Index: lld/trunk/COFF/PDB.cpp
===================================================================
--- lld/trunk/COFF/PDB.cpp
+++ lld/trunk/COFF/PDB.cpp
@@ -1056,12 +1056,10 @@
 
   // Add an Info stream.
   auto &InfoBuilder = Builder.getInfoBuilder();
-  InfoBuilder.setAge(BuildId.PDB70.Age);
-
   GUID uuid;
   memcpy(&uuid, &BuildId.PDB70.Signature, sizeof(uuid));
+  InfoBuilder.setAge(BuildId.PDB70.Age);
   InfoBuilder.setGuid(uuid);
-  InfoBuilder.setSignature(time(nullptr));
   InfoBuilder.setVersion(pdb::PdbRaw_ImplVer::PdbImplVC70);
 
   // Add an empty DBI stream.
Index: llvm/trunk/include/llvm/DebugInfo/PDB/Native/InfoStreamBuilder.h
===================================================================
--- llvm/trunk/include/llvm/DebugInfo/PDB/Native/InfoStreamBuilder.h
+++ llvm/trunk/include/llvm/DebugInfo/PDB/Native/InfoStreamBuilder.h
@@ -40,6 +40,10 @@
   void setGuid(codeview::GUID G);
   void addFeature(PdbRaw_FeatureSig Sig);
 
+  uint32_t getAge() const { return Age; }
+  codeview::GUID getGuid() const { return Guid; }
+  Optional<uint32_t> getSignature() const { return Signature; }
+
   uint32_t finalize();
 
   Error finalizeMsfLayout();
@@ -52,8 +56,8 @@
 
   std::vector<PdbRaw_FeatureSig> Features;
   PdbRaw_ImplVer Ver;
-  uint32_t Sig;
   uint32_t Age;
+  Optional<uint32_t> Signature;
   codeview::GUID Guid;
 
   NamedStreamMap &NamedStreams;
Index: llvm/trunk/lib/DebugInfo/PDB/Native/InfoStreamBuilder.cpp
===================================================================
--- llvm/trunk/lib/DebugInfo/PDB/Native/InfoStreamBuilder.cpp
+++ llvm/trunk/lib/DebugInfo/PDB/Native/InfoStreamBuilder.cpp
@@ -25,15 +25,17 @@
 
 InfoStreamBuilder::InfoStreamBuilder(msf::MSFBuilder &Msf,
                                      NamedStreamMap &NamedStreams)
-    : Msf(Msf), Ver(PdbRaw_ImplVer::PdbImplVC70), Sig(-1), Age(0),
-      NamedStreams(NamedStreams) {}
+    : Msf(Msf), Ver(PdbRaw_ImplVer::PdbImplVC70), Age(0),
+      NamedStreams(NamedStreams) {
+  ::memset(&Guid, 0, sizeof(Guid));
+}
 
 void InfoStreamBuilder::setVersion(PdbRaw_ImplVer V) { Ver = V; }
 
-void InfoStreamBuilder::setSignature(uint32_t S) { Sig = S; }
-
 void InfoStreamBuilder::setAge(uint32_t A) { Age = A; }
 
+void InfoStreamBuilder::setSignature(uint32_t S) { Signature = S; }
+
 void InfoStreamBuilder::setGuid(GUID G) { Guid = G; }
 
 void InfoStreamBuilder::addFeature(PdbRaw_FeatureSig Sig) {
@@ -56,10 +58,10 @@
   BinaryStreamWriter Writer(*InfoS);
 
   InfoStreamHeader H;
-  H.Age = Age;
-  H.Signature = Sig;
+  // Leave the build id fields 0 so they can be set as the last step before
+  // committing the file to disk.
+  ::memset(&H, 0, sizeof(H));
   H.Version = Ver;
-  H.Guid = Guid;
   if (auto EC = Writer.writeObject(H))
     return EC;
 
Index: llvm/trunk/lib/DebugInfo/PDB/Native/PDBFileBuilder.cpp
===================================================================
--- llvm/trunk/lib/DebugInfo/PDB/Native/PDBFileBuilder.cpp
+++ llvm/trunk/lib/DebugInfo/PDB/Native/PDBFileBuilder.cpp
@@ -178,6 +178,8 @@
   auto OutFileOrError = FileOutputBuffer::create(Filename, Filesize);
   if (auto E = OutFileOrError.takeError())
     return E;
+  FileOutputBuffer *FOB = OutFileOrError->get();
+
   FileBufferByteStream Buffer(std::move(*OutFileOrError),
                               llvm::support::little);
   BinaryStreamWriter Writer(Buffer);
@@ -242,5 +244,20 @@
       return EC;
   }
 
+  auto InfoStreamBlocks = Layout.StreamMap[StreamPDB];
+  assert(!InfoStreamBlocks.empty());
+  uint64_t InfoStreamFileOffset =
+      blockToOffset(InfoStreamBlocks.front(), Layout.SB->BlockSize);
+  InfoStreamHeader *H = reinterpret_cast<InfoStreamHeader *>(
+      FOB->getBufferStart() + InfoStreamFileOffset);
+
+  // Set the build id at the very end, after every other byte of the PDB
+  // has been written.
+  // FIXME: Use a hash of the PDB rather than time(nullptr) for the signature.
+  H->Age = Info->getAge();
+  H->Guid = Info->getGuid();
+  Optional<uint32_t> Sig = Info->getSignature();
+  H->Signature = Sig.hasValue() ? *Sig : time(nullptr);
+
   return Buffer.commit();
 }