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
@@ -84,6 +84,17 @@
 static cl::opt<unsigned> DomConditionsMaxUses("dom-conditions-max-uses",
                                               cl::Hidden, cl::init(20));
 
+// According to the LangRef, branching on a poison condition is absolutely
+// immediate full UB.  However, historically we haven't implemented that
+// consistently as we have an important transformation (non-trivial unswitch)
+// which introduces instances of branch on poison/undef to otherwise well
+// defined programs.  This flag exists to let us test optimization benefit
+// of exploiting the specified behavior (in combination with enabling the
+// unswitch fix.)
+static cl::opt<bool> BranchOnPoisonAsUB("branch-on-poison-as-ub",
+                                        cl::Hidden, cl::init(false));
+
+
 /// Returns the bitwidth of the given scalar or pointer type. For vector types,
 /// returns the element type's bitwidth.
 static unsigned getBitWidth(Type *Ty, const DataLayout &DL) {
@@ -5468,7 +5479,16 @@
   case Instruction::SRem:
     Operands.insert(I->getOperand(1));
     break;
-
+  case Instruction::Switch:
+    if (BranchOnPoisonAsUB)
+      Operands.insert(cast<SwitchInst>(I)->getCondition());
+    break;
+  case Instruction::Br: {
+    auto *BR = cast<BranchInst>(I);
+    if (BranchOnPoisonAsUB && BR->isConditional())
+      Operands.insert(BR->getCondition());
+    break;
+  }
   default:
     break;
   }