Index: lld/ELF/Driver.cpp
===================================================================
--- lld/ELF/Driver.cpp
+++ lld/ELF/Driver.cpp
@@ -1062,6 +1062,19 @@
   if (errorCount())
     return;
 
+  // Handle a rarely-used, -just-symbols option.
+  //
+  // This option allows you to link your output against other existing
+  // programs, so that if you load both other program and your output
+  // to memory, your output can call other program's symbols.
+  //
+  // What we are doing here is to read defined symbols from given ELF
+  // files and add them as absolute symbols.
+  for (auto *Arg : Args.filtered(OPT_just_symbols))
+    if (Optional<MemoryBufferRef> MB = readFile(Arg->getValue()))
+      for (std::pair<StringRef, uint64_t> P : readSymbols<ELFT>(*MB))
+        Symtab->addAbsoluteOptional(P.first, P.second);
+
   // Handle undefined symbols in DSOs.
   Symtab->scanShlibUndefined<ELFT>();
 
Index: lld/ELF/InputFiles.h
===================================================================
--- lld/ELF/InputFiles.h
+++ lld/ELF/InputFiles.h
@@ -59,6 +59,10 @@
 // Opens a given file.
 llvm::Optional<MemoryBufferRef> readFile(StringRef Path);
 
+// For --just-symbol.
+template <class ELFT>
+std::vector<std::pair<StringRef, uint64_t>> readSymbols(MemoryBufferRef);
+
 // The root class of input files.
 class InputFile {
 public:
Index: lld/ELF/InputFiles.cpp
===================================================================
--- lld/ELF/InputFiles.cpp
+++ lld/ELF/InputFiles.cpp
@@ -1049,6 +1049,38 @@
   }
 }
 
+// Reads defined symbols from a given ELF file, and returns their
+// names and values. This is used for -just-symbols.
+template <class ELFT>
+std::vector<std::pair<StringRef, uint64_t>>
+elf::readSymbols(MemoryBufferRef MB) {
+  typedef typename ELFT::Shdr Elf_Shdr;
+  typedef typename ELFT::Sym Elf_Sym;
+  typedef typename ELFT::SymRange Elf_Sym_Range;
+
+  StringRef ObjName = MB.getBufferIdentifier();
+  ELFFile<ELFT> Obj = check(ELFFile<ELFT>::create(MB.getBuffer()));
+  ArrayRef<Elf_Shdr> Sections = check(Obj.sections(), ObjName);
+
+  for (const Elf_Shdr &Sec : Sections) {
+    if (Sec.sh_type != SHT_SYMTAB)
+      continue;
+
+    Elf_Sym_Range Syms = check(Obj.symbols(&Sec), ObjName);
+    uint32_t FirstNonLocal = Sec.sh_info;
+    StringRef StringTable =
+        check(Obj.getStringTableForSymtab(Sec, Sections), ObjName);
+
+    std::vector<std::pair<StringRef, uint64_t>> Ret;
+    for (const Elf_Sym &Sym : Syms.slice(FirstNonLocal))
+      if (Sym.st_shndx != SHN_UNDEF)
+        Ret.emplace_back(check(Sym.getName(StringTable), ObjName),
+                         Sym.st_value);
+    return Ret;
+  }
+  return {};
+}
+
 template void ArchiveFile::parse<ELF32LE>();
 template void ArchiveFile::parse<ELF32BE>();
 template void ArchiveFile::parse<ELF64LE>();
@@ -1083,3 +1115,12 @@
 template void BinaryFile::parse<ELF32BE>();
 template void BinaryFile::parse<ELF64LE>();
 template void BinaryFile::parse<ELF64BE>();
+
+template std::vector<std::pair<StringRef, uint64_t>>
+    elf::readSymbols<ELF32LE>(MemoryBufferRef);
+template std::vector<std::pair<StringRef, uint64_t>>
+    elf::readSymbols<ELF32BE>(MemoryBufferRef);
+template std::vector<std::pair<StringRef, uint64_t>>
+    elf::readSymbols<ELF64LE>(MemoryBufferRef);
+template std::vector<std::pair<StringRef, uint64_t>>
+    elf::readSymbols<ELF64BE>(MemoryBufferRef);
Index: lld/ELF/Options.td
===================================================================
--- lld/ELF/Options.td
+++ lld/ELF/Options.td
@@ -147,6 +147,8 @@
 defm init: Eq<"init">, HelpText<"Specify an initializer function">,
   MetaVarName<"<symbol>">;
 
+defm just_symbols: Eq<"just-symbols">, HelpText<"Just link symbols">;
+
 defm library: Eq<"library">, HelpText<"Root name of library to use">,
   MetaVarName<"<libName>">;
 
Index: lld/ELF/SymbolTable.h
===================================================================
--- lld/ELF/SymbolTable.h
+++ lld/ELF/SymbolTable.h
@@ -48,6 +48,8 @@
                               uint8_t Visibility = llvm::ELF::STV_HIDDEN,
                               uint8_t Binding = llvm::ELF::STB_GLOBAL);
 
+  void addAbsoluteOptional(StringRef Name, uint64_t Value);
+
   template <class ELFT> Symbol *addUndefined(StringRef Name);
   template <class ELFT>
   Symbol *addUndefined(StringRef Name, bool IsLocal, uint8_t Binding,
Index: lld/ELF/SymbolTable.cpp
===================================================================
--- lld/ELF/SymbolTable.cpp
+++ lld/ELF/SymbolTable.cpp
@@ -141,6 +141,14 @@
   return cast<DefinedRegular>(Sym->body());
 }
 
+void SymbolTable::addAbsoluteOptional(StringRef Name, uint64_t Value) {
+  outs() << Name << "=" << Value << "\n";
+  SymbolBody *B = find(Name);
+  if (B->isUndefined())
+    replaceBody<DefinedRegular>(B->symbol(), nullptr, Name, false, 0,
+                                STT_NOTYPE, Value, 0, nullptr);
+}
+
 // Set a flag for --trace-symbol so that we can print out a log message
 // if a new symbol with the same name is inserted into the symbol table.
 void SymbolTable::trace(StringRef Name) {
Index: lld/test/ELF/Inputs/just-symbols.s
===================================================================
--- /dev/null
+++ lld/test/ELF/Inputs/just-symbols.s
@@ -0,0 +1,3 @@
+.globl foo
+foo:
+  ret
Index: lld/test/ELF/just-symbols.s
===================================================================
--- /dev/null
+++ lld/test/ELF/just-symbols.s
@@ -0,0 +1,15 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %S/Inputs/just-symbols.s -o %t1
+# RUN: ld.lld %t1 -o %t1.exe -Ttext=0x10000
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2
+# RUN: ld.lld %t2 -just-symbols=%t1.exe -o %t2.exe
+# RUN: llvm-readelf -symbols %t2.exe | FileCheck %s
+
+# CHECK: 0000000000010000     0 NOTYPE  GLOBAL DEFAULT  ABS foo
+
+.globl _start, foo
+_start:
+  call foo
+  ret