Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -25,6 +25,7 @@
 #include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/GlobalAlias.h"
@@ -791,19 +792,16 @@
 
   // If assumptions conflict with each other or previous known bits, then we
   // have a logical fallacy. This should only happen when a program has
-  // undefined behavior. We can't assert/crash, so clear out the known bits and
-  // hope for the best.
-
-  // FIXME: Publish a warning/remark that we have encountered UB or the compiler
-  // is broken.
-
-  // FIXME: Implement a stronger version of "I give up" by invalidating/clearing
-  // the assumption cache. This should indicate that the cache is corrupted so
-  // future callers will not waste time repopulating it with faulty assumptions.
-
+  // undefined behavior. We can't assert/crash, so clear out the known bits,
+  // warn the user, and hope for the best.
   if ((KnownZero & KnownOne) != 0) {
     KnownZero.clearAllBits();
     KnownOne.clearAllBits();
+
+    Q.CxtI->getContext().diagnose(DiagnosticInfoOptimizationFailure(
+        *Q.CxtI->getFunction(), Q.CxtI->getDebugLoc(),
+        "Detected conflicting code assumptions. Program has undefined behavior "
+        "or internal compiler error."));
   }
 }
 
Index: test/Transforms/InstSimplify/assume.ll
===================================================================
--- test/Transforms/InstSimplify/assume.ll
+++ test/Transforms/InstSimplify/assume.ll
@@ -1,5 +1,10 @@
 ; NOTE: Assertions have been autogenerated by update_test_checks.py
-; RUN: opt -instsimplify -S < %s | FileCheck %s
+; RUN: opt -instsimplify -S < %s 2>&1 | FileCheck %s
+
+; Verify that warnings are emitted for the 2nd and 3rd tests.
+
+; CHECK: warning: <unknown>:0:0: Detected conflicting code assumptions.
+; CHECK: warning: <unknown>:0:0: Detected conflicting code assumptions.
 
 define void @test1() {
 ; CHECK-LABEL: @test1(
@@ -28,8 +33,7 @@
 
 ; Similar to above: there's no way to know which assumption is truthful,
 ; so just don't crash. The second icmp+assume gets processed later, so that
-; determines the return value. This can be improved by permanently invalidating
-; the cached assumptions for this function. 
+; determines the return value.
 
 define i8 @conflicting_assumptions(i8 %x) {
 ; CHECK-LABEL: @conflicting_assumptions(