[Local] Zap blockaddress without users in ConstantFoldTerminator.

If the blockaddress is not destoryed, the destination block will still
be marked as having its address taken, limiting further transformations.

I think there are other places where the dead blockaddress constants are kept
around, I'll look into that as follow up.

Reviewers: craig.topper, brzycki, davide

Reviewed By: brzycki, davide

Differential Revision: https://reviews.llvm.org/D64936

llvm-svn: 366633

llvm/lib/Transforms/Utils/Local.cpp

@@ -324,8 +324,14 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
       Value *Address = IBI->getAddress();
       IBI->eraseFromParent();
       if (DeleteDeadConditions)
+        // Delete pointer cast instructions.
         RecursivelyDeleteTriviallyDeadInstructions(Address, TLI);
 
+      // Also zap the blockaddress constant if there are no users remaining,
+      // otherwise the destination is still marked as having its address taken.
+      if (BA->use_empty())
+        BA->destroyConstant();
+
       // If we didn't find our destination in the IBI successor list, then we
       // have undefined behavior.  Replace the unconditional branch with an
       // 'unreachable' instruction.

llvm/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]