Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

[trentxintong]

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.

[asbirlea]

Thank you for adding this!

unittests/Transforms/Utils/BasicBlockUtils.cpp

80

MergeBlockIntoPredecessor(BB) The other arguments are nullptr by default.

[trentxintong]

Address @asbirlea comments

lib/Transforms/Utils/BasicBlockUtils.cpp

bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI) {
 
   return Changed;
 }
 
 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();
   if (!PredBB) return false;
 
   // Don't break self-loops.
   if (PredBB == BB) return false;
   // Don't break unwinding instructions.

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

 sw.epilog:                                        ; preds = %sw.bb
   ret void
 }
 
 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]
 
 L1:                                               ; preds = %entry
   ret void
 L2:                                               ; preds = %entry
   ret void
 }

unittests/Transforms/Utils/BasicBlockUtils.cpp

TEST(BasicBlockUtils, SplitBlockPredecessors) {
   auto *F = M->getFunction("basic_func");
   DominatorTree DT(*F);
 
   // Make sure the dominator tree is properly updated if calling this on the
   // entry block.
   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, nullptr, nullptr, nullptr));
+}