8253037: G1: Improve check for string dedup

Combine dedup enabled and is_string into a single test, using the already in-hand klass of the object.

Reviewed-by: ayang, tschatzl

Author: Kim Barrett

src/hotspot/share/gc/g1/g1FullGCMarker.inline.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,7 @@
 #ifndef SHARE_GC_G1_G1FULLGCMARKER_INLINE_HPP
 #define SHARE_GC_G1_G1FULLGCMARKER_INLINE_HPP
 
+#include "classfile/javaClasses.inline.hpp"
 #include "gc/g1/g1Allocator.inline.hpp"
 #include "gc/g1/g1ConcurrentMarkBitMap.inline.hpp"
 #include "gc/g1/g1FullGCMarker.hpp"
@@ -57,7 +58,8 @@ inline bool G1FullGCMarker::mark_object(oop obj) {
   }
 
   // Check if deduplicatable string.
-  if (G1StringDedup::is_enabled()) {
+  if (G1StringDedup::is_enabled() &&
+      java_lang_String::is_instance_inlined(obj)) {
     G1StringDedup::enqueue_from_mark(obj, _worker_id);
   }
   return true;

src/hotspot/share/gc/g1/g1ParScanThreadState.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/systemDictionary.hpp"
 #include "gc/g1/g1Allocator.inline.hpp"
 #include "gc/g1/g1CollectedHeap.inline.hpp"
 #include "gc/g1/g1CollectionSet.hpp"
@@ -75,10 +76,17 @@ G1ParScanThreadState::G1ParScanThreadState(G1CollectedHeap* g1h,
     _old_gen_is_full(false),
     _partial_objarray_chunk_size(ParGCArrayScanChunk),
     _partial_array_stepper(n_workers),
+    _string_klass_or_null(G1StringDedup::is_enabled()
+                          ? SystemDictionary::String_klass()
+                          : nullptr),
     _num_optional_regions(optional_cset_length),
     _numa(g1h->numa()),
     _obj_alloc_stat(NULL)
 {
+  // Verify klass comparison with _string_klass_or_null is sufficient
+  // to determine whether dedup is enabled and the object is a String.
+  assert(SystemDictionary::String_klass()->is_final(), "precondition");
+
   // We allocate number of young gen regions in the collection set plus one
   // entries, since entry 0 keeps track of surviving bytes for non-young regions.
   // We also add a few elements at the beginning and at the end in
@@ -510,7 +518,10 @@ oop G1ParScanThreadState::do_copy_to_survivor_space(G1HeapRegionAttr const regio
       return obj;
     }
 
-    if (G1StringDedup::is_enabled()) {
+    // StringDedup::is_enabled() and java_lang_String::is_instance_inline
+    // test of the obj, combined into a single comparison, using the klass
+    // already in hand and avoiding the null check in is_instance.
+    if (klass == _string_klass_or_null) {
       const bool is_from_young = region_attr.is_young();
       const bool is_to_young = dest_attr.is_young();
       assert(is_from_young == from_region->is_young(),

src/hotspot/share/gc/g1/g1ParScanThreadState.hpp

@@ -42,6 +42,7 @@ class G1OopStarChunkedList;
 class G1PLABAllocator;
 class G1EvacuationRootClosures;
 class HeapRegion;
+class Klass;
 class outputStream;
 
 class G1ParScanThreadState : public CHeapObj<mtGC> {
@@ -83,6 +84,8 @@ class G1ParScanThreadState : public CHeapObj<mtGC> {
   // Size (in elements) of a partial objArray task chunk.
   int _partial_objarray_chunk_size;
   PartialArrayTaskStepper _partial_array_stepper;
+  // Used to check whether string dedup should be applied to an object.
+  Klass* _string_klass_or_null;
 
   G1RedirtyCardsQueue& redirty_cards_queue()     { return _rdcq; }
   G1CardTable* ct()                              { return _ct; }

src/hotspot/share/gc/g1/g1StringDedup.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -41,29 +41,23 @@ void G1StringDedup::initialize() {
 }
 
 bool G1StringDedup::is_candidate_from_mark(oop obj) {
-  if (java_lang_String::is_instance_inlined(obj)) {
-    bool from_young = G1CollectedHeap::heap()->heap_region_containing(obj)->is_young();
-    if (from_young && obj->age() < StringDeduplicationAgeThreshold) {
-      // Candidate found. String is being evacuated from young to old but has not
-      // reached the deduplication age threshold, i.e. has not previously been a
-      // candidate during its life in the young generation.
-      return true;
-    }
-  }
-
-  // Not a candidate
-  return false;
+  bool from_young = G1CollectedHeap::heap()->heap_region_containing(obj)->is_young();
+  // Candidate if string is being evacuated from young to old but has not
+  // reached the deduplication age threshold, i.e. has not previously been a
+  // candidate during its life in the young generation.
+  return from_young && (obj->age() < StringDeduplicationAgeThreshold);
 }
 
 void G1StringDedup::enqueue_from_mark(oop java_string, uint worker_id) {
   assert(is_enabled(), "String deduplication not enabled");
+  assert(java_lang_String::is_instance(java_string), "not a String");
   if (is_candidate_from_mark(java_string)) {
     G1StringDedupQueue::push(worker_id, java_string);
   }
 }
 
 bool G1StringDedup::is_candidate_from_evacuation(bool from_young, bool to_young, oop obj) {
-  if (from_young && java_lang_String::is_instance_inlined(obj)) {
+  if (from_young) {
     if (to_young && obj->age() == StringDeduplicationAgeThreshold) {
       // Candidate found. String is being evacuated from young to young and just
       // reached the deduplication age threshold.
@@ -83,6 +77,7 @@ bool G1StringDedup::is_candidate_from_evacuation(bool from_young, bool to_young,
 
 void G1StringDedup::enqueue_from_evacuation(bool from_young, bool to_young, uint worker_id, oop java_string) {
   assert(is_enabled(), "String deduplication not enabled");
+  assert(java_lang_String::is_instance(java_string), "not a String");
   if (is_candidate_from_evacuation(from_young, to_young, java_string)) {
     G1StringDedupQueue::push(worker_id, java_string);
   }

src/hotspot/share/gc/g1/g1StringDedup.hpp

@@ -65,8 +65,8 @@ class G1StringDedup : public StringDedup {
 
   // Candidate selection policies, returns true if the given object is
   // candidate for string deduplication.
-  static bool is_candidate_from_mark(oop obj);
-  static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
+  static bool is_candidate_from_mark(oop java_string);
+  static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop java_string);
 
 public:
   // Initialize string deduplication.
@@ -75,6 +75,7 @@ class G1StringDedup : public StringDedup {
   // Enqueues a deduplication candidate for later processing by the deduplication
   // thread. Before enqueuing, these functions apply the appropriate candidate
   // selection policy to filters out non-candidates.
+  // Precondition for both is that java_string is a String.
   static void enqueue_from_mark(oop java_string, uint worker_id);
   static void enqueue_from_evacuation(bool from_young, bool to_young,
                                       unsigned int queue, oop java_string);