Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/Loads.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
 #include "llvm/Analysis/VectorUtils.h"
 #include "llvm/IR/CallSite.h"
 #include "llvm/IR/ConstantRange.h"
@@ -793,13 +794,15 @@
   // 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.
-
   if ((KnownZero & KnownOne) != 0) {
     KnownZero.clearAllBits();
     KnownOne.clearAllBits();
+
+    Instruction *CxtI = const_cast<Instruction *>(Q.CxtI);
+    OptimizationRemarkEmitter ORE(CxtI->getFunction());
+    ORE.emit(OptimizationRemarkMissed("value-tracking", "bad assumption", CxtI)
+             << "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 -pass-remarks-missed=.* | FileCheck %s
+
+; Verify that warnings are emitted for the 2nd and 3rd tests.
+
+; CHECK: remark: /tmp/s.c:1:13: Detected conflicting code assumptions. Program has undefined behavior or internal compiler error.
+; CHECK: remark: /tmp/s.c:4:10: Detected conflicting code assumptions. Program has undefined behavior or internal compiler error.
 
 define void @test1() {
 ; CHECK-LABEL: @test1(
@@ -15,12 +20,12 @@
 ; return value. There's no way to win (we can't undo transforms that happened
 ; based on half-truths), so just don't crash.
 
-define i64 @PR31809() {
+define i64 @PR31809() !dbg !7 {
 ; CHECK-LABEL: @PR31809(
 ; CHECK-NEXT:    ret i64 3
 ;
   %a = alloca i32
-  %t1 = ptrtoint i32* %a to i64
+  %t1 = ptrtoint i32* %a to i64, !dbg !9
   %cond = icmp eq i64 %t1, 3
   call void @llvm.assume(i1 %cond)
   ret i64 %t1
@@ -30,14 +35,14 @@
 ; so just don't crash. The second icmp+assume gets processed later, so that
 ; determines the return value.
 
-define i8 @conflicting_assumptions(i8 %x) {
+define i8 @conflicting_assumptions(i8 %x) !dbg !10 {
 ; CHECK-LABEL: @conflicting_assumptions(
 ; CHECK-NEXT:    call void @llvm.assume(i1 false)
 ; CHECK-NEXT:    [[COND2:%.*]] = icmp eq i8 %x, 4
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[COND2]])
 ; CHECK-NEXT:    ret i8 5
 ;
-  %add = add i8 %x, 1
+  %add = add i8 %x, 1, !dbg !11
   %cond1 = icmp eq i8 %x, 3
   call void @llvm.assume(i1 %cond1)
   %cond2 = icmp eq i8 %x, 4
@@ -47,3 +52,21 @@
 
 declare void @llvm.assume(i1) nounwind
 
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 4.0.0 (trunk 282540) (llvm/trunk 282542)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
+!1 = !DIFile(filename: "/tmp/s.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"PIC Level", i32 2}
+!6 = !{!"clang version 4.0.0 (trunk 282540) (llvm/trunk 282542)"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, variables: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 1, column: 13, scope: !7)
+!10 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 3, type: !8, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, unit: !0, variables: !2)
+!11 = !DILocation(line: 4, column: 10, scope: !10)
+!12 = !DILocation(line: 4, column: 3, scope: !10)
+