Index: llvm/trunk/lib/Analysis/MemorySSA.cpp
===================================================================
--- llvm/trunk/lib/Analysis/MemorySSA.cpp
+++ llvm/trunk/lib/Analysis/MemorySSA.cpp
@@ -1125,6 +1125,8 @@
 void MemorySSA::renamePass(DomTreeNode *Root, MemoryAccess *IncomingVal,
                            SmallPtrSetImpl<BasicBlock *> &Visited,
                            bool SkipVisited, bool RenameAllUses) {
+  assert(Root && "Trying to rename accesses in an unreachable block");
+
   SmallVector<RenamePassData, 32> WorkStack;
   // Skip everything if we already renamed this block and we are skipping.
   // Note: You can't sink this into the if, because we need it to occur
Index: llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
+++ llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
@@ -72,7 +72,10 @@
     // potential phi node. This will insert phi nodes if we cycle in order to
     // break the cycle and have an operand.
     for (auto *Pred : predecessors(BB))
-      PhiOps.push_back(getPreviousDefFromEnd(Pred, CachedPreviousDef));
+      if (MSSA->DT->isReachableFromEntry(Pred))
+        PhiOps.push_back(getPreviousDefFromEnd(Pred, CachedPreviousDef));
+      else
+        PhiOps.push_back(MSSA->getLiveOnEntryDef());
 
     // Now try to simplify the ops to avoid placing a phi.
     // This may return null if we never created a phi yet, that's okay
Index: llvm/trunk/test/Analysis/MemorySSA/pr41640.ll
===================================================================
--- llvm/trunk/test/Analysis/MemorySSA/pr41640.ll
+++ llvm/trunk/test/Analysis/MemorySSA/pr41640.ll
@@ -0,0 +1,103 @@
+; RUN: opt -S -licm -enable-mssa-loop-dependency %s | FileCheck %s
+; REQUIRES: asserts
+target datalayout = "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64"
+target triple = "s390x-ibm-linux"
+
+@g_248 = external dso_local local_unnamed_addr global i32, align 4
+@g_976 = external dso_local global i64, align 8
+@g_1087 = external dso_local global i32**, align 8
+
+; CHECK-LABEL: @f1()
+define dso_local fastcc void @f1() unnamed_addr #0 {
+label0:
+  br i1 undef, label %thread-pre-split.i.preheader, label %label5
+
+thread-pre-split.i.preheader:                     ; preds = %label0
+  br label %thread-pre-split.i
+
+thread-pre-split.i.us:                            ; preds = %.critedge1.i.us
+  br i1 undef, label %.preheader.i.us.preheader, label %label2
+
+.preheader.i.us.preheader:                        ; preds = %thread-pre-split.i.us
+  br label %.preheader.i.us
+
+.preheader.i.us:                                  ; preds = %._crit_edge.i.us, %.preheader.i.us.preheader
+  br i1 undef, label %.lr.ph.i.us, label %._crit_edge.i.us
+
+.lr.ph.i.us:                                      ; preds = %.preheader.i.us
+  br label %label1
+
+label1:                                      ; preds = %label1, %.lr.ph.i.us
+  br i1 undef, label %label1, label %._crit_edge.i.us
+
+._crit_edge.i.us:                                 ; preds = %label1, %.preheader.i.us
+  br i1 undef, label %.preheader.i.us, label %._crit_edge5.i.us
+
+._crit_edge5.i.us:                                ; preds = %._crit_edge.i.us
+  br label %label2
+
+label2:                                  ; preds = %._crit_edge5.i.us, %thread-pre-split.i.us
+  tail call void @foo16()
+  br i1 undef, label %.lr.ph8.i.us.preheader, label %label4
+
+.lr.ph8.i.us.preheader:                           ; preds = %label2
+  br label %.lr.ph8.i.us
+
+.lr.ph8.i.us:                                     ; preds = %.lr.ph8.i.us, %.lr.ph8.i.us.preheader
+  %tmp3 = load volatile i64, i64* @g_976, align 8
+  br i1 undef, label %.lr.ph8.i.us, label %._crit_edge9.i.us
+
+._crit_edge9.i.us:                                ; preds = %.lr.ph8.i.us
+  br label %label4
+
+label4:                                      ; preds = %._crit_edge9.i.us, %label2
+  br i1 true, label %f9.exit, label %.critedge1.i.us
+
+.critedge1.i.us:                                  ; preds = %label4
+  br i1 undef, label %thread-pre-split.i.us, label %f9.exit
+
+label5:                                      ; preds = %label0
+  unreachable
+
+thread-pre-split.i:                               ; preds = %.critedge1.i, %thread-pre-split.i.preheader
+  br i1 undef, label %.preheader.i.preheader, label %.critedge1.i
+
+.preheader.i.preheader:                           ; preds = %thread-pre-split.i
+  br label %.preheader.i
+
+.preheader.i:                                     ; preds = %._crit_edge.i, %.preheader.i.preheader
+  br i1 undef, label %.lr.ph.i, label %._crit_edge.i
+
+.lr.ph.i:                                         ; preds = %.preheader.i
+  br label %label6
+
+label6:                                      ; preds = %label6, %.lr.ph.i
+  br i1 undef, label %label6, label %._crit_edge.i.loopexit
+
+._crit_edge.i.loopexit:                           ; preds = %label6
+  br label %._crit_edge.i
+
+._crit_edge.i:                                    ; preds = %._crit_edge.i.loopexit, %.preheader.i
+  br i1 undef, label %.preheader.i, label %._crit_edge5.i
+
+._crit_edge5.i:                                   ; preds = %._crit_edge.i
+  br label %.critedge1.i
+
+.critedge1.i:                                     ; preds = %._crit_edge5.i, %thread-pre-split.i
+  %tmp7 = load i32, i32* @g_248, align 4
+  %tmp8 = xor i32 %tmp7, 55987
+  store i32 %tmp8, i32* @g_248, align 4
+  br i1 undef, label %thread-pre-split.i, label %f9.exit.loopexit
+
+f9.exit.loopexit:                                 ; preds = %.critedge1.i
+  br label %f9.exit
+
+f9.exit:                                          ; preds = %f9.exit.loopexit, %.critedge1.i.us, %label4
+  %tmp9 = load volatile i32**, i32*** @g_1087, align 8
+  ret void
+}
+
+declare dso_local void @foo16() local_unnamed_addr #1
+
+attributes #0 = { "target-features"="+transactional-execution,+vector" }
+attributes #1 = { "use-soft-float"="false" }