Index: llvm/include/llvm/ADT/StringMap.h
===================================================================
--- llvm/include/llvm/ADT/StringMap.h
+++ llvm/include/llvm/ADT/StringMap.h
@@ -359,6 +359,11 @@
     return find(Key) == end() ? 0 : 1;
   }
 
+  template <typename InputTy>
+  size_type count(const StringMapEntry<InputTy> &MapEntry) const {
+    return count(MapEntry.getKey());
+  }
+
   /// insert - Insert the specified key/value pair into the map.  If the key
   /// already exists in the map, return false and ignore the request, otherwise
   /// insert it and return true.
Index: llvm/include/llvm/ADT/StringSet.h
===================================================================
--- llvm/include/llvm/ADT/StringSet.h
+++ llvm/include/llvm/ADT/StringSet.h
@@ -45,6 +45,12 @@
       for (auto It = Begin; It != End; ++It)
         base::insert(std::make_pair(*It, '\0'));
     }
+
+    template <typename InputTy>
+    std::pair<typename base::iterator, bool>
+    insert(const StringMapEntry<InputTy> &MapEntry) {
+      return insert(MapEntry.getKey());
+    }
   };
 
 } // end namespace llvm
Index: llvm/test/tools/llvm-objdump/X86/warn-missing-disasm-func.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-objdump/X86/warn-missing-disasm-func.test
@@ -0,0 +1,12 @@
+## Warn if --disassemble-functions specifies an unknown symbol
+# RUN: yaml2obj %s | llvm-objdump - --disassemble-functions=foo 2>&1 | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+...
+
+# CHECK: warning: Failed to disassemble missing function foo.
Index: llvm/tools/llvm-objdump/llvm-objdump.cpp
===================================================================
--- llvm/tools/llvm-objdump/llvm-objdump.cpp
+++ llvm/tools/llvm-objdump/llvm-objdump.cpp
@@ -18,6 +18,7 @@
 #include "llvm-objdump.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SetOperations.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/Triple.h"
@@ -375,6 +376,11 @@
   errs().flush();
 }
 
+void warn(const Twine &Message) {
+  SmallVector<char, 256> MessageVec;
+  warn(Message.toStringRef(MessageVec));
+}
+
 LLVM_ATTRIBUTE_NORETURN void report_error(StringRef File, Twine Message) {
   WithColor::error(errs(), ToolName)
       << "'" << File << "': " << Message << ".\n";
@@ -1090,6 +1096,7 @@
 
   // Sort all the symbols, this allows us to use a simple binary search to find
   // a symbol near an address.
+  StringSet<> FoundDisasmFuncsSet;
   for (std::pair<const SectionRef, SectionSymbolsTy> &SecSyms : AllSymbols)
     array_pod_sort(SecSyms.second.begin(), SecSyms.second.end());
   array_pod_sort(AbsoluteSymbols.begin(), AbsoluteSymbols.end());
@@ -1176,6 +1183,8 @@
       if (!DisasmFuncsSet.empty() &&
           !DisasmFuncsSet.count(std::get<1>(Symbols[SI])))
         continue;
+      else
+        FoundDisasmFuncsSet.insert(std::get<1>(Symbols[SI]));
 
       uint64_t Start = std::get<0>(Symbols[SI]);
       if (Start < SectionAddr || StopAddress <= Start)
@@ -1400,6 +1409,11 @@
       }
     }
   }
+  auto MissingDisasmFuncsSet =
+      set_difference(DisasmFuncsSet, FoundDisasmFuncsSet);
+  if (!MissingDisasmFuncsSet.empty())
+    for (const StringRef &MissingDisasmFunc : MissingDisasmFuncsSet.keys())
+      warn("Failed to disassemble missing function " + MissingDisasmFunc);
 }
 
 static void disassembleObject(const ObjectFile *Obj, bool InlineRelocs) {