Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

Summary:
Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

This is a missing small optimization in MergeBlockIntoPredecessor.

This helps with one simplifycfg test which expects this case to be handled.

Reviewers: davide, spatel, brzycki, asbirlea

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48284

llvm-svn: 334992

lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -119,7 +119,14 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DominatorTree *DT,
                                      LoopInfo *LI,
                                      MemoryDependenceResults *MemDep) {
   // Don't merge away blocks who have their address taken.
-  if (BB->hasAddressTaken()) return false;
+  if (BB->hasAddressTaken()) {
+    // If the block has its address taken, it may be a tree of dead constants
+    // hanging off of it.  These shouldn't keep the block alive.
+    BlockAddress *BA = BlockAddress::get(BB);
+    BA->removeDeadConstantUsers();
+    if (!BA->use_empty())
+      return false;
+  }
 
   // Can't merge if there are multiple predecessors, or no predecessors.
   BasicBlock *PredBB = BB->getUniquePredecessor();

test/Transforms/SimplifyCFG/dce-cond-after-folding-terminator.ll

@@ -37,10 +37,7 @@ define void @test_indirectbr(i32 %x) {
 entry:
 ; CHECK-LABEL: @test_indirectbr(
 ; CHECK-NEXT: entry:
-; Ideally this should now check:
-;   CHK-NEXT: ret void
-; But that doesn't happen yet. Instead:
-; CHECK-NEXT: br label %L1
+; CHECK-NEXT: ret void
 
   %label = bitcast i8* blockaddress(@test_indirectbr, %L1) to i8*
   indirectbr i8* %label, [label %L1, label %L2]

unittests/Transforms/Utils/BasicBlockUtils.cpp

@@ -50,3 +50,31 @@ TEST(BasicBlockUtils, SplitBlockPredecessors) {
   SplitBlockPredecessors(&F->getEntryBlock(), {}, "split.entry", &DT);
   EXPECT_TRUE(DT.verify());
 }
+
+TEST(BasicBlockUtils, MergeBlockIntoPredecessor) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C,
+                                      R"(
+
+      define i32 @f(i8* %str) {
+      entry:
+        %dead = extractvalue [1 x i8*] [ i8* blockaddress(@f, %L0) ], 0
+        br label %L0
+      L0:
+        ret i32 0
+      }
+      )");
+
+  // First remove the dead instruction to empty the usage of the constant
+  // containing blockaddress(@f, %L0)
+  Function *F = M->getFunction("f");
+  auto BBI = F->begin();
+  Instruction *DI = &*((*BBI).begin());
+  EXPECT_TRUE(DI->use_empty());
+  DI->eraseFromParent();
+
+  // Get L0 and make sure that it can be merged into entry block.
+  ++BBI;
+  BasicBlock *BB = &(*BBI);
+  EXPECT_TRUE(MergeBlockIntoPredecessor(BB));
+}