[Utils] Extract EliminateUnreachableBlocks (NFC)

Summary:
Extract the functionality of eliminating unreachable basic blocks
within a function, previously encapsulated within the
-unreachableblockelim pass, and make it available as a function within
BlockUtils.h. No functional change intended other than making the logic
reusable.

Exposing this logic makes it easier to implement
https://reviews.llvm.org/D59068, which fixes coroutines bug
https://bugs.llvm.org/show_bug.cgi?id=40979.

Reviewers: mkazantsev, wmi, davidxl, silvas, davide

Reviewed By: davide

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59069

llvm-svn: 355846

1 files changed, 58 insertions, 0 deletions

diff --git a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
index 2d3731c03e0..bf8228e99d8 100644
--- a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
@@ -25,6 +25,64 @@ static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
   return Mod;
 }
 
+TEST(BasicBlockUtils, EliminateUnreachableBlocks) {
+  LLVMContext C;
+
+  std::unique_ptr<Module> M = parseIR(
+    C,
+    "define i32 @has_unreachable(i1 %cond) {\n"
+    "entry:\n"
+    "  br i1 %cond, label %bb0, label %bb1\n"
+    "bb0:\n"
+    "  br label %bb1\n"
+    "bb1:\n"
+    "  %phi = phi i32 [ 0, %entry ], [ 1, %bb0 ]"
+    "  ret i32 %phi\n"
+    "bb2:\n"
+    "  ret i32 42\n"
+    "}\n"
+    "\n"
+    );
+
+  auto *F = M->getFunction("has_unreachable");
+  DominatorTree DT(*F);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+
+  EXPECT_EQ(F->size(), (size_t)4);
+  bool Result = EliminateUnreachableBlocks(*F, &DTU);
+  EXPECT_TRUE(Result);
+  EXPECT_EQ(F->size(), (size_t)3);
+  EXPECT_TRUE(DT.verify());
+}
+
+TEST(BasicBlockUtils, NoUnreachableBlocksToEliminate) {
+  LLVMContext C;
+
+  std::unique_ptr<Module> M = parseIR(
+    C,
+    "define i32 @no_unreachable(i1 %cond) {\n"
+    "entry:\n"
+    "  br i1 %cond, label %bb0, label %bb1\n"
+    "bb0:\n"
+    "  br label %bb1\n"
+    "bb1:\n"
+    "  %phi = phi i32 [ 0, %entry ], [ 1, %bb0 ]"
+    "  ret i32 %phi\n"
+    "}\n"
+    "\n"
+    );
+
+  auto *F = M->getFunction("no_unreachable");
+  DominatorTree DT(*F);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+
+  EXPECT_EQ(F->size(), (size_t)3);
+  bool Result = EliminateUnreachableBlocks(*F, &DTU);
+  EXPECT_FALSE(Result);
+  EXPECT_EQ(F->size(), (size_t)3);
+  EXPECT_TRUE(DT.verify());
+}
+
 TEST(BasicBlockUtils, SplitBlockPredecessors) {
   LLVMContext C;