Disable left-trimming when optimizing compile jobs exist

... as these jobs may have references to the array backing store and
expect them to stay valid.


Bug: chromium:1211215
Change-Id: Ia48519e993306223afab8d11a94d6d8fc150a11d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928502Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74901}

src/compiler-dispatcher/optimizing-compile-dispatcher.cc

@@ -46,7 +46,6 @@ class OptimizingCompileDispatcher::CompileTask : public CancelableTask {
         worker_thread_runtime_call_stats_(
             isolate->counters()->worker_thread_runtime_call_stats()),
         dispatcher_(dispatcher) {
-    base::MutexGuard lock_guard(&dispatcher_->ref_count_mutex_);
     ++dispatcher_->ref_count_;
   }
 
@@ -98,12 +97,7 @@ class OptimizingCompileDispatcher::CompileTask : public CancelableTask {
 };
 
 OptimizingCompileDispatcher::~OptimizingCompileDispatcher() {
-#ifdef DEBUG
-  {
-    base::MutexGuard lock_guard(&ref_count_mutex_);
-    DCHECK_EQ(0, ref_count_);
-  }
-#endif
+  DCHECK_EQ(0, ref_count_);
   DCHECK_EQ(0, input_queue_length_);
   DeleteArray(input_queue_);
 }
@@ -219,6 +213,14 @@ void OptimizingCompileDispatcher::InstallOptimizedFunctions() {
   }
 }
 
+bool OptimizingCompileDispatcher::HasJobs() {
+  DCHECK_EQ(ThreadId::Current(), isolate_->thread_id());
+  // Note: This relies on {output_queue_} being mutated by a background thread
+  // only when {ref_count_} is not zero. Also, {ref_count_} is never incremented
+  // by a background thread.
+  return !(ref_count_ == 0 && output_queue_.empty());
+}
+
 void OptimizingCompileDispatcher::QueueForOptimization(
     OptimizedCompilationJob* job) {
   DCHECK(IsQueueAvailable());

src/compiler-dispatcher/optimizing-compile-dispatcher.h

@@ -52,6 +52,9 @@ class V8_EXPORT_PRIVATE OptimizingCompileDispatcher {
 
   static bool Enabled() { return FLAG_concurrent_recompilation; }
 
+  // This method must be called on the main thread.
+  bool HasJobs();
+
  private:
   class CompileTask;
 
@@ -88,7 +91,7 @@ class V8_EXPORT_PRIVATE OptimizingCompileDispatcher {
 
   int blocked_jobs_;
 
-  int ref_count_;
+  std::atomic<int> ref_count_;
   base::Mutex ref_count_mutex_;
   base::ConditionVariable ref_count_zero_;
 

src/heap/heap.cc

@@ -23,6 +23,7 @@
 #include "src/codegen/compilation-cache.h"
 #include "src/common/assert-scope.h"
 #include "src/common/globals.h"
+#include "src/compiler-dispatcher/optimizing-compile-dispatcher.h"
 #include "src/debug/debug.h"
 #include "src/deoptimizer/deoptimizer.h"
 #include "src/execution/isolate-utils-inl.h"
@@ -3163,6 +3164,9 @@ bool Heap::CanMoveObjectStart(HeapObject object) {
 
   if (IsLargeObject(object)) return false;
 
+  // Compilation jobs may have references to the object.
+  if (isolate()->optimizing_compile_dispatcher()->HasJobs()) return false;
+
   // We can move the object start if the page was already swept.
   return Page::FromHeapObject(object)->SweepingDone();
 }