Fix printing of ActRecInfo

If it's an AR for a constructor, the high-order bit is set, and it gets printed
as -2 billion. This is confusing; at first I thought it was a bug. Print it out
in a readable way.

hphp/runtime/vm/bytecode.h

@@ -271,11 +271,11 @@ struct ActRec {
    */
 
   int32_t numArgs() const {
-    return m_numArgsAndCtorFlag & ~(1u << 31);
+    return decodeNumArgs(m_numArgsAndCtorFlag).first;
   }
 
   bool isFromFPushCtor() const {
-    return m_numArgsAndCtorFlag & (1u << 31);
+    return decodeNumArgs(m_numArgsAndCtorFlag).second;
   }
 
   static inline uint32_t
@@ -284,6 +284,11 @@ struct ActRec {
     return numArgs | (isFPushCtor << 31);
   }
 
+  static inline std::pair<uint32_t,bool>
+  decodeNumArgs(uint32_t numArgs) {
+    return { numArgs & ~(1u << 31), numArgs & (1u << 31) };
+  }
+
   void initNumArgs(uint32_t numArgs, bool isFPushCtor = false) {
     m_numArgsAndCtorFlag = encodeNumArgs(numArgs, isFPushCtor);
   }

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

@@ -201,7 +201,10 @@ struct ActRecInfo : IRExtraData {
   int32_t numArgs;
 
   std::string show() const {
-    return folly::to<std::string>(numArgs, invName ? " M" : "");
+    auto numArgsAndCtorFlag = ActRec::decodeNumArgs(numArgs);
+    return folly::to<std::string>(numArgsAndCtorFlag.first,
+                                  numArgsAndCtorFlag.second ? ",ctor" : "",
+                                  invName ? " M" : "");
   }
 };
 

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

@@ -1579,7 +1579,7 @@ void HhbcTranslator::emitFPushCtorCommon(SSATmp* cls,
     fn = gen(LdClsCtor, cls);
   }
   SSATmp* obj2 = gen(IncRef, obj);
-  int32_t numArgsAndCtorFlag = numParams | (1 << 31);
+  int32_t numArgsAndCtorFlag = ActRec::encodeNumArgs(numParams, true);
   emitFPushActRec(fn, obj2, numArgsAndCtorFlag, nullptr);
 }