Index: include/llvm/Transforms/Utils/Local.h
===================================================================
--- include/llvm/Transforms/Utils/Local.h
+++ include/llvm/Transforms/Utils/Local.h
@@ -16,6 +16,7 @@
 #define LLVM_TRANSFORMS_UTILS_LOCAL_H
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/TinyPtrVector.h"
@@ -333,6 +334,16 @@
 /// DIExpression.
 void salvageDebugInfo(Instruction &I);
 
+/// Assuming the instruction \p From is going to be deleted, insert replacement
+/// dbg.value intrinsics for each debug user of \p From. The newly-inserted
+/// dbg.values refer to \p To instead of \p From. Each replacement dbg.value
+/// has the same location and variable as the debug user it replaces, has a
+/// DIExpression determined by the result of \p RewriteExpr applied to an old
+/// debug user of \p From, and is placed before \p InsertBefore.
+void insertReplacementDbgValues(
+    Instruction &From, Instruction &To, Instruction &InsertBefore,
+    function_ref<DIExpression *(DbgInfoIntrinsic &OldDII)> RewriteExpr);
+
 /// Remove all instructions from a basic block other than it's terminator
 /// and any present EH pad instructions.
 unsigned removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB);
Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -268,17 +268,12 @@
       // the second cast (CI). CSrc will then have a good chance of being dead.
       auto *Res = CastInst::Create(NewOpc, CSrc->getOperand(0), CI.getType());
 
-      // If the eliminable cast has debug users, insert a debug value after the
-      // cast pointing to the new Value.
-      SmallVector<DbgInfoIntrinsic *, 1> CSrcDbgInsts;
-      findDbgUsers(CSrcDbgInsts, CSrc);
-      if (CSrcDbgInsts.size()) {
-        DIBuilder DIB(*CI.getModule());
-        for (auto *DII : CSrcDbgInsts)
-          DIB.insertDbgValueIntrinsic(
-              Res, DII->getVariable(), DII->getExpression(),
-              DII->getDebugLoc().get(), &*std::next(CI.getIterator()));
-      }
+      // Replace debug users of the eliminable cast by emitting debug values
+      // which refer to the new cast.
+      insertReplacementDbgValues(
+          *CSrc, *Res, *std::next(CI.getIterator()),
+          [](DbgInfoIntrinsic &OldDII) { return OldDII.getExpression(); });
+
       return Res;
     }
   }
Index: lib/Transforms/Utils/Local.cpp
===================================================================
--- lib/Transforms/Utils/Local.cpp
+++ lib/Transforms/Utils/Local.cpp
@@ -1671,6 +1671,24 @@
   }
 }
 
+void llvm::insertReplacementDbgValues(
+    Instruction &From, Instruction &To, Instruction &InsertBefore,
+    function_ref<DIExpression *(DbgInfoIntrinsic &OldDII)> RewriteExpr) {
+  // Collect all debug users of From.
+  SmallVector<DbgInfoIntrinsic *, 1> Users;
+  findDbgUsers(Users, &From);
+  if (Users.empty())
+    return;
+
+  // Insert a replacement debug value before each old debug user. It's assumed
+  // that the old debug users will be erased later.
+  DIBuilder DIB(*From.getModule());
+  for (auto *OldDII : Users)
+    DIB.insertDbgValueIntrinsic(&To, OldDII->getVariable(),
+                                RewriteExpr(*OldDII),
+                                OldDII->getDebugLoc().get(), &InsertBefore);
+}
+
 unsigned llvm::removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB) {
   unsigned NumDeadInst = 0;
   // Delete the instructions backwards, as it has a reduced likelihood of