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/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:            get_address_of_global_var
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:   - Type:            START
+; CHECK-NEXT:     StartFunction:   2
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - Offset:          
Index: wasm/Driver.cpp
===================================================================
--- wasm/Driver.cpp
+++ wasm/Driver.cpp
@@ -247,11 +247,12 @@
   Config->CheckSignatures =
       Args.hasFlag(OPT_check_signatures, OPT_no_check_signatures, false);
   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");
   std::vector<StringRef> Undefined = args::getStrings(Args, OPT_undefined);
   Config->ImportMemory = Args.hasArg(OPT_import_memory);
   Config->OutputFile = Args.getLastArgValue(OPT_o);
-  Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->SearchPaths = args::getStrings(Args, OPT_L);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->StripDebug = Args.hasArg(OPT_strip_debug);
@@ -282,10 +283,10 @@
     error("undefined symbols specified for relocatable output file");
 
   if (!Config->Relocatable) {
-    if (Config->Entry.empty())
-      Config->Entry = "_start";
-    static WasmSignature Signature = {{}, WASM_TYPE_NORESULT};
-    addSyntheticUndefinedFunction(Config->Entry, &Signature);
+    if (!Config->Entry.empty()) {
+      static WasmSignature Signature = {{}, WASM_TYPE_NORESULT};
+      addSyntheticUndefinedFunction(Config->Entry, &Signature);
+    }
 
     // Handle the `--undefined <sym>` options.
     for (StringRef S : Undefined)
@@ -312,6 +313,8 @@
 
   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/Writer.cpp
===================================================================
--- wasm/Writer.cpp
+++ wasm/Writer.cpp
@@ -336,7 +336,19 @@
   }
 }
 
-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)