[heap] Move write rx->rw->rx transition in Scavenger to parallel jobs.

Bug: chromium:774108,v8:6792
Change-Id: I1bb1e47ebf16e55e63ed4bbd9ce9eb0c01e8b754
Reviewed-on: https://chromium-review.googlesource.com/735440
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48965}

src/heap/heap-inl.h

@@ -605,16 +605,19 @@ CodeSpaceMemoryModificationScope::~CodeSpaceMemoryModificationScope() {
 
 CodePageMemoryModificationScope::CodePageMemoryModificationScope(
     MemoryChunk* chunk)
-    : chunk_(chunk) {
-  if (FLAG_write_protect_code_memory &&
+    : chunk_(chunk),
+      scope_active_(FLAG_write_protect_code_memory &&
                     chunk_->IsFlagSet(MemoryChunk::IS_EXECUTABLE)) {
+  if (scope_active_) {
+    DCHECK(chunk_->owner()->identity() == CODE_SPACE ||
+           (chunk_->owner()->identity() == LO_SPACE &&
+            chunk_->IsFlagSet(MemoryChunk::IS_EXECUTABLE)));
     chunk_->SetReadAndWritable();
   }
 }
 
 CodePageMemoryModificationScope::~CodePageMemoryModificationScope() {
-  if (FLAG_write_protect_code_memory &&
-      chunk_->IsFlagSet(MemoryChunk::IS_EXECUTABLE)) {
+  if (scope_active_) {
     chunk_->SetReadAndExecutable();
   }
 }

src/heap/heap.cc

@@ -1915,7 +1915,6 @@ void Heap::Scavenge() {
   }
 
   {
-    CodeSpaceMemoryModificationScope code_modification(this);
     MarkCompactCollector::Sweeper* sweeper =
         &mark_compact_collector()->sweeper();
     // Pause the concurrent sweeper.

src/heap/heap.h

@@ -2534,6 +2534,7 @@ class CodePageMemoryModificationScope {
 
  private:
   MemoryChunk* chunk_;
+  bool scope_active_;
 };
 
 // Visitor class to verify interior pointers in spaces that do not contain

src/heap/scavenger.cc

@@ -98,6 +98,7 @@ void Scavenger::AddPageToSweeperIfNecessary(MemoryChunk* page) {
 void Scavenger::ScavengePage(MemoryChunk* page) {
   PreferredSweepingPage(page);
 
+  CodePageMemoryModificationScope memory_modification_scope(page);
   RememberedSet<OLD_TO_NEW>::Iterate(
       page,
       [this](Address addr) { return CheckAndScavengeObject(heap_, addr); },