Index: include/llvm/Analysis/MemorySSAUpdater.h
===================================================================
--- include/llvm/Analysis/MemorySSAUpdater.h
+++ include/llvm/Analysis/MemorySSAUpdater.h
@@ -99,7 +99,7 @@
   /// load a
   /// Where a mayalias b, *does* require RenameUses be set to true.
   void insertDef(MemoryDef *Def, bool RenameUses = false);
-  void insertUse(MemoryUse *Use);
+  void insertUse(MemoryUse *Use, bool RenameUses = false);
   /// Update the MemoryPhi in `To` following an edge deletion between `From` and
   /// `To`. If `To` becomes unreachable, a call to removeBlocks should be made.
   void removeEdge(BasicBlock *From, BasicBlock *To);
Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -218,7 +218,7 @@
   return recursePhi(Same);
 }
 
-void MemorySSAUpdater::insertUse(MemoryUse *MU) {
+void MemorySSAUpdater::insertUse(MemoryUse *MU, bool RenameUses) {
   InsertedPHIs.clear();
   MU->setDefiningAccess(getPreviousDef(MU));
   // Unlike for defs, there is no extra work to do.  Because uses do not create
@@ -229,6 +229,33 @@
   //
   // 2. There is no def below us, and therefore, there is no extra renaming work
   // to do.
+
+  // Above rule 1. is broken by having unreachable blocks, where the unnecessary
+  // Phis were optimized out. Adding the Use may re-insert those Phis. Hence,
+  // when inserting Uses outside of the MSSA creation process, and new Phis
+  // were added, rename all uses if we are asked. This only happens in the
+  // presence of unreachable blocks.
+  if (RenameUses && InsertedPHIs.size()) {
+    SmallPtrSet<BasicBlock *, 16> Visited;
+    BasicBlock *StartBlock = MU->getBlock();
+
+    if (auto *Defs = MSSA->getWritableBlockDefs(StartBlock)) {
+      MemoryAccess *FirstDef = &*Defs->begin();
+      // Convert to incoming value if it's a memorydef. A phi *is* already an
+      // incoming value.
+      if (auto *MD = dyn_cast<MemoryDef>(FirstDef))
+        FirstDef = MD->getDefiningAccess();
+
+      MSSA->renamePass(MU->getBlock(), FirstDef, Visited);
+      // We just inserted a phi into this block, so the incoming value will
+      // become the phi anyway, so it does not matter what we pass.
+      for (auto &MP : InsertedPHIs) {
+        MemoryPhi *Phi = dyn_cast_or_null<MemoryPhi>(MP);
+        if (Phi)
+          MSSA->renamePass(Phi->getBlock(), nullptr, Visited);
+      }
+    }
+  }
 }
 
 // Set every incoming edge {BB, MP->getBlock()} of MemoryPhi MP to NewDef.
Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -1392,7 +1392,7 @@
         MSSAU->insertDef(MemDef, /*RenameUses=*/true);
       else {
         auto *MemUse = cast<MemoryUse>(NewMemAcc);
-        MSSAU->insertUse(MemUse);
+        MSSAU->insertUse(MemUse, /*RenameUses=*/true);
       }
     }
   }
@@ -2119,9 +2119,11 @@
     PreheaderLoadMemoryAccess = MSSAU->createMemoryAccessInBB(
         PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
     MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
-    MSSAU->insertUse(NewMemUse);
+    MSSAU->insertUse(NewMemUse, /*RenameUses=*/true);
   }
 
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
   // Rewrite all the loads in the loop and remember all the definitions from
   // stores in the loop.
   Promoter.run(LoopUses);
Index: test/Analysis/MemorySSA/PR42940.ll
===================================================================
--- /dev/null
+++ test/Analysis/MemorySSA/PR42940.ll
@@ -0,0 +1,36 @@
+; RUN: opt -licm -enable-mssa-loop-dependency -verify-memoryssa  -S %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_77 = external dso_local global i16, align 2
+
+; CHECK-LABEL: @f1()
+define void @f1() {
+entry:
+  store i16 undef, i16* @g_77, align 2
+  br label %loop_pre
+
+unreachablelabel: ; No predecessors
+  br label %loop_pre
+
+loop_pre:
+  br label %for.cond.header
+
+for.cond.header:
+  store i32 0, i32* undef, align 4
+  br i1 undef, label %for.body, label %for.end
+
+for.body:
+  %tmp1 = load volatile i16, i16* undef, align 2
+  br label %for.end
+
+for.end:
+  br i1 undef, label %func.exit, label %for.cond.header
+
+func.exit:
+  ret void
+
+}
+