Index: llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
+++ llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp
@@ -156,8 +156,10 @@
     DenseMap<BasicBlock *, TrackingVH<MemoryAccess>> &CachedPreviousDef) {
   auto *Defs = MSSA->getWritableBlockDefs(BB);
 
-  if (Defs)
+  if (Defs) {
+    CachedPreviousDef.insert({BB, &*Defs->rbegin()});
     return &*Defs->rbegin();
+  }
 
   return getPreviousDefRecursive(BB, CachedPreviousDef);
 }
Index: llvm/trunk/test/Analysis/MemorySSA/pr40749_2.ll
===================================================================
--- llvm/trunk/test/Analysis/MemorySSA/pr40749_2.ll
+++ llvm/trunk/test/Analysis/MemorySSA/pr40749_2.ll
@@ -0,0 +1,62 @@
+; RUN: opt -S -licm -loop-unswitch -enable-mssa-loop-dependency -verify-memoryssa %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_92 = external dso_local local_unnamed_addr global i16, align 2
+@g_993 = external dso_local local_unnamed_addr global i32, align 4
+
+; CHECK-LABEL: @ff6
+define dso_local fastcc void @ff6(i16 %arg1) unnamed_addr #0 {
+bb:
+  %tmp6.i = icmp sgt i16 %arg1, 0
+  br label %bb10
+
+bb10:                                             ; preds = %bb81.loopexit, %bb
+  %tmp17 = load i16, i16* @g_92, align 2
+  %tmp18 = add i16 %tmp17, 1
+  store i16 %tmp18, i16* @g_92, align 2
+  br label %bb19
+
+bb19:                                             ; preds = %bb42, %bb10
+  br label %bb24.preheader
+
+bb24.preheader:                                   ; preds = %bb75, %bb19
+  store i32 0, i32* @g_993, align 4
+  br i1 %tmp6.i, label %bb24.preheader.split.us, label %bb24.preheader.split
+
+bb24.preheader.split.us:                          ; preds = %bb24.preheader
+  br label %bb61.us
+
+bb67.us.loopexit:                                 ; preds = %bb65.us
+  br label %bb75
+
+bb61.us:                                          ; preds = %bb65.us, %bb24.preheader.split.us
+  br i1 false, label %bb65.us, label %bb81.loopexit
+
+bb65.us:                                          ; preds = %bb61.us
+  br i1 false, label %bb61.us, label %bb67.us.loopexit
+
+bb24.preheader.split:                             ; preds = %bb24.preheader
+  br label %bb27
+
+bb27:                                             ; preds = %bb24.preheader.split
+  br i1 false, label %bb42, label %bb67
+
+bb42:                                             ; preds = %bb27
+  br label %bb19
+
+bb67:                                             ; preds = %bb27
+  br label %bb75
+
+bb75:                                             ; preds = %bb67, %bb67.us.loopexit
+  br i1 undef, label %bb24.preheader, label %bb84.loopexit
+
+bb81.loopexit:                                    ; preds = %bb61.us
+  br label %bb10
+
+bb84.loopexit:                                    ; preds = %bb75
+  ret void
+}
+
+attributes #0 = { "target-features"="+transactional-execution,+vector" }