Commit 0b5b8066 authored by Ted Campbell's avatar Ted Campbell
Browse files

Bug 1600705 - Avoid storing nullptr closed-over-bindings data for LazyScripts. r=jorendorff 

The closed-over-binding data is generated for each scope with a nullptr
delimiter per scope. This is wasteful when there are no closed-over bindings.
This patch removes trailing nullptr entries and for leaf-functions may result
in the script-data allocation being avoided altogether.

The FullParseHandler::nextLazyClosedOverBinding() will return nullptr after
the end of the gcthings data so that the delazification process is otherwise
unchanged.

Differential Revision: https://phabricator.services.mozilla.com/D57885

--HG--
extra : moz-landing-system : lando
parent e6a17a63

js/src/frontend/FullParseHandler.h

+8 −2
Original line number Diff line number Diff line
@@ -1056,10 +1056,16 @@ class FullParseHandler { 
                .as<JSFunction>();

  }

  JSAtom* nextLazyClosedOverBinding() {

    auto gcthings = lazyOuterFunction_->gcthings();



    // Trailing nullptrs were elided in PerHandlerParser::finishFunction().

    if (lazyClosedOverBindingIndex >= gcthings.Length()) {

      return nullptr;

    }



    // These entries are either JSAtom* or nullptr, so use the 'asCell()'

    // accessor which is faster.

    gc::Cell* cell =

        lazyOuterFunction_->gcthings()[lazyClosedOverBindingIndex++].asCell();

    gc::Cell* cell = gcthings[lazyClosedOverBindingIndex++].asCell();

    MOZ_ASSERT_IF(cell, cell->is<JSAtom>());

    return static_cast<JSAtom*>(cell);

  }

js/src/frontend/Parser.cpp

+9 −0
Original line number Diff line number Diff line
@@ -1739,6 +1739,15 @@ bool PerHandlerParser<SyntaxParseHandler>::finishFunction( 
    return false;

  }



  // Elide nullptr sentinels from end of binding list. These are inserted for

  // each scope regardless of if any bindings are actually closed over.

  {

    AtomVector& COB = pc_->closedOverBindingsForLazy();

    while (!COB.empty() && (COB.back() == nullptr)) {

      COB.popBack();

    }

  }



  // There are too many bindings or inner functions to be saved into the

  // LazyScript. Do a full parse.

  if (pc_->closedOverBindingsForLazy().length() >=

js/src/vm/JSScript.cpp

+0 −2
Original line number Diff line number Diff line
@@ -4210,8 +4210,6 @@ PrivateScriptData::PrivateScriptData(uint32_t ngcthings) 


  // Layout and initialize the gcthings array.

  {

    MOZ_ASSERT(ngcthings > 0);



    static_assert(alignof(PrivateScriptData) >= alignof(JS::GCCellPtr),

                  "Incompatible alignment");

    initElements<JS::GCCellPtr>(cursor, ngcthings);