diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2967,11 +2967,13 @@
 
       // Take the minimum of all incoming values.  This can't infinitely loop
       // because of our depth threshold.
-      Tmp = ComputeNumSignBits(PN->getIncomingValue(0), Depth + 1, Q);
-      for (unsigned i = 1, e = NumIncomingValues; i != e; ++i) {
+      Query RecQ = Q;
+      Tmp = TyBits;
+      for (unsigned i = 0, e = NumIncomingValues; i != e; ++i) {
         if (Tmp == 1) return Tmp;
+        RecQ.CxtI = PN->getIncomingBlock(i)->getTerminator();
         Tmp = std::min(
-            Tmp, ComputeNumSignBits(PN->getIncomingValue(i), Depth + 1, Q));
+            Tmp, ComputeNumSignBits(PN->getIncomingValue(i), Depth + 1, RecQ));
       }
       return Tmp;
     }
@@ -4930,7 +4932,20 @@
         return true;
     }
 
-    if (!canCreateUndefOrPoison(Opr) && all_of(Opr->operands(), OpCheck))
+    if (const auto *PN = dyn_cast<PHINode>(V)) {
+      unsigned Num = PN->getNumIncomingValues();
+      bool IsWellDefined = true;
+      for (unsigned i = 0; i < Num; ++i) {
+        auto *TI = PN->getIncomingBlock(i)->getTerminator();
+        if (!isGuaranteedNotToBeUndefOrPoison(PN->getIncomingValue(i), TI, DT,
+                                              Depth + 1, PoisonOnly)) {
+          IsWellDefined = false;
+          break;
+        }
+      }
+      if (IsWellDefined)
+        return true;
+    } else if (!canCreateUndefOrPoison(Opr) && all_of(Opr->operands(), OpCheck))
       return true;
   }
 
diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -768,6 +768,30 @@
   }
 }
 
+TEST_F(ValueTrackingTest, isGuaranteedNotToBePoison_phi) {
+  parseAssembly("declare i32 @any_i32(i32)"
+                "define void @test() {\n"
+                "ENTRY:\n"
+                "  br label %LOOP\n"
+                "LOOP:\n"
+                "  %A = phi i32 [0, %ENTRY], [%A.next, %NEXT]\n"
+                "  %A.next = call i32 @any_i32(i32 %A)\n"
+                "  %cond = icmp eq i32 %A.next, 0\n"
+                "  br i1 %cond, label %NEXT, label %EXIT\n"
+                "NEXT:\n"
+                "  br label %LOOP\n"
+                "EXIT:\n"
+                "  ret void\n"
+                "}\n");
+  DominatorTree DT(*F);
+  for (auto &BB : *F) {
+    if (BB.getName() == "LOOP") {
+      EXPECT_EQ(isGuaranteedNotToBePoison(A, A, &DT), true)
+          << "isGuaranteedNotToBePoison does not hold";
+    }
+  }
+}
+
 TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison) {
   parseAssembly("declare void @f(i32 noundef)"
                 "define void @test(i32 %x) {\n"