpdater

Summary:
This patch refines the logic of `recalculate()` in the `DomTreeUpdater` in the following two aspects:
1. Previously, `recalculate()` tests whether there are pending updates/BBs awaiting deletion and then do recalculation under Lazy UpdateStrategy; and do recalculation immediately under Eager UpdateStrategy. (The former behavior is inherited from the `DeferredDominance` class). This is an inconsistency between two strategies and there is no obvious reason to do this. So the behavior is changed to always recalculate available trees when calling `recalculate()`.
2. Fix the issue of when DTU under Lazy UpdateStrategy holds nothing but with BBs awaiting deletion, after calling `recalculate()`, BBs awaiting deletion aren't flushed. An additional unittest is added to cover this case.

Reviewers: kuhar, dmgreen, brzycki, grosser, davide

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50173

llvm-svn: 338822

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/unittests/IR/DomTreeUpdaterTest.cpp b/llvm/unittests/IR/DomTreeUpdaterTest.cpp
index 03b6b529fd8..db2bb80a79a 100644
--- a/llvm/unittests/IR/DomTreeUpdaterTest.cpp
+++ b/llvm/unittests/IR/DomTreeUpdaterTest.cpp
@@ -699,3 +699,29 @@ TEST(DomTreeUpdater, LazyUpdateStepTest) {
   DTU.flush();
   ASSERT_TRUE(DT.verify());
 }
+
+TEST(DomTreeUpdater, NoTreeTest) {
+  StringRef FuncName = "f";
+  StringRef ModuleString = R"(
+                           define i32 @f() {
+                           bb0:
+                              ret i32 0
+                           }
+                           )";
+  // Make the module.
+  LLVMContext Context;
+  std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
+  Function *F = M->getFunction(FuncName);
+
+  // Make the DTU.
+  DomTreeUpdater DTU(nullptr, nullptr, DomTreeUpdater::UpdateStrategy::Lazy);
+  ASSERT_FALSE(DTU.hasDomTree());
+  ASSERT_FALSE(DTU.hasPostDomTree());
+  Function::iterator FI = F->begin();
+  BasicBlock *BB0 = &*FI++;
+  // Test whether PendingDeletedBB is flushed after the recalculation.
+  DTU.deleteBB(BB0);
+  ASSERT_TRUE(DTU.hasPendingDeletedBB());
+  DTU.recalculate(*F);
+  ASSERT_FALSE(DTU.hasPendingDeletedBB());
+}