diff --git a/hphp/compiler/analysis/emitter.cpp b/hphp/compiler/analysis/emitter.cpp
index 752c04c78..46788889c 100644
--- a/hphp/compiler/analysis/emitter.cpp
+++ b/hphp/compiler/analysis/emitter.cpp
@@ -5238,6 +5238,7 @@ void EmitterVisitor::emitPostponedMeths() {
     PostponedMeth& p = m_postponedMeths.front();
     FunctionScopePtr funcScope = p.m_meth->getFunctionScope();
     FuncEmitter* fe = p.m_fe;
+    bool allowOverride = false;
     if (!fe) {
       assert(p.m_top);
       const StringData* methName =
@@ -5253,6 +5254,10 @@ void EmitterVisitor::emitPostponedMeths() {
       funcScope->userAttributes();
     for (FunctionScope::UserAttributeMap::const_iterator it = userAttrs.begin();
          it != userAttrs.end(); ++it) {
+      if (it->first == "__Overridable") {
+        allowOverride = true;
+        continue;
+      }
       const StringData* uaName = StringData::GetStaticString(it->first);
       ExpressionPtr uaValue = it->second;
       assert(uaValue);
@@ -5394,6 +5399,8 @@ void EmitterVisitor::emitPostponedMeths() {
     ModifierExpressionPtr mod(p.m_meth->getModifiers());
     Attr attrs = buildAttrs(mod, p.m_meth->isRef());
 
+    if (allowOverride) attrs = attrs | AttrAllowOverride;
+
     if (funcScope->mayUseVV()) {
       attrs = attrs | AttrMayUseVV;
     }
@@ -5926,6 +5933,10 @@ bool EmitterVisitor::canEmitBuiltinCall(FunctionCallPtr fn,
     if (func->allowOverride() && !Option::WholeProgram) {
       return false;
     }
+
+    if (Func* f = Unit::lookupFunc(StringData::GetStaticString(name))) {
+      if (!f->info()) return false;
+    }
     return true;
   }
   return false;
@@ -6974,20 +6985,7 @@ static Unit* emitHHBCNativeFuncUnit(const HhbcExtFuncInfo* builtinFuncs,
   MD5 md5("11111111111111111111111111111111");
   UnitEmitter* ue = new UnitEmitter(md5);
   ue->setFilepath(StringData::GetStaticString(""));
-  ue->initMain(0, 0);
-  FuncEmitter* mfe = ue->getMain();
-  ue->emitOp(OpInt);
-  ue->emitInt64(1);
-  ue->emitOp(OpRetC);
-  mfe->setMaxStackCells(1);
-  mfe->finish(ue->bcPos(), false);
-  ue->recordFunction(mfe);
-
-  TypedValue mainReturn;
-  mainReturn.m_data.num = 1;
-  mainReturn.m_type = KindOfInt64;
-  ue->setMainReturn(&mainReturn);
-  ue->setMergeOnly(true);
+  ue->addTrivialPseudoMain();
 
   /*
     Special function used by FPushCuf* when its argument
@@ -7011,6 +7009,11 @@ static Unit* emitHHBCNativeFuncUnit(const HhbcExtFuncInfo* builtinFuncs,
     const ClassInfo::MethodInfo* mi = ClassInfo::FindFunction(name);
     assert(mi &&
       "MethodInfo not found; may be a problem with the .idl.json files");
+    if (Unit::lookupFunc(name)) {
+      // already provided by systemlib, rename to allow the php
+      // version to delegate if necessary
+      name = StringData::GetStaticString("__builtin_" + name->toCPPString());
+    }
     FuncEmitter* fe = ue->newFuncEmitter(name, /*top*/ true);
     Offset base = ue->bcPos();
     fe->setBuiltinFunc(mi, bif, nif, base);
@@ -7097,21 +7100,7 @@ static Unit* emitHHBCNativeClassUnit(const HhbcExtClassInfo* builtinClasses,
   MD5 md5("eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee");
   UnitEmitter* ue = new UnitEmitter(md5);
   ue->setFilepath(StringData::GetStaticString(""));
-  ue->initMain(0, 0);
-  FuncEmitter* mfe = ue->getMain();
-  ue->emitOp(OpInt);
-  ue->emitInt64(1);
-  ue->emitOp(OpRetC);
-  Offset past = ue->bcPos();
-  mfe->setMaxStackCells(1);
-  mfe->finish(past, false);
-  ue->recordFunction(mfe);
-
-  TypedValue mainReturn;
-  mainReturn.m_data.num = 1;
-  mainReturn.m_type = KindOfInt64;
-  ue->setMainReturn(&mainReturn);
-  ue->setMergeOnly(true);
+  ue->addTrivialPseudoMain();
 
   MetaInfoBuilder metaInfo;
 
@@ -7400,41 +7389,6 @@ static void batchCommit(std::vector<UnitEmitter*>& ues) {
   ues.clear();
 }
 
-static void emitSystemLib() {
-  if (!Option::WholeProgram) return;
-
-  string slib = get_systemlib();
-  if (slib.empty()) return;
-
-  Option::WholeProgram = false;
-  SystemLib::s_inited = false;
-
-  SCOPE_EXIT {
-    SystemLib::s_inited = true;
-    Option::WholeProgram = true;
-  };
-
-  AnalysisResultPtr ar(new AnalysisResult());
-  Scanner scanner(slib.c_str(), slib.size(),
-                  RuntimeOption::GetScannerType(), "/:systemlib.php");
-  Parser parser(scanner, "/:systemlib.php", ar, slib.size());
-  parser.parse();
-  FileScopePtr fsp = parser.getFileScope();
-  fsp->setOuterScope(ar);
-
-  ar->loadBuiltins();
-  ar->setPhase(AnalysisResult::AnalyzeAll);
-  fsp->analyzeProgram(ar);
-
-  int md5len;
-  char* md5str = string_md5(slib.c_str(), slib.size(), false, md5len);
-  MD5 md5(md5str);
-  free(md5str);
-
-  UnitEmitter* ue = emitHHBCUnitEmitter(ar, fsp, md5);
-  Repo::get().commitUnit(ue, UnitOriginFile);
-}
-
 /**
  * This is the entry point for offline bytecode generation.
  */
@@ -7489,8 +7443,6 @@ void emitAllHHBC(AnalysisResultPtr ar) {
         break;
       }
     }
-
-    emitSystemLib();
   } else {
     dispatcher.waitEmpty();
   }
@@ -7537,6 +7489,7 @@ Unit* hphp_compiler_parse(const char* code, int codeLen, const MD5& md5,
     }
     SCOPE_EXIT { SymbolTable::Purge(); };
 
+    UnitEmitter* ue = nullptr;
     // Check if this file contains raw hip hop bytecode instead of php.
     // For now this is just dictated by file extension, and doesn't ever
     // commit to the repo.
@@ -7544,23 +7497,25 @@ Unit* hphp_compiler_parse(const char* code, int codeLen, const MD5& md5,
       if (const char* dot = strrchr(filename, '.')) {
         const char hhbc_ext[] = "hhas";
         if (!strcmp(dot + 1, hhbc_ext)) {
-          return assemble_file(filename, md5);
+          ue = assemble_string(code, codeLen, filename, md5);
         }
       }
     }
 
-    AnalysisResultPtr ar(new AnalysisResult());
-    Scanner scanner(code, codeLen, RuntimeOption::GetScannerType(), filename);
-    Parser parser(scanner, filename, ar, codeLen);
-    parser.parse();
-    FileScopePtr fsp = parser.getFileScope();
-    fsp->setOuterScope(ar);
+    if (!ue) {
+      AnalysisResultPtr ar(new AnalysisResult());
+      Scanner scanner(code, codeLen, RuntimeOption::GetScannerType(), filename);
+      Parser parser(scanner, filename, ar, codeLen);
+      parser.parse();
+      FileScopePtr fsp = parser.getFileScope();
+      fsp->setOuterScope(ar);
 
-    ar->loadBuiltins();
-    ar->setPhase(AnalysisResult::AnalyzeAll);
-    fsp->analyzeProgram(ar);
+      ar->loadBuiltins();
+      ar->setPhase(AnalysisResult::AnalyzeAll);
+      fsp->analyzeProgram(ar);
 
-    UnitEmitter* ue = emitHHBCUnitEmitter(ar, fsp, md5);
+      ue = emitHHBCUnitEmitter(ar, fsp, md5);
+    }
     Repo::get().commitUnit(ue, unitOrigin);
     Unit* unit = ue->create();
     delete ue;
diff --git a/hphp/compiler/analysis/function_scope.cpp b/hphp/compiler/analysis/function_scope.cpp
index 661bba8a3..b66602ba0 100644
--- a/hphp/compiler/analysis/function_scope.cpp
+++ b/hphp/compiler/analysis/function_scope.cpp
@@ -80,7 +80,7 @@ FunctionScope::FunctionScope(AnalysisResultConstPtr ar, bool method,
   // Support for systemlib functions implemented in PHP
   if (!m_method &&
       m_userAttributes.find("__Overridable") != m_userAttributes.end()) {
-      setAllowOverride();
+    setAllowOverride();
   }
 }
 
diff --git a/hphp/compiler/compiler.cpp b/hphp/compiler/compiler.cpp
index d65abe385..bda7a904c 100644
--- a/hphp/compiler/compiler.cpp
+++ b/hphp/compiler/compiler.cpp
@@ -837,6 +837,9 @@ void hhbcTargetInit(const CompilerOptions &po, AnalysisResultPtr ar) {
   RuntimeOption::RepoJournal = "memory";
   RuntimeOption::EnableHipHopSyntax = Option::EnableHipHopSyntax;
   RuntimeOption::EvalJitEnableRenameFunction = Option::JitEnableRenameFunction;
+
+  // Turn off commits, because we don't want systemlib to get included
+  RuntimeOption::RepoCommit = false;
 }
 
 int hhbcTarget(const CompilerOptions &po, AnalysisResultPtr ar,
@@ -868,7 +871,7 @@ int hhbcTarget(const CompilerOptions &po, AnalysisResultPtr ar,
   /* without this, emitClass allows classes with interfaces to be
      hoistable */
   SystemLib::s_inited = true;
-
+  RuntimeOption::RepoCommit = true;
   Option::AutoInline = -1;
 
   if (po.optimizeLevel > 0) {
diff --git a/hphp/hhvm/process_init.cpp b/hphp/hhvm/process_init.cpp
index 0bb349c64..ac0cc9421 100644
--- a/hphp/hhvm/process_init.cpp
+++ b/hphp/hhvm/process_init.cpp
@@ -105,47 +105,39 @@ void ProcessInit() {
   int db = RuntimeOption::EvalDumpBytecode;
   bool rp = RuntimeOption::AlwaysUseRelativePath;
   bool sf = RuntimeOption::SafeFileAccess;
+  bool ah = RuntimeOption::EvalAllowHhas;
   RuntimeOption::EvalDumpBytecode &= ~1;
   RuntimeOption::AlwaysUseRelativePath = false;
   RuntimeOption::SafeFileAccess = false;
+  RuntimeOption::EvalAllowHhas = true;
 
   Transl::TargetCache::requestInit();
 
-  Unit* nativeFuncUnit = build_native_func_unit(hhbc_ext_funcs,
-                                                hhbc_ext_funcs_count);
-  SystemLib::s_nativeFuncUnit = nativeFuncUnit;
   String currentDir = g_vmContext->getCwd();
 
-  if (RuntimeOption::RepoAuthoritative) {
-    auto file = g_vmContext->lookupPhpFile(String("/:systemlib.php").get(),
-                                         currentDir.data(), nullptr);
-    if (!file) {
-      // Die a horrible death.
-      Logger::Error("Unable to find/load systemlib.php");
-      _exit(1);
-    }
-    file->incRef();
-    SystemLib::s_unit = file->unit();
-  } else {
-    string slib = get_systemlib();
+  string hhas;
+  string slib = get_systemlib(&hhas);
 
-    if (slib.empty()) {
-      // Die a horrible death.
-      Logger::Error("Unable to find/load systemlib.php");
-      _exit(1);
-    }
-    SystemLib::s_unit = compile_string(slib.c_str(), slib.size());
+  if (slib.empty()) {
+    // Die a horrible death.
+    Logger::Error("Unable to find/load systemlib.php");
+    _exit(1);
+  }
+  SystemLib::s_unit = compile_string(slib.c_str(), slib.size(),
+                                     "systemlib.php");
+  if (!hhas.empty()) {
+    SystemLib::s_hhas_unit = compile_string(hhas.c_str(), hhas.size(),
+                                            "systemlib.hhas");
   }
-
-  // Restore most settings before merging anything,
-  // because of optimizations that depend on them
-  RuntimeOption::AlwaysUseRelativePath = rp;
-  RuntimeOption::SafeFileAccess = sf;
 
   // Load the systemlib unit to build the Class objects
   SystemLib::s_unit->merge();
+  if (SystemLib::s_hhas_unit) {
+    SystemLib::s_hhas_unit->merge();
+  }
 
-  // load builtins
+  SystemLib::s_nativeFuncUnit = build_native_func_unit(hhbc_ext_funcs,
+                                                       hhbc_ext_funcs_count);
   SystemLib::s_nativeFuncUnit->merge();
   SystemLib::s_nullFunc =
     Unit::lookupFunc(StringData::GetStaticString("86null"));
@@ -188,8 +180,10 @@ void ProcessInit() {
   Stack::ValidateStackSize();
   SystemLib::s_inited = true;
 
-  // Restore last to avoid dumping system things
+  RuntimeOption::AlwaysUseRelativePath = rp;
+  RuntimeOption::SafeFileAccess = sf;
   RuntimeOption::EvalDumpBytecode = db;
+  RuntimeOption::EvalAllowHhas = ah;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
diff --git a/hphp/runtime/base/program_functions.cpp b/hphp/runtime/base/program_functions.cpp
index 0caae61d2..4619cbc59 100644
--- a/hphp/runtime/base/program_functions.cpp
+++ b/hphp/runtime/base/program_functions.cpp
@@ -1151,16 +1151,26 @@ String canonicalize_path(CStrRef p, const char* root, int rootLen) {
   return path;
 }
 
+static string systemlib_split(string slib, string* hhas) {
+  auto pos = slib.find("\n<?hhas\n");
+  if (pos != string::npos) {
+    if (hhas) *hhas = slib.substr(pos + 8);
+    return slib.substr(0, pos);
+  }
+  return slib;
+}
+
 // Search for systemlib.php in the following places:
 // 1) ${HHVM_SYSTEMLIB}
 // 2) section "systemlib" in the current executable
 // and return its contents
-string get_systemlib() {
+string get_systemlib(string* hhas) {
   if (char *file = getenv("HHVM_SYSTEMLIB")) {
     std::ifstream ifs(file);
     if (ifs.good()) {
-      return std::string(std::istreambuf_iterator<char>(ifs),
-                         std::istreambuf_iterator<char>());
+      return systemlib_split(std::string(
+                               std::istreambuf_iterator<char>(ifs),
+                               std::istreambuf_iterator<char>()), hhas);
     }
   }
 
@@ -1172,8 +1182,7 @@ string get_systemlib() {
   ifs.seekg(desc.m_start, std::ios::beg);
   std::unique_ptr<char[]> data(new char[desc.m_len]);
   ifs.read(data.get(), desc.m_len);
-  string result(data.get(), desc.m_len);
-  return result;
+  return systemlib_split(string(data.get(), desc.m_len), hhas);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
diff --git a/hphp/runtime/base/program_functions.h b/hphp/runtime/base/program_functions.h
index 9ca2a94ec..e80d90cf8 100644
--- a/hphp/runtime/base/program_functions.h
+++ b/hphp/runtime/base/program_functions.h
@@ -83,7 +83,7 @@ void hphp_thread_exit();
 void hphp_session_exit();
 void hphp_process_exit() ATTRIBUTE_COLD;
 bool hphp_is_warmup_enabled();
-std::string get_systemlib();
+std::string get_systemlib(std::string* hhas = nullptr);
 
 // Generated by hphp/util/generate_buildinfo.sh.
 extern const char* const kCompilerId;
diff --git a/hphp/runtime/ext/ext_function.cpp b/hphp/runtime/ext/ext_function.cpp
index b3ac018a0..2bf90c52d 100644
--- a/hphp/runtime/ext/ext_function.cpp
+++ b/hphp/runtime/ext/ext_function.cpp
@@ -276,7 +276,7 @@ String f_create_function(CStrRef args, CStrRef code) {
 
 Variant f_func_get_arg(int arg_num) {
   CallerFrame cf;
-  ActRec* ar = cf();
+  ActRec* ar = cf.actRecForArgs();
 
   if (ar == NULL) {
     return false;
@@ -367,7 +367,7 @@ Array hhvm_get_frame_args(const ActRec* ar) {
 
 Variant f_func_get_args() {
   EagerCallerFrame cf;
-  ActRec* ar = cf();
+  ActRec* ar = cf.actRecForArgs();
   if (ar && ar->hasVarEnv() && ar->getVarEnv()->isGlobalScope()) {
     raise_warning(
       "func_get_args():  Called from the global scope - no function context"
@@ -399,7 +399,7 @@ Array func_get_args(int num_args, CArrRef params, CArrRef args) {
 
 int64_t f_func_num_args() {
   EagerCallerFrame cf;
-  ActRec* ar = cf();
+  ActRec* ar = cf.actRecForArgs();
   if (ar == NULL) {
     return -1;
   }
diff --git a/hphp/runtime/ext/ext_reflection.cpp b/hphp/runtime/ext/ext_reflection.cpp
index 62ba100ac..67afab5e8 100644
--- a/hphp/runtime/ext/ext_reflection.cpp
+++ b/hphp/runtime/ext/ext_reflection.cpp
@@ -330,7 +330,8 @@ static void set_function_info(Array &ret, const Func* func) {
   }
   if (func->isBuiltin()) {
     ret.set(s_internal, true_varNR);
-    if (func->info()->attribute & ClassInfo::HipHopSpecific) {
+    if (func->info() &&
+        func->info()->attribute & ClassInfo::HipHopSpecific) {
       ret.set(s_hphp,     true_varNR);
     }
   }
diff --git a/hphp/runtime/vm/as.cpp b/hphp/runtime/vm/as.cpp
index 89feda7a1..a2c751de0 100644
--- a/hphp/runtime/vm/as.cpp
+++ b/hphp/runtime/vm/as.cpp
@@ -75,6 +75,8 @@
  * @author Jorden DeLong <delong.j@fb.com>
  */
 
+#include "hphp/runtime/vm/as.h"
+
 #include <cstdio>
 #include <iostream>
 #include <algorithm>
@@ -90,6 +92,7 @@
 #include "hphp/runtime/vm/unit.h"
 #include "hphp/runtime/vm/hhbc.h"
 #include "hphp/runtime/base/builtin_functions.h"
+#include "hphp/system/lib/systemlib.h"
 
 TRACE_SET_MOD(hhas);
 
@@ -944,9 +947,11 @@ OpcodeParserMap opcode_parsers;
 #define IMM_FOUR(t1, t2, t3, t4) IMM_##t1; IMM_##t2; IMM_##t3; IMM_##t4
 
 // FCall and NewTuple need to know the the first imm do POP_*MANY.
-#define IMM_IVA                                   \
-  immIVA = read_opcode_arg<int64_t>(as);          \
-  as.ue->emitIVA(immIVA);                         \
+#define IMM_IVA do {                            \
+    int imm = read_opcode_arg<int64_t>(as);     \
+    as.ue->emitIVA(imm);                        \
+    if (immIVA < 0) immIVA = imm;               \
+  } while (0)
 
 #define IMM_SA   as.ue->emitInt32(as.ue->mergeLitstr(read_litstr(as)))
 #define IMM_I64A as.ue->emitInt64(read_opcode_arg<int64_t>(as))
@@ -1294,10 +1299,15 @@ enum AttrContext {
 };
 Attr parse_attribute_list(AsmState& as, AttrContext ctx) {
   as.in.skipWhitespace();
-  if (as.in.peek() != '[') return AttrNone;
+  int ret = AttrNone;
+  if (ctx == ClassAttributes || ctx == FuncAttributes) {
+    if (!SystemLib::s_inited) {
+      ret |= AttrUnique | AttrPersistent;
+    }
+  }
+  if (as.in.peek() != '[') return Attr(ret);
   as.in.getc();
 
-  int ret = AttrNone;
   std::string word;
   for (;;) {
     as.in.skipWhitespace();
@@ -1752,6 +1762,13 @@ void parse_class(AsmState& as) {
  *                ;
  */
 void parse_main(AsmState& as) {
+  if (as.emittedPseudoMain) {
+    if (!SystemLib::s_inited) {
+      as.error(".main found in systemlib");
+    } else {
+      as.error("Multiple .main directives found");
+    }
+  }
   as.in.expectWs('{');
 
   as.ue->initMain(as.in.getLineNumber(),
@@ -1801,6 +1818,17 @@ void parse_adata(AsmState& as) {
 void parse(AsmState& as) {
   as.in.skipWhitespace();
   std::string directive;
+  if (!SystemLib::s_inited) {
+    /*
+     * The SystemLib::s_hhas_unit is required to be merge-only,
+     * and we create the source by concatenating separate .hhas files
+     * Rather than choosing one to have the .main directive, we just
+     * generate a trivial pseudoMain automatically.
+     */
+    as.ue->addTrivialPseudoMain();
+    as.emittedPseudoMain = true;
+  }
+
   while (as.in.readword(directive)) {
     if (directive == ".main")        { parse_main(as);     continue; }
     if (directive == ".function")    { parse_function(as); continue; }
@@ -1819,17 +1847,14 @@ void parse(AsmState& as) {
 
 //////////////////////////////////////////////////////////////////////
 
-Unit* assemble_file(const char* filename, const MD5& md5) {
-  boost::scoped_ptr<UnitEmitter> ue(new UnitEmitter(md5));
+UnitEmitter* assemble_string(const char*code, int codeLen,
+                             const char* filename, const MD5& md5) {
+  std::unique_ptr<UnitEmitter> ue(new UnitEmitter(md5));
   StringData* sd = StringData::GetStaticString(filename);
   ue->setFilepath(sd);
 
   try {
-    std::ifstream instr(filename);
-    if (!instr.is_open()) {
-      throw std::runtime_error(std::string("couldn't open file ") +
-            filename + ": " + strerror(errno));
-    }
+    std::istringstream instr(string(code, codeLen));
     AsmState as(instr);
     as.ue = ue.get();
     parse(as);
@@ -1847,7 +1872,7 @@ Unit* assemble_file(const char* filename, const MD5& md5) {
     ue->recordFunction(fe);
   }
 
-  return ue->create();
+  return ue.release();
 }
 
 //////////////////////////////////////////////////////////////////////
diff --git a/hphp/runtime/vm/as.h b/hphp/runtime/vm/as.h
index e10d538ee..590f22d5b 100644
--- a/hphp/runtime/vm/as.h
+++ b/hphp/runtime/vm/as.h
@@ -21,16 +21,17 @@
 
 namespace HPHP {
 
-class Unit;
+class UnitEmitter;
 
 //////////////////////////////////////////////////////////////////////
 
 /*
- * Assemble the contents of `filename' and return a Unit.
+ * Assemble the contents of `filename' and return a UnitEmitter.
  *
  * Minimal documentation is available in as.cpp.
  */
-Unit* assemble_file(const char* filename, const MD5&);
+UnitEmitter* assemble_string(const char* code, int codeLen,
+                             const char* filename, const MD5&);
 
 //////////////////////////////////////////////////////////////////////
 
diff --git a/hphp/runtime/vm/bytecode.cpp b/hphp/runtime/vm/bytecode.cpp
index ed7a47462..18d4baf62 100644
--- a/hphp/runtime/vm/bytecode.cpp
+++ b/hphp/runtime/vm/bytecode.cpp
@@ -114,7 +114,7 @@ ActRec* ActRec::arGetSfp() const {
 
 bool
 ActRec::skipFrame() const {
-  return m_func && m_func->isBuiltin();
+  return m_func && m_func->skipFrame();
 }
 
 template <>
@@ -879,7 +879,7 @@ void Stack::toStringFrame(std::ostream& os, const ActRec* fp,
     os << ">";
   }
 
-  assert(!func->isBuiltin() || func->numIterators() == 0);
+  assert(!func->info() || func->numIterators() == 0);
   if (func->numIterators() > 0) {
     os << "|";
     Iter* it = &((Iter*)&tv[1])[-1];
@@ -1788,7 +1788,7 @@ bool VMExecutionContext::prepareFuncEntry(ActRec *ar, PC& pc) {
   // cppext functions/methods have their own logic for raising
   // warnings for missing arguments, so we only need to do this work
   // for non-cppext functions/methods
-  if (raiseMissingArgumentWarnings && !func->isBuiltin()) {
+  if (raiseMissingArgumentWarnings && !func->info()) {
     // need to sync m_pc to pc for backtraces/re-entry
     SYNC();
     const Func::ParamInfoVec& paramInfo = func->params();
@@ -2282,7 +2282,7 @@ void VMExecutionContext::invokeUnit(TypedValue* retval, Unit* unit) {
 void VMExecutionContext::unwindBuiltinFrame() {
   // Unwind the frame for a builtin. Currently only used for
   // hphpd_break and fb_enable_code_coverage
-  assert(m_fp->m_func->isBuiltin());
+  assert(m_fp->m_func->info());
   assert(m_fp->m_func->name()->isame(s_hphpd_break.get()) ||
          m_fp->m_func->name()->isame(s_fb_enable_code_coverage.get()));
   // Free any values that may be on the eval stack
@@ -2839,7 +2839,7 @@ bool VMExecutionContext::evalUnit(Unit* unit, bool local,
     ar->setThis(nullptr);
   }
   Func* func = unit->getMain(cls);
-  assert(!func->isBuiltin());
+  assert(!func->info());
   assert(!func->isGenerator());
   ar->m_func = func;
   ar->initNumArgs(0);
@@ -5974,7 +5974,7 @@ inline void OPTBLD_INLINE VMExecutionContext::iopFPassCW(PC& pc) {
     TRACE(1, "FPassCW: function %s(%d) param %d is by reference, "
           "raising a strict warning (attr:0x%x)\n",
           func->name()->data(), func->numParams(), paramId,
-          func->isBuiltin() ? func->info()->attribute : 0);
+          func->info() ? func->info()->attribute : 0);
     raise_strict_warning("Only variables should be passed by reference");
   }
 }
@@ -5985,7 +5985,7 @@ inline void OPTBLD_INLINE VMExecutionContext::iopFPassCE(PC& pc) {
     TRACE(1, "FPassCE: function %s(%d) param %d is by reference, "
           "throwing a fatal error (attr:0x%x)\n",
           func->name()->data(), func->numParams(), paramId,
-          func->isBuiltin() ? func->info()->attribute : 0);
+          func->info() ? func->info()->attribute : 0);
     raise_error("Cannot pass parameter %d by reference", paramId+1);
   }
 }
@@ -6200,8 +6200,8 @@ static int makeNativeRefCall(const Func* f, Ret* ret,
 
 inline void OPTBLD_INLINE VMExecutionContext::iopFCallBuiltin(PC& pc) {
   NEXT();
-  DECODE_IA(numArgs);
-  DECODE_IA(numNonDefault);
+  DECODE_IVA(numArgs);
+  DECODE_IVA(numNonDefault);
   DECODE(Id, id);
   const NamedEntity* ne = m_fp->m_func->unit()->lookupNamedEntityId(id);
   Func* func = Unit::lookupFunc(ne);
@@ -7591,12 +7591,14 @@ void VMExecutionContext::requestInit() {
 
   if (UNLIKELY(RuntimeOption::EvalJitEnableRenameFunction)) {
     SystemLib::s_unit->merge();
+    if (SystemLib::s_hhas_unit) SystemLib::s_hhas_unit->merge();
     SystemLib::s_nativeFuncUnit->merge();
     SystemLib::s_nativeClassUnit->merge();
   } else {
     // System units are always merge only, and
     // everything is persistent.
     assert(SystemLib::s_unit->isEmpty());
+    assert(!SystemLib::s_hhas_unit || SystemLib::s_hhas_unit->isEmpty());
     assert(SystemLib::s_nativeFuncUnit->isEmpty());
     assert(SystemLib::s_nativeClassUnit->isEmpty());
   }
diff --git a/hphp/runtime/vm/class.cpp b/hphp/runtime/vm/class.cpp
index eb2a01a8b..db5634c0e 100644
--- a/hphp/runtime/vm/class.cpp
+++ b/hphp/runtime/vm/class.cpp
@@ -1282,7 +1282,8 @@ void Class::setSpecial() {
   // Use 86ctor(), since no program-supplied constructor exists
   m_ctor = findSpecialMethod(this, sd86ctor);
   assert(m_ctor && "class had no user-defined constructor or 86ctor");
-  assert(m_ctor->attrs() == (AttrPublic|AttrNoInjection|AttrPhpLeafFn));
+  assert((m_ctor->attrs() & ~AttrBuiltin) ==
+         (AttrPublic|AttrNoInjection|AttrPhpLeafFn));
 }
 
 void Class::applyTraitPrecRule(const PreClass::TraitPrecRule& rule) {
diff --git a/hphp/runtime/vm/core_types.h b/hphp/runtime/vm/core_types.h
index 5cc79e0ce..033435868 100644
--- a/hphp/runtime/vm/core_types.h
+++ b/hphp/runtime/vm/core_types.h
@@ -97,6 +97,15 @@ const Slot kInvalidSlot = Slot(-1);
  * to other php functions.  It may still call other user-level
  * functions via re-entry (e.g. for destructors or autoload), and it
  * may make calls to builtins using FCallBuiltin.
+ *
+ * AttrBuiltin is set on builtin functions - whether c++ or php
+ *
+ * AttrAllowOverride is set on builtin functions that can be replaced
+ *   by user implementations
+ *
+ * AttrSkipFrame is set to indicate that the frame should be ignored
+ *   when searching for the context (eg array_map evaluates its
+ *   callback in the context of its caller).
  */
 enum Attr {
   AttrNone      = 0,             // class  property  method  //
@@ -121,6 +130,9 @@ enum Attr {
   AttrPersistent= (1 << 17),     //    X                X    //
   AttrDeepInit = (1 << 18),      //            X             //
   AttrHot = (1 << 19),           //                     X    //
+  AttrBuiltin = (1 << 20),       //                     X    //
+  AttrAllowOverride = (1 << 21), //                     X    //
+  AttrSkipFrame = (1 << 22),     //                     X    //
 };
 
 static inline Attr operator|(Attr a, Attr b) { return Attr((int)a | (int)b); }
diff --git a/hphp/runtime/vm/event_hook.cpp b/hphp/runtime/vm/event_hook.cpp
index 2dfbfcc09..65976a680 100644
--- a/hphp/runtime/vm/event_hook.cpp
+++ b/hphp/runtime/vm/event_hook.cpp
@@ -93,7 +93,7 @@ void EventHook::RunUserProfiler(const ActRec* ar, int mode) {
       if (fault.m_faultType == Fault::UserException) {
         frameinfo.set(s_exception, fault.m_userException);
       }
-    } else if (!ar->m_func->isBuiltin() &&
+    } else if (!ar->m_func->info() &&
                !ar->m_func->isGenerator()) {
       // TODO (#1131400) This is wrong for builtins
       frameinfo.set(s_return, tvAsCVarRef(g_vmContext->m_stack.topTV()));
diff --git a/hphp/runtime/vm/func.cpp b/hphp/runtime/vm/func.cpp
index 0e92b9dac..c8dced764 100644
--- a/hphp/runtime/vm/func.cpp
+++ b/hphp/runtime/vm/func.cpp
@@ -39,10 +39,6 @@ static const Trace::Module TRACEMOD = Trace::bcinterp;
 const StringData* Func::s___call = StringData::GetStaticString("__call");
 const StringData* Func::s___callStatic =
   StringData::GetStaticString("__callStatic");
-static const StringData* sd___overridable =
-  StringData::GetStaticString("__Overridable");
-static const StringData* sd___PHPBuiltin =
-  StringData::GetStaticString("__PHPBuiltin");
 
 //=============================================================================
 // Func.
@@ -394,7 +390,7 @@ bool Func::isNameBindingImmutable(const Unit* fromUnit) const {
 bool Func::byRef(int32_t arg) const {
   // Super special case. A handful of builtins are varargs functions where the
   // (not formally declared) varargs are pass-by-reference. psychedelic-kitten
-  if (arg >= m_numParams && isBuiltin() &&
+  if (arg >= m_numParams && info() &&
       (info()->attribute & (ClassInfo::RefVariableArguments |
                             ClassInfo::MixedVariableArguments))) {
     return true;
@@ -408,7 +404,7 @@ bool Func::byRef(int32_t arg) const {
 bool Func::mustBeRef(int32_t arg) const {
   // return true if the argument is required to be a reference
   // (and thus should be an lvalue)
-  if (arg >= m_numParams && isBuiltin() &&
+  if (arg >= m_numParams && info() &&
       ((info()->attribute & (ClassInfo::RefVariableArguments |
                              ClassInfo::MixedVariableArguments)) ==
                                ClassInfo::RefVariableArguments)) {
@@ -514,11 +510,6 @@ void Func::prettyPrint(std::ostream& out) const {
   }
 }
 
-bool Func::isPHPBuiltin() const {
-  return shared()->m_userAttributes.find(sd___PHPBuiltin) !=
-         shared()->m_userAttributes.end();
-}
-
 HphpArray* Func::getStaticLocals() const {
   return g_vmContext->getFuncStaticCtx(this);
 }
@@ -667,13 +658,6 @@ void Func::setCached() {
   setCachedFunc(this, isDebuggerAttached());
 }
 
-bool Func::isAllowOverride() const {
-  return (shared()->m_info &&
-    (shared()->m_info->attribute & ClassInfo::AllowOverride)) ||
-    (shared()->m_userAttributes.find(sd___overridable) !=
-     shared()->m_userAttributes.end());
-}
-
 const Func* Func::getGeneratorBody(const StringData* name) const {
   if (isNonClosureMethod()) {
     return cls()->lookupMethod(name);
@@ -744,6 +728,10 @@ void FuncEmitter::init(int line1, int line2, Offset base, Attr attrs, bool top,
   m_attrs = attrs;
   m_top = top;
   m_docComment = docComment;
+  if (!SystemLib::s_inited) {
+    m_attrs = m_attrs | AttrBuiltin;
+    if (!pce()) m_attrs = m_attrs | AttrSkipFrame;
+  }
 }
 
 void FuncEmitter::finish(Offset past, bool load) {
@@ -989,37 +977,39 @@ void FuncEmitter::setBuiltinFunc(const ClassInfo::MethodInfo* info,
   m_docComment = StringData::GetStaticString(info->docComment);
   m_line1 = 0;
   m_line2 = 0;
-  m_attrs = AttrNone;
+  m_attrs = AttrBuiltin | AttrSkipFrame;
   // TODO: Task #1137917: See if we can avoid marking most builtins with
   // "MayUseVV" and still make things work
-  m_attrs = (Attr)(m_attrs | AttrMayUseVV);
+  m_attrs = m_attrs | AttrMayUseVV;
   if (info->attribute & (ClassInfo::RefVariableArguments |
                          ClassInfo::MixedVariableArguments)) {
-    m_attrs = Attr(m_attrs | AttrVariadicByRef);
+    m_attrs = m_attrs | AttrVariadicByRef;
   }
   if (info->attribute & ClassInfo::IsReference) {
-    m_attrs = (Attr)(m_attrs | AttrReference);
+    m_attrs = m_attrs | AttrReference;
   }
   if (info->attribute & ClassInfo::NoInjection) {
-    m_attrs = (Attr)(m_attrs | AttrNoInjection);
+    m_attrs = m_attrs | AttrNoInjection;
   }
   if (pce()) {
     if (info->attribute & ClassInfo::IsStatic) {
-      m_attrs = (Attr)(m_attrs | AttrStatic);
+      m_attrs = m_attrs | AttrStatic;
     }
     if (info->attribute & ClassInfo::IsFinal) {
-      m_attrs = (Attr)(m_attrs | AttrFinal);
+      m_attrs = m_attrs | AttrFinal;
     }
     if (info->attribute & ClassInfo::IsAbstract) {
-      m_attrs = (Attr)(m_attrs | AttrAbstract);
+      m_attrs = m_attrs | AttrAbstract;
     }
     if (info->attribute & ClassInfo::IsPrivate) {
-      m_attrs = (Attr)(m_attrs | AttrPrivate);
+      m_attrs = m_attrs | AttrPrivate;
     } else if (info->attribute & ClassInfo::IsProtected) {
-      m_attrs = (Attr)(m_attrs | AttrProtected);
+      m_attrs = m_attrs | AttrProtected;
     } else {
-      m_attrs = (Attr)(m_attrs | AttrPublic);
+      m_attrs = m_attrs | AttrPublic;
     }
+  } else if (info->attribute & ClassInfo::AllowOverride) {
+    m_attrs = m_attrs | AttrAllowOverride;
   }
 
   m_returnType = info->returnType;
diff --git a/hphp/runtime/vm/func.h b/hphp/runtime/vm/func.h
index c4e453b3d..c343a43dc 100644
--- a/hphp/runtime/vm/func.h
+++ b/hphp/runtime/vm/func.h
@@ -204,8 +204,8 @@ struct Func {
   void prettyPrint(std::ostream& out) const;
 
   bool isPseudoMain() const { return m_name->empty(); }
-  bool isBuiltin() const { return (bool)info(); }
-  bool isPHPBuiltin() const;
+  bool isBuiltin() const { return m_attrs & AttrBuiltin; }
+  bool skipFrame() const { return m_attrs & AttrSkipFrame; }
   bool isMethod() const {
     return !isPseudoMain() && (bool)cls();
   }
@@ -352,7 +352,8 @@ struct Func {
   bool hasStaticLocals() const { return !shared()->m_staticVars.empty(); }
   int numStaticLocals() const { return shared()->m_staticVars.size(); }
   const ClassInfo::MethodInfo* info() const { return shared()->m_info; }
-  bool isAllowOverride() const;
+  bool isAllowOverride() const { return m_attrs & AttrAllowOverride; }
+
   const BuiltinFunction& nativeFuncPtr() const {
     return shared()->m_nativeFuncPtr;
   }
diff --git a/hphp/runtime/vm/hhbc.h b/hphp/runtime/vm/hhbc.h
index 7483a0267..9cf2b1c5d 100644
--- a/hphp/runtime/vm/hhbc.h
+++ b/hphp/runtime/vm/hhbc.h
@@ -516,7 +516,7 @@ enum SetOpOp {
   O(FPassM,          TWO(IVA,MA),      LMANY(),         ONE(FV),    FF) \
   O(FCall,           ONE(IVA),         FMANY,           ONE(RV),    CF_FF) \
   O(FCallArray,      NA,               ONE(FV),         ONE(RV),    CF_FF) \
-  O(FCallBuiltin,    THREE(IA,IA,SA),  FMANY,           ONE(RV),    CF) \
+  O(FCallBuiltin,    THREE(IVA,IVA,SA),FMANY,           ONE(RV),    CF) \
   O(CufSafeArray,    NA,               THREE(RV,CV,CV), ONE(CV),    NF) \
   O(CufSafeReturn,   NA,               THREE(RV,CV,CV), ONE(RV),    NF) \
   O(IterInit,        THREE(IA,BA,HA),  ONE(CV),         NOV,        CF) \
diff --git a/hphp/runtime/vm/runtime.cpp b/hphp/runtime/vm/runtime.cpp
index 760711df3..3891fa53b 100644
--- a/hphp/runtime/vm/runtime.cpp
+++ b/hphp/runtime/vm/runtime.cpp
@@ -328,7 +328,7 @@ Unit* build_native_class_unit(const HhbcExtClassInfo* builtinClasses,
   return g_hphp_build_native_class_unit(builtinClasses, numBuiltinClasses);
 }
 
-Unit* compile_string(const char* s, size_t sz) {
+Unit* compile_string(const char* s, size_t sz, const char* fname) {
   MD5 md5;
   int out_len;
   md5 = MD5(string_md5(s, sz, false, out_len));
@@ -337,7 +337,7 @@ Unit* compile_string(const char* s, size_t sz) {
   if (u != nullptr) {
     return u;
   }
-  return g_hphp_compiler_parse(s, sz, md5, nullptr);
+  return g_hphp_compiler_parse(s, sz, md5, fname);
 }
 
 // Returned array has refcount zero! Caller must refcount.
diff --git a/hphp/runtime/vm/runtime.h b/hphp/runtime/vm/runtime.h
index 5e567fd7f..bdd749d8b 100644
--- a/hphp/runtime/vm/runtime.h
+++ b/hphp/runtime/vm/runtime.h
@@ -174,7 +174,7 @@ frame_free_args(TypedValue* args, int count) {
 
 Unit*
 compile_file(const char* s, size_t sz, const MD5& md5, const char* fname);
-Unit* compile_string(const char* s, size_t sz);
+Unit* compile_string(const char* s, size_t sz, const char* fname = nullptr);
 Unit* build_native_func_unit(const HhbcExtFuncInfo* builtinFuncs,
                                  ssize_t numBuiltinFuncs);
 Unit* build_native_class_unit(const HhbcExtClassInfo* builtinClasses,
diff --git a/hphp/runtime/vm/translator/translator-inline.h b/hphp/runtime/vm/translator/translator-inline.h
index 2745d9866..f025145f0 100644
--- a/hphp/runtime/vm/translator/translator-inline.h
+++ b/hphp/runtime/vm/translator/translator-inline.h
@@ -129,26 +129,35 @@ struct EagerVMRegAnchor {
   }
 };
 
-static inline ActRec* regAnchorFP() {
+inline ActRec* regAnchorFP() {
   // In builtins, m_fp points to the caller's frame if called
   // through FCallBuiltin, else it points to the builtin's frame,
   // in which case, getPrevVMState() gets the caller's frame.
+  // In addition, we need to skip over php-defined builtin functions
+  // in order to find the true context.
   VMExecutionContext* context = g_vmContext;
   ActRec* cur = context->getFP();
-  if (!cur) return nullptr;
-  if (cur->skipFrame()) {
-    ActRec* prev = context->getPrevVMState(cur);
-    if (prev == cur) return nullptr;
-    return prev;
-  } else {
-    return cur;
+  while (cur && cur->skipFrame()) {
+    cur = context->getPrevVMState(cur);
   }
+  return cur;
+}
+
+inline ActRec* regAnchorFPForArgs() {
+  // Like regAnchorFP, but only account for FCallBuiltin
+  VMExecutionContext* context = g_vmContext;
+  ActRec* cur = context->getFP();
+  if (cur && cur->m_func->info()) {
+    cur = context->getPrevVMState(cur);
+  }
+  return cur;
 }
 
 struct EagerCallerFrame : public EagerVMRegAnchor {
   ActRec* operator()() {
     return regAnchorFP();
   }
+  ActRec* actRecForArgs() { return regAnchorFPForArgs(); }
 };
 
 
@@ -158,6 +167,7 @@ struct CallerFrame : public VMRegAnchor {
   ActRec* operator()() {
     return regAnchorFP();
   }
+  ActRec* actRecForArgs() { return regAnchorFPForArgs(); }
 };
 
 #define SYNC_VM_REGS_SCOPED() \
diff --git a/hphp/runtime/vm/translator/translator-x64.cpp b/hphp/runtime/vm/translator/translator-x64.cpp
index cc2eb4064..79e69534c 100644
--- a/hphp/runtime/vm/translator/translator-x64.cpp
+++ b/hphp/runtime/vm/translator/translator-x64.cpp
@@ -2284,7 +2284,7 @@ TranslatorX64::emitPrologue(Func* func, int nPassed) {
   Fixup fixup(funcBody.m_offset - func->base(), frameCells);
 
   // Emit warnings for any missing arguments
-  if (!func->isBuiltin()) {
+  if (!func->info()) {
     for (int i = nPassed; i < numParams; ++i) {
       if (paramInfo[i].funcletOff() == InvalidAbsoluteOffset) {
         emitImmReg(a, (intptr_t)func->name()->data(), argNumToRegName[0]);
@@ -2315,7 +2315,7 @@ TranslatorX64::emitPrologue(Func* func, int nPassed) {
 
 static bool
 isNativeImplCall(const Func* funcd, int numArgs) {
-  return funcd && funcd->isBuiltin() && numArgs == funcd->numParams();
+  return funcd && funcd->info() && numArgs == funcd->numParams();
 }
 
 int32_t // returns the amount by which rVmSp should be adjusted
@@ -6626,17 +6626,19 @@ bool TranslatorX64::eagerRecord(const Func* func) {
     "func_num_args",
     "array_filter",
     "array_map",
+    "WaitHandle::join",
   };
 
+  assert(func->info());
+
+  const StringData* name = func->isMethod() ?
+    func->fullName() : func->info()->name.get();
   for (int i = 0; i < sizeof(list)/sizeof(list[0]); i++) {
-    if (!strcmp(func->name()->data(), list[i])) {
+    if (!strcmp(name->data(), list[i])) {
       return true;
     }
   }
-  if (func->cls() && !strcmp(func->cls()->name()->data(), "WaitHandle")
-      && !strcmp(func->name()->data(), "join")) {
-    return true;
-  }
+
   return false;
 }
 
@@ -6680,7 +6682,7 @@ int32_t TranslatorX64::emitNativeImpl(const Func* func,
    * contains only a single opcode (NativeImpl), and there are no
    * non-argument locals.
    */
-  assert(func->numIterators() == 0 && func->isBuiltin());
+  assert(func->numIterators() == 0 && func->info());
   assert(func->numLocals() == func->numParams());
   assert(*func->getEntry() == OpNativeImpl);
   assert(instrLen(func->getEntry()) == func->past() - func->base());
@@ -9681,7 +9683,7 @@ void TranslatorX64::analyzeFCallBuiltin(Tracelet& t,
 void TranslatorX64::translateFCallBuiltin(const Tracelet& t,
                                           const NormalizedInstruction& ni) {
   int numArgs = ni.imm[0].u_IVA;
-  int numNonDefault = ni.imm[1].u_IA;
+  int numNonDefault = ni.imm[1].u_IVA;
   Id funcId = ni.imm[2].u_SA;
   const NamedEntity* ne = curUnit()->lookupNamedEntityId(funcId);
   const Func* func = Unit::lookupFunc(ne);
diff --git a/hphp/runtime/vm/translator/translator.cpp b/hphp/runtime/vm/translator/translator.cpp
index 07bd1c812..3ece7afbd 100644
--- a/hphp/runtime/vm/translator/translator.cpp
+++ b/hphp/runtime/vm/translator/translator.cpp
@@ -3125,7 +3125,7 @@ static bool shouldAnalyzeCallee(const NormalizedInstruction* fcall) {
     FTRACE(1, "analyzeCallee: target func not known\n");
     return false;
   }
-  if (target->isBuiltin()) {
+  if (target->info()) {
     FTRACE(1, "analyzeCallee: target func is a builtin\n");
     return false;
   }
@@ -3946,6 +3946,7 @@ ActRecState::getCurrentState() {
 const Func* lookupImmutableMethod(const Class* cls, const StringData* name,
                                   bool& magicCall, bool staticLookup) {
   if (!cls || RuntimeOption::EvalJitEnableRenameFunction) return nullptr;
+  if (cls->attrs() & AttrInterface) return nullptr;
   bool privateOnly = false;
   if (!RuntimeOption::RepoAuthoritative ||
       !(cls->preClass()->attrs() & AttrUnique)) {
diff --git a/hphp/runtime/vm/unit.cpp b/hphp/runtime/vm/unit.cpp
index f3cc77785..db672c9ad 100644
--- a/hphp/runtime/vm/unit.cpp
+++ b/hphp/runtime/vm/unit.cpp
@@ -181,7 +181,7 @@ Array Unit::getUserFunctions() {
     for (NamedEntityMap::const_iterator it = s_namedDataMap->begin();
          it != s_namedDataMap->end(); ++it) {
       Func* func_ = it->second.getCachedFunc();
-      if (!func_ || func_->isBuiltin() || func_->isPHPBuiltin() ||
+      if (!func_ || func_->isBuiltin() ||
           isdigit(func_->name()->data()[0])) {
         continue;
       }
@@ -2236,6 +2236,23 @@ UnitEmitter::~UnitEmitter() {
   }
 }
 
+void UnitEmitter::addTrivialPseudoMain() {
+  initMain(0, 0);
+  FuncEmitter* mfe = getMain();
+  emitOp(OpInt);
+  emitInt64(1);
+  emitOp(OpRetC);
+  mfe->setMaxStackCells(1);
+  mfe->finish(bcPos(), false);
+  recordFunction(mfe);
+
+  TypedValue mainReturn;
+  mainReturn.m_data.num = 1;
+  mainReturn.m_type = KindOfInt64;
+  setMainReturn(&mainReturn);
+  setMergeOnly(true);
+}
+
 void UnitEmitter::setBc(const uchar* bc, size_t bclen) {
   m_bc = (uchar*)malloc(bclen);
   m_bcmax = bclen;
@@ -2278,6 +2295,7 @@ Id UnitEmitter::addPreConst(const StringData* name, const TypedValue& value) {
   return id;
 }
 
+
 Id UnitEmitter::mergeLitstr(const StringData* litstr) {
   LitstrMap::const_iterator it = m_litstr2id.find(litstr);
   if (it == m_litstr2id.end()) {
diff --git a/hphp/runtime/vm/unit.h b/hphp/runtime/vm/unit.h
index 6e15293f4..d53b8e0bf 100644
--- a/hphp/runtime/vm/unit.h
+++ b/hphp/runtime/vm/unit.h
@@ -717,6 +717,7 @@ class UnitEmitter {
   explicit UnitEmitter(const MD5& md5);
   ~UnitEmitter();
 
+  void addTrivialPseudoMain();
   int repoId() const { return m_repoId; }
   void setRepoId(int repoId) { m_repoId = repoId; }
   int64_t sn() const { return m_sn; }
diff --git a/hphp/system/lib/gen_systemlib.php b/hphp/system/lib/gen_systemlib.php
index fb5f2195c..5779b9f9d 100644
--- a/hphp/system/lib/gen_systemlib.php
+++ b/hphp/system/lib/gen_systemlib.php
@@ -10,12 +10,15 @@ $outputPath = $argv[1];
 function processPhpFile($phpfile, $systemlib_php) {
   $firstchar = true;
   $contents = file_get_contents($phpfile);
-  $i = 0;
-  $k = strpos($contents, "\n") + 1;
-  $header = trim(substr($contents, 0, $k));
-  if ($header !== "<?php") {
-    echo "ERROR: Unexpected header in file $phpfile\n";
-    throw new Exception("Unexpected header in file $phpfile");
+  if (preg_match('/\.hhas$/', $phpfile)) {
+    $k = 0;
+  } else {
+    $k = strpos($contents, "\n") + 1;
+    $header = trim(substr($contents, 0, $k));
+    if ($header !== "<?php") {
+      echo "ERROR: Unexpected header in file $phpfile\n";
+      throw new Exception("Unexpected header in file $phpfile");
+    }
   }
   fwrite($systemlib_php, substr($contents, $k));
 }
@@ -24,7 +27,7 @@ function populatePhpFiles($input_files) {
   $php_files = array();
   foreach ($input_files as $file) {
     $key = strtolower(basename($file));
-    if (!preg_match('/\.php$/', $file)) {
+    if (!preg_match('/\.(php|hhas)$/', $file)) {
       $errMsg = "ERROR: Encountered non-php file ($file)";
       echo $errMsg . "\n";
       throw new Exception($errMsg);
@@ -66,11 +69,20 @@ function genSystemlib($input_files) {
         unset($phpfiles[$initialFile]);
       }
     }
-    foreach ($phpfiles as $phpfile) {
-      processPhpFile($phpfile, $systemlib_php);
+    foreach ($phpfiles as $key => $phpfile) {
+      if (preg_match('/\.php$/', $phpfile)) {
+        processPhpFile($phpfile, $systemlib_php);
+        unset($phpfiles[$key]);
+      }
     }
     fwrite($systemlib_php, "\n");
-
+    if (count($phpfiles)) {
+      fwrite($systemlib_php, "\n\n<?hhas\n\n");
+      foreach ($phpfiles as $key => $phpfile) {
+        processPhpFile($phpfile, $systemlib_php);
+        unset($phpfiles[$key]);
+      }
+    }
     fclose($systemlib_php);
     $systemlib_php = null;
     chmod($systemlib_php_tempnam, 0644);
diff --git a/hphp/system/lib/systemlib.cpp b/hphp/system/lib/systemlib.cpp
index 5e9ab13b3..56980bafa 100644
--- a/hphp/system/lib/systemlib.cpp
+++ b/hphp/system/lib/systemlib.cpp
@@ -34,6 +34,7 @@ namespace HPHP {
 
 bool SystemLib::s_inited = false;
 HPHP::Unit* SystemLib::s_unit = nullptr;
+HPHP::Unit* SystemLib::s_hhas_unit = nullptr;
 HPHP::Unit* SystemLib::s_nativeFuncUnit = nullptr;
 HPHP::Unit* SystemLib::s_nativeClassUnit = nullptr;
 HPHP::Func* SystemLib::s_nullFunc = nullptr;
diff --git a/hphp/system/lib/systemlib.h b/hphp/system/lib/systemlib.h
index 925736348..4eb48f618 100644
--- a/hphp/system/lib/systemlib.h
+++ b/hphp/system/lib/systemlib.h
@@ -73,6 +73,7 @@ class SystemLib {
  public:
   static bool s_inited;
   static HPHP::Unit* s_unit;
+  static HPHP::Unit* s_hhas_unit;
   static HPHP::Unit* s_nativeFuncUnit;
   static HPHP::Unit* s_nativeClassUnit;
 
diff --git a/hphp/test/quick/Setprofile.php.expectf b/hphp/test/quick/Setprofile.php.expectf
index c914b27f7..ca54ce84c 100644
--- a/hphp/test/quick/Setprofile.php.expectf
+++ b/hphp/test/quick/Setprofile.php.expectf
@@ -198,11 +198,7 @@ array(3) {
       [0]=>
       array(4) {
         [0]=>
-        array(6) {
-          ["file"]=>
-          string(%d) "%S"
-          ["line"]=>
-          int(42)
+        array(4) {
           ["function"]=>
           string(9) "initTrace"
           ["class"]=>