diff --git a/llvm/include/llvm/Analysis/ValueTracking.h b/llvm/include/llvm/Analysis/ValueTracking.h
--- a/llvm/include/llvm/Analysis/ValueTracking.h
+++ b/llvm/include/llvm/Analysis/ValueTracking.h
@@ -601,6 +601,9 @@
   /// canCreatePoison returns false.
   bool canCreatePoison(const Instruction *I);
 
+  /// Return true if V is poison given that ValAssumedPoison is already poison.
+  bool impliesPoison(const Value *ValAssumedPoison, const Value *V);
+
   /// Return true if this function can prove that V is never undef value
   /// or poison value.
   //
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
@@ -4677,6 +4677,65 @@
   }
 }
 
+bool llvm::impliesPoison(const Value *ValAssumedPoison, const Value *V) {
+  if (isGuaranteedNotToBeUndefOrPoison(ValAssumedPoison))
+    // The assumption is false, so we can say anything
+    return true;
+
+  // Construct a set of values which are known to be poison from
+  // ValAssumedPoison.
+  SmallSet<const Value *, 4> PoisonValues;
+  PoisonValues.insert(ValAssumedPoison);
+  const Instruction *PoisonI = dyn_cast<Instruction>(ValAssumedPoison);
+  unsigned Depth = 0;
+
+  while (PoisonI && Depth < MaxDepth) {
+    // We'd like to know whether an operand of PoisonI is also poison.
+    if (canCreatePoison(PoisonI))
+      // PoisonI can be a poison-generating instruction, so don't look further
+      break;
+
+    const Value *NextVal = nullptr;
+    bool MoreThanOneCandidate = false;
+    for (auto &Op : PoisonI->operands()) {
+      if (!isGuaranteedNotToBeUndefOrPoison(Op.get())) {
+        // Op can be poison.
+        if (NextVal) {
+          // There is more than one operand that can make PoisonI poison.
+          MoreThanOneCandidate = true;
+          break;
+        }
+        NextVal = Op.get();
+      }
+    }
+
+    if (NextVal == nullptr) {
+      // All operands are non-poison, so PoisonI cannot be poison!
+      return true;
+    } else if (MoreThanOneCandidate)
+      break;
+
+    Depth++;
+    PoisonValues.insert(NextVal);
+    PoisonI = dyn_cast<Instruction>(NextVal);
+  }
+
+  if (PoisonValues.count(V) != 0)
+    // V should be poison.
+    return true;
+
+  // Let's look one level further, by seeing its arguments if I was an
+  // instruction.
+  auto *I = dyn_cast<Instruction>(V);
+  if (I && propagatesFullPoison(I)) {
+    for (auto &Op : I->operands())
+      if (PoisonValues.count(Op.get()))
+        return true;
+  }
+
+  return false;
+}
+
 bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V,
                                             const Instruction *CtxI,
                                             const DominatorTree *DT) {
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
@@ -44,11 +44,13 @@
     if (!F)
       return;
 
-    A = nullptr;
+    A = B = nullptr;
     for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) {
       if (I->hasName()) {
         if (I->getName() == "A")
           A = &*I;
+        else if (I->getName() == "B")
+          B = &*I;
       }
     }
     ASSERT_TRUE(A) << "@test must have an instruction %A";
@@ -57,6 +59,7 @@
   LLVMContext Context;
   std::unique_ptr<Module> M;
   Instruction *A = nullptr;
+  Instruction *B = nullptr;
 };
 
 class MatchSelectPatternTest : public ValueTrackingTest {
@@ -667,6 +670,59 @@
   EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 1u);
 }
 
+TEST_F(ValueTrackingTest, impliesPoisonTest_Identity) {
+  parseAssembly("define void @test(i32 %x, i32 %y) {\n"
+                "  %A = add i32 %x, %y\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(A, A), 1u);
+}
+
+TEST_F(ValueTrackingTest, impliesPoisonTest_ICmp) {
+  parseAssembly("define void @test(i32 %x) {\n"
+                "  %B = icmp eq i32 %x, 0\n"
+                "  %A = icmp eq i32 %x, 1\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(B, A), 1u);
+}
+
+TEST_F(ValueTrackingTest, impliesPoisonTest_ICmpUnknown) {
+  parseAssembly("define void @test(i32 %x, i32 %y) {\n"
+                "  %B = icmp eq i32 %x, %y\n"
+                "  %A = icmp eq i32 %x, 1\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(B, A), 0u);
+}
+
+TEST_F(ValueTrackingTest, impliesPoisonTest_AddNswOkay) {
+  parseAssembly("define void @test(i32 %x) {\n"
+                "  %B = add nsw i32 %x, 1\n"
+                "  %A = add i32 %B, 1\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(B, A), 1u);
+}
+
+TEST_F(ValueTrackingTest, impliesPoisonTest_AddNswOkay2) {
+  parseAssembly("define void @test(i32 %x) {\n"
+                "  %B = add i32 %x, 1\n"
+                "  %A = add nsw i32 %B, 1\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(B, A), 1u);
+}
+
+TEST_F(ValueTrackingTest, impliesPoisonTest_AddNsw) {
+  parseAssembly("define void @test(i32 %x) {\n"
+                "  %B = add nsw i32 %x, 1\n"
+                "  %A = add i32 %x, 1\n"
+                "  ret void\n"
+                "}");
+  EXPECT_EQ(impliesPoison(B, A), 0u);
+}
+
 TEST(ValueTracking, canCreatePoison) {
   std::string AsmHead =
       "declare i32 @g(i32)\n"