Bug 1565945 - Compute LazyScript::TreatAsRunOnce flag the same as JSScript. r=jandem

This has equivalent overall behavior as before, but computes the
precise LazyScript::TreatAsRunOnce flag rather than checking the extra
conditions during delazification. The heuristics can be computed using
just the LazyScript information so the result is the same.

Differential Revision: https://phabricator.services.mozilla.com/D38025

--HG--
extra : moz-landing-system : lando

js/src/frontend/BytecodeCompiler.cpp

@@ -998,6 +998,8 @@ static bool CompileLazyFunctionImpl(JSContext* cx, Handle<LazyScript*> lazy,
     return false;
   }
 
+  MOZ_ASSERT(lazy->treatAsRunOnce() == script->treatAsRunOnce());
+
   delazificationCompletion.complete();
   assertException.reset();
   return true;

js/src/frontend/BytecodeEmitter.cpp

@@ -2219,18 +2219,14 @@ MOZ_NEVER_INLINE bool BytecodeEmitter::emitSwitch(SwitchStatement* switchStmt) {
 }
 
 bool BytecodeEmitter::isRunOnceLambda() {
-  // The run once lambda flags set by the parser are approximate, and we look
-  // at properties of the function itself before deciding to emit a function
-  // as a run once lambda.
-
-  if (!(parent && parent->emittingRunOnceLambda) &&
-      (emitterMode != LazyFunction || !lazyScript->treatAsRunOnce())) {
-    return false;
+  if (lazyScript) {
+    MOZ_ASSERT_IF(sc->asFunctionBox()->shouldSuppressRunOnce(),
+                  !lazyScript->treatAsRunOnce());
+    return lazyScript->treatAsRunOnce();
   }
 
-  FunctionBox* funbox = sc->asFunctionBox();
-  return !funbox->argumentsHasLocalBinding() && !funbox->isGenerator() &&
-         !funbox->isAsync() && !funbox->explicitName();
+  return parent && parent->emittingRunOnceLambda &&
+         !sc->asFunctionBox()->shouldSuppressRunOnce();
 }
 
 bool BytecodeEmitter::allocateResumeIndex(BytecodeOffset offset,

js/src/frontend/FunctionEmitter.cpp

@@ -113,7 +113,7 @@ bool FunctionEmitter::emitLazy() {
   }
 
   funbox_->setEnclosingScopeForInnerLazyFunction(bce_->innermostScope());
-  if (bce_->emittingRunOnceLambda) {
+  if (bce_->emittingRunOnceLambda && !funbox_->shouldSuppressRunOnce()) {
     fun_->lazyScript()->setTreatAsRunOnce();
   }
 

js/src/frontend/SharedContext.h

@@ -553,6 +553,15 @@ class FunctionBox : public ObjectBox, public SharedContext {
     return !strict() && hasSimpleParameterList();
   }
 
+  bool shouldSuppressRunOnce() const {
+    // These heuristics suppress the run-once optimization if we expect that
+    // script-cloning will have more impact than TI type-precision would gain.
+    //
+    // See also: Bug 864218
+    return explicitName() || argumentsHasLocalBinding() || isGenerator() ||
+           isAsync();
+  }
+
   // Return whether this or an enclosing function is being parsed and
   // validated as asm.js. Note: if asm.js validation fails, this will be false
   // while the function is being reparsed. This flag can be used to disable