Index: llvm/lib/Transforms/Scalar/Sink.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/Sink.cpp
+++ llvm/lib/Transforms/Scalar/Sink.cpp
@@ -135,15 +135,21 @@
   for (Use &U : Inst->uses()) {
     Instruction *UseInst = cast<Instruction>(U.getUser());
     BasicBlock *UseBlock = UseInst->getParent();
+    // Don't worry about dead users.
+    if (!DT.isReachableFromEntry(UseBlock))
+      continue;
     if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
       // PHI nodes use the operand in the predecessor block, not the block with
       // the PHI.
       unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo());
       UseBlock = PN->getIncomingBlock(Num);
     }
-    if (SuccToSinkTo)
+    if (SuccToSinkTo) {
       SuccToSinkTo = DT.findNearestCommonDominator(SuccToSinkTo, UseBlock);
-    else
+      // findNearestCommonDominator may fail for some reasons.
+      if (!SuccToSinkTo)
+        return false;
+    } else
       SuccToSinkTo = UseBlock;
     // The current basic block needs to dominate the candidate.
     if (!DT.dominates(BB, SuccToSinkTo))
Index: llvm/test/Transforms/Sink/dead-user.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/Sink/dead-user.ll
@@ -0,0 +1,42 @@
+; Compiler should not be broken with a dead user.
+; RUN: opt -sink -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test(
+; CHECK:       bb.0:
+; CHECK-NEXT:    %conv = sext i16 %p1 to i32
+; CHECK-NEXT:    br i1 undef, label %bb.1, label %bb.3
+
+; CHECK:       bb.1:                                             ; preds = %bb.0
+; CHECK-NEXT:    br label %bb.2
+
+; CHECK:       bb.2:                                             ; preds = %bb.2, %bb.1
+; CHECK-NEXT:    %and.2 = and i32 undef, %conv
+; CHECK-NEXT:    br label %bb.2
+
+; CHECK:       bb.3:                                             ; preds = %bb.3, %bb.0
+; CHECK-NEXT:    %and.3 = and i32 undef, %conv
+; CHECK-NEXT:    br label %bb.3
+
+; CHECK:       dead:                                             ; preds = %dead
+; CHECK-NEXT:    %and.dead = and i32 undef, %conv
+; CHECK-NEXT:    br label %dead
+define void @test(i16 %p1) {
+bb.0:
+  %conv = sext i16 %p1 to i32
+  br i1 undef, label %bb.1, label %bb.3
+
+bb.1:                                             ; preds = %bb.0
+  br label %bb.2
+
+bb.2:                                             ; preds = %bb.2, %bb.1
+  %and.2 = and i32 undef, %conv
+  br label %bb.2
+
+bb.3:                                             ; preds = %bb.3, %bb.0
+  %and.3 = and i32 undef, %conv
+  br label %bb.3
+
+dead:                                             ; preds = %dead
+  %and.dead = and i32 undef, %conv
+  br label %dead
+}