[UnifyLoopExits] Never generate phis of only undef values (#99924)

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

@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes='unify-loop-exits' -S | FileCheck %s
+
+define fastcc void @undef_phi(i64 %i5247, i1 %i4530, i1 %i4936.not) {
+; CHECK-LABEL: define fastcc void @undef_phi(
+; CHECK-SAME: i64 [[I5247:%.*]], i1 [[I4530:%.*]], i1 [[I4936_NOT:%.*]]) {
+; CHECK-NEXT:  [[BB:.*:]]
+; CHECK-NEXT:    br label %[[MBB3932:.*]]
+; CHECK:       [[MBB3932]]:
+; CHECK-NEXT:    br label %[[MBB4454:.*]]
+; CHECK:       [[MBB4321:.*]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[I5247]] to i32
+; CHECK-NEXT:    [[I5290:%.*]] = icmp eq i32 [[TMP0]], 0
+; CHECK-NEXT:    br i1 [[I5290]], label %[[MBB3932]], label %[[LOOP_EXIT_GUARD:.*]]
+; CHECK:       [[MBB4454]]:
+; CHECK-NEXT:    br i1 [[I4530]], label %[[MBB4535:.*]], label %[[LOOP_EXIT_GUARD1:.*]]
+; CHECK:       [[MBB4531:.*]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[MBB4535]]:
+; CHECK-NEXT:    br i1 [[I4936_NOT]], label %[[LOOP_EXIT_GUARD1]], label %[[MBB4454]]
+; CHECK:       [[MBB5291:.*]]:
+; CHECK-NEXT:    [[I5293:%.*]] = insertvalue [2 x i32] zeroinitializer, i32 [[DOTMOVED:%.*]], 1
+; 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]] ], [ 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:    [[GUARD_LOOP_EXIT_GUARD:%.*]] = phi i1 [ true, %[[MBB4454]] ], [ false, %[[MBB4535]] ]
+; CHECK-NEXT:    br i1 [[GUARD_LOOP_EXIT_GUARD]], label %[[LOOP_EXIT_GUARD]], label %[[MBB4321]]
+;
+mbb:
+  br label %mbb3932
+
+mbb3932:                                           ; preds = %mbb4321, %mbb
+  br label %mbb4454
+
+mbb4321:                                           ; preds = %mbb4535
+  %0 = trunc i64 %i5247 to i32
+  %i5290 = icmp eq i32 %0, 0
+  br i1 %i5290, label %mbb3932, label %mbb5291
+
+mbb4454:                                           ; preds = %mbb4535, %mbb3932
+  br i1 %i4530, label %mbb4535, label %mbb4531
+
+mbb4531:                                           ; preds = %mbb4454
+  ret void
+
+mbb4535:                                           ; preds = %mbb4454
+  br i1 %i4936.not, label %mbb4321, label %mbb4454
+
+mbb5291:                                           ; preds = %mbb4321
+  %i5293 = insertvalue [2 x i32] zeroinitializer, i32 %0, 1
+  store volatile [2 x i32] %i5293, ptr addrspace(5) null, align 4
+  ret void
+}