[BasicBlockUtils] Never emit PHIs that are known to be undef

There are (rare) cases we used to emit a PHI node with only `undef`
inputs, that's not helpful.

Author: jdoerfert

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -1909,22 +1909,29 @@ static void reconnectPhis(BasicBlock *Out, BasicBlock *GuardBlock,
     auto NewPhi =
         PHINode::Create(Phi->getType(), Incoming.size(),
                         Phi->getName() + ".moved", FirstGuardBlock->begin());
+    bool AllUndef = true;
     for (auto *In : Incoming) {
       Value *V = UndefValue::get(Phi->getType());
       if (In == Out) {
         V = NewPhi;
       } else if (Phi->getBasicBlockIndex(In) != -1) {
         V = Phi->removeIncomingValue(In, false);
+        AllUndef &= isa<UndefValue>(V);
       }
       NewPhi->addIncoming(V, In);
     }
     assert(NewPhi->getNumIncomingValues() == Incoming.size());
+    Value *NewV = NewPhi;
+    if (AllUndef) {
+      NewPhi->eraseFromParent();
+      NewV = UndefValue::get(Phi->getType());
+    }
     if (Phi->getNumOperands() == 0) {
-      Phi->replaceAllUsesWith(NewPhi);
+      Phi->replaceAllUsesWith(NewV);
       I = Phi->eraseFromParent();
       continue;
     }
-    Phi->addIncoming(NewPhi, GuardBlock);
+    Phi->addIncoming(NewV, GuardBlock);
     ++I;
   }
 }

llvm/test/Transforms/UnifyLoopExits/undef-phis.ll

@@ -23,12 +23,11 @@ define fastcc void @undef_phi(i64 %i5247, i1 %i4530, i1 %i4936.not) {
 ; CHECK-NEXT:    store volatile [2 x i32] [[I5293]], ptr addrspace(5) null, align 4
 ; CHECK-NEXT:    ret void
 ; CHECK:       [[LOOP_EXIT_GUARD]]:
-; CHECK-NEXT:    [[DOTMOVED]] = phi i32 [ [[TMP0]], %[[MBB4321]] ], [ [[DOTMOVED_MOVED:%.*]], %[[LOOP_EXIT_GUARD1]] ]
+; CHECK-NEXT:    [[DOTMOVED]] = phi i32 [ [[TMP0]], %[[MBB4321]] ], [ undef, %[[LOOP_EXIT_GUARD1]] ]
 ; CHECK-NEXT:    [[GUARD_MBB4531:%.*]] = phi i1 [ false, %[[MBB4321]] ], [ [[GUARD_MBB4531_MOVED:%.*]], %[[LOOP_EXIT_GUARD1]] ]
 ; CHECK-NEXT:    br i1 [[GUARD_MBB4531]], label %[[MBB4531]], label %[[MBB5291]]
 ; CHECK:       [[LOOP_EXIT_GUARD1]]:
 ; CHECK-NEXT:    [[GUARD_MBB4531_MOVED]] = phi i1 [ true, %[[MBB4454]] ], [ undef, %[[MBB4535]] ]
-; CHECK-NEXT:    [[DOTMOVED_MOVED]] = phi i32 [ poison, %[[MBB4454]] ], [ undef, %[[MBB4535]] ]
 ; CHECK-NEXT:    [[GUARD_LOOP_EXIT_GUARD:%.*]] = phi i1 [ true, %[[MBB4454]] ], [ false, %[[MBB4535]] ]
 ; CHECK-NEXT:    br i1 [[GUARD_LOOP_EXIT_GUARD]], label %[[LOOP_EXIT_GUARD]], label %[[MBB4321]]
 ;