Improve side exits from HHIR to tx64-generated translations

When a side exit from an HHIR-generated translation was taken, a
BIND_JMP service request was being used, which would make it create a
new HHIR translation at the exiting bytecode. This translation would
then generate a REQ_RETRANSLATE_NO_IR to produce a tx64 translation.

This diff avoids the useless HHIR translation generated for the
exiting bytecode in case no translation for this bytecode exists when
the service request is made.  This is done through a new
BIND_JMP_NO_IR request that directly generates a translation for the
exiting bytecode without HHIR if no translation for the SrcKey exists.

src/runtime/vm/translator/abi-x64.h

@@ -166,6 +166,13 @@ const PhysReg serviceReqArgRegs[] = {
   REQ(BIND_JMPCC_SECOND) \
   REQ(BIND_REQUIRE)      \
   \
+  /*
+   * BIND_JMP_NO_IR is similar to BIND_JMP except that, if a new translation
+   * needs to be generated, it'll force that HHIR is not used.
+   * This is only used when HHIR is turned on.
+   */ \
+  REQ(BIND_JMP_NO_IR)  \
+  \
   /*
    * When all translations don't support the incoming types, a
    * retranslate request is made.
@@ -195,7 +202,11 @@ const PhysReg serviceReqArgRegs[] = {
   REQ(STACK_OVERFLOW) \
   \
   /*
-   * TODO: explain this
+   * When HHIR is in use, this requests a retranslation that does not use HHIR.
+   * This is only used when HHIR is turned on.
+   *
+   * Note that, when EvalJitUseIR is enabled, RETRANSLATE requests will attempt
+   * to use HHIR.
    */ \
   REQ(RETRANSLATE_NO_IR) \
   /*

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

@@ -1640,9 +1640,11 @@ Address CodeGenerator::cgExitTrace(IRInstruction* inst) {
 
   switch(exitType) {
     case TraceExitType::Normal:
-    case TraceExitType::Slow:
       m_tx64->emitBindJmp(outputAsm, destSK);
       break;
+    case TraceExitType::Slow:
+      m_tx64->emitBindJmp(outputAsm, destSK, REQ_BIND_JMP_NO_IR);
+      break;
     case TraceExitType::SlowNoProgress:
       m_tx64->emitReqRetransNoIR(outputAsm, destSK);
       break;

src/runtime/vm/translator/translator-x64.cpp

@@ -1181,7 +1181,8 @@ reqName(int req) {
  * a translation.
  */
 TCA
-TranslatorX64::getTranslation(const SrcKey *sk, bool align) {
+TranslatorX64::getTranslation(const SrcKey *sk, bool align,
+                              bool forceNoHHIR /* = false */) {
   curFunc()->validate();
   SKTRACE(2, *sk, "getTranslation: curUnit %s funcId %llx offset %d\n",
           curUnit()->filepath()->data(),
@@ -1222,7 +1223,8 @@ TranslatorX64::getTranslation(const SrcKey *sk, bool align) {
       // Since we are holding the write lease, we know that sk is properly
       // initialized, except that it has no translations (due to
       // replaceOldTranslations)
-      return retranslate(*sk, align, RuntimeOption::EvalJitUseIR);
+      return retranslate(*sk, align,
+                         RuntimeOption::EvalJitUseIR && !forceNoHHIR);
     }
   }
 
@@ -1241,7 +1243,7 @@ TranslatorX64::getTranslation(const SrcKey *sk, bool align) {
 
   ASSERT(getTransRec(start)->kind == TransAnchor);
 
-  return retranslate(*sk, align, RuntimeOption::EvalJitUseIR);
+  return retranslate(*sk, align, RuntimeOption::EvalJitUseIR && !forceNoHHIR);
 }
 
 TCA
@@ -2251,8 +2253,9 @@ void TranslatorX64::drawCFG(std::ofstream& out) const {
  *   u:dest from toSmash.
  */
 TCA
-TranslatorX64::bindJmp(TCA toSmash, SrcKey destSk, bool isAddr) {
-  TCA tDest = getTranslation(&destSk, false);
+TranslatorX64::bindJmp(TCA toSmash, SrcKey destSk, bool isAddr,
+                       bool forceNoHHIR /* = false */) {
+  TCA tDest = getTranslation(&destSk, false, forceNoHHIR);
   if (!tDest) return NULL;
   LeaseHolder writer(s_writeLease);
   if (!writer) return tDest;
@@ -2869,14 +2872,16 @@ TranslatorX64::enterTC(SrcKey sk) {
 
       case REQ_BIND_SIDE_EXIT:
       case REQ_BIND_JMP:
+      case REQ_BIND_JMP_NO_IR:
       case REQ_BIND_ADDR: {
         TCA toSmash = (TCA)args[0];
         Offset off = args[1];
         sk = SrcKey(curFunc(), off);
         if (requestNum == REQ_BIND_SIDE_EXIT) {
           SKTRACE(3, sk, "side exit taken!\n");
         }
-        start = bindJmp(toSmash, sk, requestNum == REQ_BIND_ADDR);
+        start = bindJmp(toSmash, sk, requestNum == REQ_BIND_ADDR,
+                        requestNum == REQ_BIND_JMP_NO_IR);
       } break;
 
       case REQ_BIND_JMPCC_FIRST: {

src/runtime/vm/translator/translator-x64.h

@@ -701,13 +701,13 @@ PSEUDOINSTRS
   static bool isSmashable(Asm &a, int nBytes);
   static void smash(Asm &a, TCA src, TCA dest);
 
-  TCA getTranslation(const SrcKey *sk, bool align);
+  TCA getTranslation(const SrcKey *sk, bool align, bool forceNoHHIR = false);
   TCA retranslate(SrcKey sk, bool align, bool useHHIR);
   TCA retranslateOpt(TransID transId, bool align);
   TCA retranslateAndPatchNoIR(SrcKey sk,
                               bool   align,
                               TCA    toSmash);
-  TCA bindJmp(TCA toSmash, SrcKey dest, bool isAddr = false);
+  TCA bindJmp(TCA toSmash, SrcKey dest, bool isAddr, bool forceNoHHIR = false);
   TCA bindJmpccFirst(TCA toSmash,
                      Offset offTrue, Offset offFalse,
                      bool toTake,