Optimize ContSend, ContRaise, and ContNext for m_received being null

Make sure m_received is always null at ContSend, ContRaise,
and ContNext, and this information to generate better code for these
bytecode instructions.

hphp/doc/ir.specification

@@ -1389,6 +1389,10 @@ DbgAssertPtr S0:PtrToGen
   the same logic as DbgAssertRefCount.  Internally this uses always_assert();
   failures cause an abort (whatever always_assert does).
 
+DbgAssertType<T> S0:Cell
+
+  Assert that S0 is of type T at runtime.  If the assertion fails,
+  execution is aborted via a hardware exception.
 
 18. Iterators
 

hphp/runtime/ext/ext_continuation.cpp

@@ -141,7 +141,9 @@ void c_Continuation::t_raise(CVarRef v) {
 void c_Continuation::t_raised() {
   if (m_should_throw) {
     m_should_throw = false;
-    throw_exception(m_received);
+    Variant e(Variant::nullInit);
+    m_received.swap(e);
+    throw_exception(e);
   }
 }
 

hphp/runtime/vm/bytecode.cpp

@@ -7128,7 +7128,7 @@ inline void OPTBLD_INLINE VMExecutionContext::iopContReceive(PC& pc) {
   TypedValue* fr = cont->m_received.asTypedValue();
   TypedValue* to = m_stack.allocTV();
   memcpy(to, fr, sizeof(TypedValue));
-  tvWriteUninit(fr);
+  tvWriteNull(fr);
 }
 
 inline void OPTBLD_INLINE VMExecutionContext::iopContRetC(PC& pc) {

hphp/runtime/vm/translator/hopt/codegen.cpp

@@ -5059,6 +5059,12 @@ void traceCallback(ActRec* fp, Cell* sp, int64_t pcOff, void* rip) {
   checkFrame(fp, sp, /*checkLocals*/true);
 }
 
+void CodeGenerator::cgDbgAssertType(IRInstruction* inst) {
+  ConditionCode cc = emitTypeTest(inst->getTypeParam(),
+                                  m_regs[inst->getSrc(0)].getReg(1), true);
+  ifThen(m_as, cc, [&] { m_as.ud2(); });
+}
+
 void CodeGenerator::cgVerifyParamCls(IRInstruction* inst) {
   SSATmp* objClass = inst->getSrc(0);
   assert(!objClass->isConst());

hphp/runtime/vm/translator/hopt/hhbctranslator.cpp

@@ -1003,7 +1003,7 @@ void HhbcTranslator::emitContReceive() {
   gen(ContRaiseCheck, getExitSlowTrace(), cont);
   auto const valOffset = cns(CONTOFF(m_received));
   push(gen(LdProp, Type::Cell, cont, valOffset));
-  gen(StProp, cont, valOffset, m_tb->genDefUninit());
+  gen(StProp, cont, valOffset, m_tb->genDefNull());
 }
 
 void HhbcTranslator::emitContRetC() {
@@ -1026,10 +1026,11 @@ void HhbcTranslator::emitContNext() {
   assert(getCurClass());
   SSATmp* cont = gen(LdThis, m_tb->getFp());
   gen(ContPreNext, getExitSlowTrace(), cont);
-
-  auto const oldVal = gen(LdProp, Type::Cell, cont, cns(CONTOFF(m_received)));
-  gen(StProp, cont, cns(CONTOFF(m_received)), m_tb->genDefInitNull());
-  gen(DecRef, oldVal);
+  if (RuntimeOption::EvalHHIRGenerateAsserts) {
+    // We're guaranteed to have a Null in m_received at this point
+    auto const oldVal = gen(LdProp, Type::Cell, cont, cns(CONTOFF(m_received)));
+    gen(DbgAssertType, Type::InitNull, oldVal);
+  }
 }
 
 void HhbcTranslator::emitContSendImpl(bool raise) {
@@ -1037,16 +1038,16 @@ void HhbcTranslator::emitContSendImpl(bool raise) {
   SSATmp* cont = gen(LdThis, m_tb->getFp());
   gen(ContStartedCheck, getExitSlowTrace(), cont);
   gen(ContPreNext, getExitSlowTrace(), cont);
-
   gen(AssertLoc, Type::Cell, LocalId(0), m_tb->getFp());
   auto const newVal = gen(IncRef, ldLoc(0));
-  auto const oldVal = gen(LdProp, Type::Cell, cont, cns(CONTOFF(m_received)));
+  if (RuntimeOption::EvalHHIRGenerateAsserts) {
+    // We're guaranteed to have a Null in m_received at this point
+    auto const oldVal = gen(LdProp, Type::Cell, cont, cns(CONTOFF(m_received)));
+    gen(DbgAssertType, Type::InitNull, oldVal);
+  }
   gen(StProp, cont, cns(CONTOFF(m_received)), newVal);
-  gen(DecRef, oldVal);
   if (raise) {
-    gen(
-      StRaw, cont, cns(RawMemSlot::ContShouldThrow), cns(true)
-    );
+    gen(StRaw, cont, cns(RawMemSlot::ContShouldThrow), cns(true));
   }
 }
 

hphp/runtime/vm/translator/hopt/ir.h

@@ -587,6 +587,7 @@ O(ArrayIdx,                    D(Cell), C(TCA) S(Arr) S(Int,Str) S(Cell),     \
                                                         E|N|CRc|PRc|Refs|Mem) \
 O(DbgAssertRefCount,                ND, SUnk,                            N|E) \
 O(DbgAssertPtr,                     ND, S(PtrToGen),                     N|E) \
+O(DbgAssertType,                    ND, S(Cell),                           E) \
 O(Nop,                              ND, NA,                               NF) \
 /* */