[JumpThreading] Re-enable JumpThreading for guards

Summary:
JumpThreading for guards feature has been reverted at https://reviews.llvm.org/rL295200
due to the following problem: the feature used the following algorithm for detection of
diamond patters:

1. Find a block with 2 predecessors;
2. Check that these blocks have a common single parent;
3. Check that the parent's terminator is a branch instruction.

The problem is that these checks are insufficient. They may pass for a non-diamond
construction in case if those two predecessors are actually the same block. This may
happen if parent's terminator is a br (either conditional or unconditional) to a block
that ends with "switch" instruction with exactly two branches going to one block.

This patch re-enables the JumpThreading for guards and fixes this issue by adding the
check that those found predecessors are actually different blocks. This guarantees that
parent's terminator is a conditional branch with exactly 2 different successors, which
is now ensured by assertions. It also adds two more tests for this situation (with parent's
terminator being a conditional and an unconditional branch).

Patch by Max Kazantsev!

Reviewers: anna, sanjoy, reames

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30036

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295410 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 47 insertions, 0 deletions

diff --git a/unittests/Transforms/Utils/Cloning.cpp b/unittests/Transforms/Utils/Cloning.cpp
index 634aa9e7e65..a216959751f 100644
--- a/unittests/Transforms/Utils/Cloning.cpp
+++ b/unittests/Transforms/Utils/Cloning.cpp
@@ -201,6 +201,53 @@ TEST_F(CloneInstruction, CallingConvention) {
   delete F2;
 }
 
+TEST_F(CloneInstruction, DuplicateInstructionsToSplit) {
+  Type *ArgTy1[] = {Type::getInt32PtrTy(context)};
+  FunctionType *FT = FunctionType::get(Type::getVoidTy(context), ArgTy1, false);
+  V = new Argument(Type::getInt32Ty(context));
+
+  Function *F = Function::Create(FT, Function::ExternalLinkage);
+
+  BasicBlock *BB1 = BasicBlock::Create(context, "", F);
+  IRBuilder<> Builder1(BB1);
+
+  BasicBlock *BB2 = BasicBlock::Create(context, "", F);
+  IRBuilder<> Builder2(BB2);
+
+  Builder1.CreateBr(BB2);
+
+  Instruction *AddInst = cast<Instruction>(Builder2.CreateAdd(V, V));
+  Instruction *MulInst = cast<Instruction>(Builder2.CreateMul(AddInst, V));
+  Instruction *SubInst = cast<Instruction>(Builder2.CreateSub(MulInst, V));
+  Builder2.CreateRetVoid();
+
+  ValueToValueMapTy Mapping;
+
+  auto Split = DuplicateInstructionsInSplitBetween(BB2, BB1, SubInst, Mapping);
+
+  EXPECT_TRUE(Split);
+  EXPECT_EQ(Mapping.size(), 2u);
+  EXPECT_TRUE(Mapping.find(AddInst) != Mapping.end());
+  EXPECT_TRUE(Mapping.find(MulInst) != Mapping.end());
+
+  auto AddSplit = dyn_cast<Instruction>(Mapping[AddInst]);
+  EXPECT_TRUE(AddSplit);
+  EXPECT_EQ(AddSplit->getOperand(0), V);
+  EXPECT_EQ(AddSplit->getOperand(1), V);
+  EXPECT_EQ(AddSplit->getParent(), Split);
+
+  auto MulSplit = dyn_cast<Instruction>(Mapping[MulInst]);
+  EXPECT_TRUE(MulSplit);
+  EXPECT_EQ(MulSplit->getOperand(0), AddSplit);
+  EXPECT_EQ(MulSplit->getOperand(1), V);
+  EXPECT_EQ(MulSplit->getParent(), Split);
+
+  EXPECT_EQ(AddSplit->getNextNode(), MulSplit);
+  EXPECT_EQ(MulSplit->getNextNode(), Split->getTerminator());
+
+  delete F;
+}
+
 class CloneFunc : public ::testing::Test {
 protected:
   void SetUp() override {