Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -54,6 +54,7 @@
   bool AsNeeded = false;
   bool Bsymbolic;
   bool BsymbolicFunctions;
+  bool BuildId;
   bool Demangle = true;
   bool DiscardAll;
   bool DiscardLocals;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -223,6 +223,7 @@
   Config->AllowMultipleDefinition = Args.hasArg(OPT_allow_multiple_definition);
   Config->Bsymbolic = Args.hasArg(OPT_Bsymbolic);
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
+  Config->BuildId = Args.hasArg(OPT_build_id);
   Config->Demangle = !Args.hasArg(OPT_no_demangle);
   Config->DiscardAll = Args.hasArg(OPT_discard_all);
   Config->DiscardLocals = Args.hasArg(OPT_discard_locals);
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -12,6 +12,8 @@
 def Bstatic: Flag<["-"], "Bstatic">,
   HelpText<"Do not link against shared libraries">;
 
+def build_id : Flag<["--"], "build-id">;
+
 def L : JoinedOrSeparate<["-"], "L">, MetaVarName<"<dir>">,
   HelpText<"Directory to search for libraries">;
 
@@ -170,7 +172,6 @@
 def start_group_paren: Flag<["-"], "(">;
 
 // Options listed below are silently ignored for now for compatibility.
-def build_id : Flag<["--"], "build-id">;
 def fatal_warnings : Flag<["--"], "fatal-warnings">;
 def no_add_needed : Flag<["--"], "no-add-needed">;
 def no_fatal_warnings : Flag<["--"], "no-fatal-warnings">;
Index: ELF/OutputSections.h
===================================================================
--- ELF/OutputSections.h
+++ ELF/OutputSections.h
@@ -14,6 +14,7 @@
 
 #include "llvm/MC/StringTableBuilder.h"
 #include "llvm/Object/ELF.h"
+#include "llvm/Support/MD5.h"
 
 #include "Config.h"
 
@@ -529,12 +530,26 @@
   std::vector<FdeData> FdeList;
 };
 
+template <class ELFT>
+class BuildIdSection final : public OutputSectionBase<ELFT> {
+public:
+  BuildIdSection();
+  void writeTo(uint8_t *Buf) override;
+  void update(ArrayRef<uint8_t> Buf);
+  void writeBuildId();
+
+private:
+  llvm::MD5 Hash;
+  uint8_t *Md5Buf;
+};
+
 // All output sections that are hadnled by the linker specially are
 // globally accessible. Writer initializes them, so don't use them
 // until Writer is initialized.
 template <class ELFT> struct Out {
   typedef typename llvm::object::ELFFile<ELFT>::uintX_t uintX_t;
   typedef typename llvm::object::ELFFile<ELFT>::Elf_Phdr Elf_Phdr;
+  static BuildIdSection<ELFT> *BuildId;
   static DynamicSection<ELFT> *Dynamic;
   static EhFrameHeader<ELFT> *EhFrameHdr;
   static GnuHashTableSection<ELFT> *GnuHashTab;
@@ -559,6 +574,7 @@
   static OutputSectionBase<ELFT> *ProgramHeaders;
 };
 
+template <class ELFT> BuildIdSection<ELFT> *Out<ELFT>::BuildId;
 template <class ELFT> DynamicSection<ELFT> *Out<ELFT>::Dynamic;
 template <class ELFT> EhFrameHeader<ELFT> *Out<ELFT>::EhFrameHdr;
 template <class ELFT> GnuHashTableSection<ELFT> *Out<ELFT>::GnuHashTab;
Index: ELF/OutputSections.cpp
===================================================================
--- ELF/OutputSections.cpp
+++ ELF/OutputSections.cpp
@@ -1566,6 +1566,32 @@
 }
 
 template <class ELFT>
+BuildIdSection<ELFT>::BuildIdSection()
+    : OutputSectionBase<ELFT>(".note.gnu.build-id", SHT_NOTE, 0) {
+  // 16 bytes for the note section header and 16 bytes for MD5.
+  this->Header.sh_size = 32;
+}
+
+template <class ELFT> void BuildIdSection<ELFT>::writeTo(uint8_t *Buf) {
+  const endianness E = ELFT::TargetEndianness;
+  write32<E>(Buf, 4);         // Name size
+  write32<E>(Buf + 4, 16);    // Content size
+  write32<E>(Buf + 8, 3);     // Type (NT_GNU_BUILD_ID = 3)
+  memcpy(Buf + 12, "GNU", 4); // Name string
+  Md5Buf = Buf + 16;
+}
+
+template <class ELFT> void BuildIdSection<ELFT>::update(ArrayRef<uint8_t> Buf) {
+  Hash.update(Buf);
+}
+
+template <class ELFT> void BuildIdSection<ELFT>::writeBuildId() {
+  MD5::MD5Result Res;
+  Hash.final(Res);
+  memcpy(Md5Buf, Res, 16);
+}
+
+template <class ELFT>
 MipsReginfoOutputSection<ELFT>::MipsReginfoOutputSection()
     : OutputSectionBase<ELFT>(".reginfo", SHT_MIPS_REGINFO, SHF_ALLOC) {
   this->Header.sh_addralign = 4;
@@ -1669,6 +1695,11 @@
 template class SymbolTableSection<ELF64LE>;
 template class SymbolTableSection<ELF64BE>;
 
+template class BuildIdSection<ELF32LE>;
+template class BuildIdSection<ELF32BE>;
+template class BuildIdSection<ELF64LE>;
+template class BuildIdSection<ELF64BE>;
+
 template uint32_t getLocalRelTarget(const ObjectFile<ELF32LE> &,
                                     const ELFFile<ELF32LE>::Elf_Rel &,
                                     uint32_t);
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -75,6 +75,7 @@
   bool openFile();
   void writeHeader();
   void writeSections();
+  void writeBuildId();
   bool isDiscarded(InputSectionBase<ELFT> *IS) const;
   StringRef getOutputSectionName(InputSectionBase<ELFT> *S) const;
   bool needsInterpSection() const {
@@ -143,6 +144,7 @@
   ProgramHeaders.updateAlign(sizeof(uintX_t));
 
   // Instantiate optional output sections if they are needed.
+  std::unique_ptr<BuildIdSection<ELFT>> BuildId;
   std::unique_ptr<GnuHashTableSection<ELFT>> GnuHashTab;
   std::unique_ptr<GotPltSection<ELFT>> GotPlt;
   std::unique_ptr<HashTableSection<ELFT>> HashTab;
@@ -151,6 +153,8 @@
   std::unique_ptr<SymbolTableSection<ELFT>> SymTabSec;
   std::unique_ptr<OutputSection<ELFT>> MipsRldMap;
 
+  if (Config->BuildId)
+    BuildId.reset(new BuildIdSection<ELFT>);
   if (Config->GnuHash)
     GnuHashTab.reset(new GnuHashTableSection<ELFT>);
   if (Config->SysvHash)
@@ -175,6 +179,7 @@
     MipsRldMap->updateAlign(sizeof(uintX_t));
   }
 
+  Out<ELFT>::BuildId = BuildId.get();
   Out<ELFT>::DynStrTab = &DynStrTab;
   Out<ELFT>::DynSymTab = &DynSymTab;
   Out<ELFT>::Dynamic = &Dynamic;
@@ -221,6 +226,7 @@
   writeSections();
   if (HasError)
     return;
+  writeBuildId();
   check(Buffer->commit());
 }
 
@@ -1126,6 +1132,7 @@
 
   // This order is not the same as the final output order
   // because we sort the sections using their attributes below.
+  Add(Out<ELFT>::BuildId);
   Add(Out<ELFT>::SymTab);
   Add(Out<ELFT>::ShStrTab);
   Add(Out<ELFT>::StrTab);
@@ -1547,6 +1554,29 @@
       Sec->writeTo(Buf + Sec->getFileOff());
 }
 
+template <class ELFT> void Writer<ELFT>::writeBuildId() {
+  BuildIdSection<ELFT> *S = Out<ELFT>::BuildId;
+  if (!S)
+    return;
+
+  // Compute the md5sum of all sections except .debug_* sections.
+  // We skip debug sections because they tend to be very large
+  // and their contents are very likely to be the same as long as
+  // other sections are the same.
+  uint8_t *Start = Buffer->getBufferStart();
+  uint8_t *Last = Start;
+  for (OutputSectionBase<ELFT> *Sec : OutputSections) {
+    uint8_t *End = Start + Sec->getFileOff();
+    if (!Sec->getName().startswith(".debug_"))
+      S->update({Last, End});
+    Last = End;
+  }
+  S->update({Last, Start + FileSize});
+
+  // Fill the MD5 field in the .note.gnu.build-id section.
+  S->writeBuildId();
+}
+
 template void elf::writeResult<ELF32LE>(SymbolTable<ELF32LE> *Symtab);
 template void elf::writeResult<ELF32BE>(SymbolTable<ELF32BE> *Symtab);
 template void elf::writeResult<ELF64LE>(SymbolTable<ELF64LE> *Symtab);