Index: test/wasm/data-layout.ll
===================================================================
--- test/wasm/data-layout.ll
+++ test/wasm/data-layout.ll
@@ -1,6 +1,6 @@
 ; RUN: llc -filetype=obj %p/Inputs/hello.ll -o %t.hello.o
 ; RUN: llc -filetype=obj %s -o %t.o
-; RUN: lld -flavor wasm --emit-relocs --allow-undefined -o %t.wasm %t.o %t.hello.o
+; RUN: lld -flavor wasm --emit-relocs --allow-undefined --entry '' -o %t.wasm %t.o %t.hello.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
Index: test/wasm/entry.ll
===================================================================
--- test/wasm/entry.ll
+++ test/wasm/entry.ll
@@ -12,11 +12,13 @@
   ret void
 }
 
-; CHECK:   - Type:            EXPORT
-; CHECK:     Exports:         
-; CHECK:       - Name:            memory
-; CHECK:         Kind:            MEMORY
-; CHECK:         Index:           0
-; CHECK:       - Name:            entry
-; CHECK:         Kind:            FUNCTION
-; CHECK:         Index:           0
+; CHECK:        - Type:            EXPORT
+; CHECK-NEXT:     Exports:
+; CHECK-NEXT:       - Name:            memory
+; CHECK-NEXT:         Kind:            MEMORY
+; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:       - Name:            entry
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   0
Index: test/wasm/load-undefined.ll
===================================================================
--- /dev/null
+++ test/wasm/load-undefined.ll
@@ -0,0 +1,22 @@
+; Verify that the -u / --undefined option is able to pull in symbols from
+; an archive, and doesn't error when uses to pull in a symbol already loaded.
+;
+; RUN: llc -filetype=obj -mtriple=wasm32-unknown-uknown-wasm %S/Inputs/ret64.ll -o %t.o
+; RUN: llc -filetype=obj -mtriple=wasm32-unknown-uknown-wasm %S/Inputs/ret32.ll -o %t2.o
+; RUN: llvm-ar rcs %t2.a %t2.o
+; RUN: lld -flavor wasm %t2.a %t.o -o %t.wasm --entry '' -u ret32 --undefined ret64
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+
+; CHECK:        - Type:            EXPORT
+; CHECK-NEXT:     Exports:
+; CHECK-NEXT:       - Name:            memory
+; CHECK-NEXT:         Kind:            MEMORY
+; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:       - Name:            ret32
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:       - Name:            ret64
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:   - Type:
Index: test/wasm/local-symbols.ll
===================================================================
--- test/wasm/local-symbols.ll
+++ test/wasm/local-symbols.ll
@@ -54,6 +54,8 @@
 ; CHECK-NEXT:       - Name:            _start
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   1
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Functions:       
 ; CHECK-NEXT:       - Locals:          
Index: test/wasm/weak-alias-overide.ll
===================================================================
--- test/wasm/weak-alias-overide.ll
+++ test/wasm/weak-alias-overide.ll
@@ -65,6 +65,8 @@
 ; CHECK-NEXT:       - Name:            call_bar
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           3
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   1
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Functions:       
 ; CHECK-NEXT:       - Locals:          
Index: test/wasm/weak-alias.ll
===================================================================
--- test/wasm/weak-alias.ll
+++ test/wasm/weak-alias.ll
@@ -59,6 +59,8 @@
 ; CHECK-NEXT:       - Name:            call_bar
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   0
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Functions:       
 ; CHECK-NEXT:       - Locals:          
Index: test/wasm/weak-external.ll
===================================================================
--- test/wasm/weak-external.ll
+++ test/wasm/weak-external.ll
@@ -66,6 +66,8 @@
 ; CHECK-NEXT:       - Name:            _start
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   2
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - Offset:          
Index: wasm/Config.h
===================================================================
--- wasm/Config.h
+++ wasm/Config.h
@@ -39,6 +39,7 @@
   llvm::StringRef OutputFile;
 
   std::vector<llvm::StringRef> SearchPaths;
+  std::vector<llvm::StringRef> Undefined;
   std::set<llvm::StringRef> AllowUndefinedSymbols;
   std::vector<std::pair<Symbol *, WasmGlobal>> SyntheticGlobals;
 };
Index: wasm/Driver.cpp
===================================================================
--- wasm/Driver.cpp
+++ wasm/Driver.cpp
@@ -292,11 +292,13 @@
 
   Config->AllowUndefined = Args.hasArg(OPT_allow_undefined);
   Config->EmitRelocs = Args.hasArg(OPT_emit_relocs);
-  Config->Entry = Args.getLastArgValue(OPT_entry);
+  Config->Relocatable = Args.hasArg(OPT_relocatable);
+  Config->Entry = Args.getLastArgValue(OPT_entry,
+                                       Config->Relocatable ? "" : "_start");
   Config->ImportMemory = Args.hasArg(OPT_import_memory);
   Config->OutputFile = Args.getLastArgValue(OPT_o);
-  Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->SearchPaths = getArgs(Args, OPT_L);
+  Config->Undefined = getArgs(Args, OPT_undefined);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->StripDebug = Args.hasArg(OPT_strip_debug);
   Config->Sysroot = Args.getLastArgValue(OPT_sysroot);
@@ -323,9 +325,11 @@
     error("entry point specified for relocatable output file");
 
   if (!Config->Relocatable) {
-    if (Config->Entry.empty())
-      Config->Entry = "_start";
-    addSyntheticUndefinedFunction(Config->Entry);
+    if (!Config->Entry.empty())
+      addSyntheticUndefinedFunction(Config->Entry);
+    // Handle the `--undefined <sym>` options.
+    for (StringRef S : Config->Undefined)
+      addSyntheticUndefinedFunction(S);
 
     addSyntheticGlobal("__stack_pointer", 0);
   }
@@ -348,6 +352,8 @@
 
   if (!Config->Entry.empty()) {
     Symbol *Sym = Symtab->find(Config->Entry);
+    if (!Sym || !Sym->isDefined())
+      fatal("entry point not found: " + Config->Entry);
     if (!Sym->isFunction())
       fatal("entry point is not a function: " + Sym->getName());
   }
Index: wasm/Options.td
===================================================================
--- wasm/Options.td
+++ wasm/Options.td
@@ -6,6 +6,11 @@
 class J<string name>: Joined<["--", "-"], name>;
 class S<string name>: Separate<["--", "-"], name>;
 
+multiclass Eq<string name> {
+  def "": Separate<["--", "-"], name>;
+  def _eq: Joined<["--", "-"], name # "=">, Alias<!cast<Separate>(NAME)>;
+}
+
 def L: JoinedOrSeparate<["-"], "L">, MetaVarName<"<dir>">,
   HelpText<"Add a directory to the library search path">;
 
@@ -50,6 +55,9 @@
 
 def sysroot: J<"sysroot=">, HelpText<"Set the system root">;
 
+defm undefined: Eq<"undefined">,
+  HelpText<"Force undefined symbol during linking">;
+
 def z: JoinedOrSeparate<["-"], "z">, MetaVarName<"<option>">,
   HelpText<"Linker option extensions">;
 
@@ -87,3 +95,4 @@
 def alias_relocatable_r: Flag<["-"], "r">, Alias<relocatable>;
 def alias_entry_e: JoinedOrSeparate<["-"], "e">, Alias<entry>;
 def alias_entry_entry: J<"entry=">, Alias<entry>;
+def alias_undefined_u: JoinedOrSeparate<["-"], "u">, Alias<undefined>;
Index: wasm/Writer.cpp
===================================================================
--- wasm/Writer.cpp
+++ wasm/Writer.cpp
@@ -305,22 +305,6 @@
     writeExport(OS, MemoryExport);
   }
 
-  if (ExportMain) {
-    Symbol *Sym = Symtab->find(Config->Entry);
-    if (Sym->isDefined()) {
-      if (!Sym->isFunction())
-        fatal("entry point is not a function: " + Sym->getName());
-
-      if (!ExportOther) {
-        WasmExport MainExport;
-        MainExport.Name = Config->Entry;
-        MainExport.Kind = WASM_EXTERNAL_FUNCTION;
-        MainExport.Index = Sym->getOutputIndex();
-        writeExport(OS, MainExport);
-      }
-    }
-  }
-
   if (ExportOther) {
     for (ObjFile *File : Symtab->ObjectFiles) {
       for (Symbol *Sym : File->getSymbols()) {
@@ -345,7 +329,19 @@
   }
 }
 
-void Writer::createStartSection() {}
+void Writer::createStartSection() {
+  if (Config->Entry.empty())
+    return;
+
+  const Symbol *Sym = Symtab->find(Config->Entry);
+  assert(Sym && Sym->isDefined() && Sym->isFunction());
+
+  SyntheticSection *Section = createSyntheticSection(WASM_SEC_START);
+  raw_ostream &OS = Section->getStream();
+
+  log("Start: " + Sym->getName());
+  writeUleb128(OS, Sym->getOutputIndex(), "start index");
+}
 
 void Writer::createElemSection() {
   if (!NumElements)