src/share/vm/opto/graphKit.cpp
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@@ -3825,10 +3825,118 @@
// Final sync IdealKit and GraphKit.
final_sync(ideal);
}
+/*
+ * Determine if the G1 pre-barrier can be removed. The pre-barrier is
+ * required by SATB to make sure all objects live at the start of the
+ * marking are kept alive, all reference updates need to any previous
+ * reference stored before writing.
+ *
+ * If the previous value is NULL there is no need to save the old value.
+ * References that are NULL are filtered during runtime by the barrier
+ * code to avoid unnecessary queuing.
+ *
+ * However in the case of newly allocated objects it might be possible to
+ * prove that the reference about to be overwritten is NULL during compile
+ * time and avoid adding the barrier code completely.
+ *
+ * The compiler needs to determine that the object in which a field is about
+ * to be written is newly allocated, and that no prior store to the same field
+ * has happened since the allocation.
+ *
+ * Returns true iff the pre-barrier can be removed
+ */
+bool GraphKit::g1_can_remove_pre_barrier(PhaseTransform* phase, Node* adr,
+ BasicType bt, uint adr_idx) {
+ intptr_t offset = 0;
+ Node* base = AddPNode::Ideal_base_and_offset(adr, phase, offset);
+ AllocateNode* alloc = AllocateNode::Ideal_allocation(base, phase);
+
+ if (offset == Type::OffsetBot) {
+ return false; // cannot unalias unless there are precise offsets
+ }
+
+ if (alloc == NULL) {
+ return false; // No allocation found
+ }
+
+ intptr_t size_in_bytes = type2aelembytes(bt);
+
+ Node* mem = memory(adr_idx); // start searching here...
+
+ for (int cnt = 0; cnt < 50; cnt++) {
+
+ if (mem->is_Store()) {
+
+ Node* st_adr = mem->in(MemNode::Address);
+ intptr_t st_offset = 0;
+ Node* st_base = AddPNode::Ideal_base_and_offset(st_adr, phase, st_offset);
+
+ if (st_base == NULL) {
+ break; // inscrutable pointer
+ }
+
+ // Break we have found a store with same base and offset as ours so break
+ if (st_base == base && st_offset == offset) {
+ break;
+ }
+
+ if (st_offset != offset && st_offset != Type::OffsetBot) {
+ const int MAX_STORE = BytesPerLong;
+ if (st_offset >= offset + size_in_bytes ||
+ st_offset <= offset - MAX_STORE ||
+ st_offset <= offset - mem->as_Store()->memory_size()) {
+ // Success: The offsets are provably independent.
+ // (You may ask, why not just test st_offset != offset and be done?
+ // The answer is that stores of different sizes can co-exist
+ // in the same sequence of RawMem effects. We sometimes initialize
+ // a whole 'tile' of array elements with a single jint or jlong.)
+ mem = mem->in(MemNode::Memory);
+ continue; // advance through independent store memory
+ }
+ }
+
+ if (st_base != base &&
+ MemNode::detect_ptr_independence(base, alloc, st_base,
+ AllocateNode::Ideal_allocation(st_base, phase),
+ phase)) {
+ // Success: The bases are provably independent.
+ mem = mem->in(MemNode::Memory);
+ continue; // advance through independent store memory
+ }
+ } else if (mem->is_Proj() && mem->in(0)->is_Initialize()) {
+
+ InitializeNode* st_init = mem->in(0)->as_Initialize();
+ AllocateNode* st_alloc = st_init->allocation();
+
+ if (st_alloc == NULL) {
+ break; // something degenerated
+ }
+
+ // Make sure that we are looking at the same allocation site
+ if (alloc == st_alloc) {
+ // Check that the initialization is storing NULL so that no previous store
+ // has been moved up and directly write a reference
+ Node* captured_store = st_init->find_captured_store(offset, type2aelembytes(T_OBJECT), phase);
+ if (captured_store != NULL && captured_store != st_init->zero_memory()) {
+ return false;
+ }
+
+ return true;
+ }
+ }
+
+ // Unless there is an explicit 'continue', we must bail out here,
+ // because 'mem' is an inscrutable memory state (e.g., a call).
+ break;
+ }
+
+ return false;
+}
+
// G1 pre/post barriers
void GraphKit::g1_write_barrier_pre(bool do_load,
Node* obj,
Node* adr,
uint alias_idx,
@@ -3844,10 +3952,16 @@
// We need to generate the load of the previous value
assert(obj != NULL, "must have a base");
assert(adr != NULL, "where are loading from?");
assert(pre_val == NULL, "loaded already?");
assert(val_type != NULL, "need a type");
+
+ if (use_ReduceInitialCardMarks() &&
+ g1_can_remove_pre_barrier(&_gvn, adr, bt, alias_idx)) {
+ return;
+ }
+
} else {
// In this case both val_type and alias_idx are unused.
assert(pre_val != NULL, "must be loaded already");
// Nothing to be done if pre_val is null.
if (pre_val->bottom_type() == TypePtr::NULL_PTR) return;
@@ -3925,10 +4039,85 @@
// Final sync IdealKit and GraphKit.
final_sync(ideal);
}
+/*
+ * G1 similar to any GC with a Young Generation requires a way to keep track of
+ * references from Old Generation to Young Generation to make sure all live
+ * objects are found. G1 also requires to keep track of object references
+ * between different regions to enable evacuation of old regions, which is done
+ * as part of mixed collections. References are tracked in remembered sets and
+ * is continuously updated as reference are written to with the help of the
+ * post-barrier.
+ *
+ * To reduce the number of updates to the remembered set the post-barrier
+ * filters updates to fields in objects located in the Young Generation,
+ * the same region as the reference, when the NULL is being written or
+ * if the card is already marked as dirty by an earlier write.
+ *
+ * Under certain circumstances it is possible to avoid generating the
+ * post-barrier completely if it is possible during compile time to prove
+ * the object is newly allocated and that no safepoint exists between the
+ * allocation and the store.
+ *
+ * In the case of slow allocation the allocation code must handle the barrier
+ * as part of the allocation in the case the allocated object is not located
+ * in the nursery, this would happen for humongous objects. This is similar to
+ * how CMS is required to handle this case, see the comments for the method
+ * CollectedHeap::new_store_pre_barrier and OptoRuntime::new_store_pre_barrier.
+ * A deferred card mark is required for these objects and handled in the above
+ * mentioned methods.
+ *
+ * Returns true iff the post barrier can be removed
+ */
+bool GraphKit::g1_can_remove_post_barrier(PhaseTransform* phase, Node* store,
+ Node* adr) {
+ intptr_t offset = 0;
+ Node* base = AddPNode::Ideal_base_and_offset(adr, phase, offset);
+ AllocateNode* alloc = AllocateNode::Ideal_allocation(base, phase);
+
+ if (offset == Type::OffsetBot) {
+ return false; // cannot unalias unless there are precise offsets
+ }
+
+ if (alloc == NULL) {
+ return false; // No allocation found
+ }
+
+ // Start search from Store node
+ Node* mem = store;
+ for (int cnt = 0; cnt < 50; cnt++) { // While we can dance past unrelated stores...
+
+ if (mem->is_Store()) {
+ // We can walk through all stores as we don't care about the previous value
+ // Walk through the Control instead of the Memory to make sure we detect
+ // if there is another allocation between the store and the allocation
+ // we are looking for as this would imply a safepoint.
+ mem = mem->in(MemNode::Control);
+ continue;
+ } else if (mem->is_Proj() && mem->in(0)->is_Initialize()) {
+
+ InitializeNode* st_init = mem->in(0)->as_Initialize();
+ AllocateNode* st_alloc = st_init->allocation();
+ if (st_alloc == NULL) {
+ break; // something degenerated
+ }
+ // Make sure we are looking at the same allocation
+ if (alloc == st_alloc) {
+ return true;
+ }
+ }
+
+ // Unless there is an explicit 'continue', we must bail out here,
+ // because 'mem' is an inscrutable memory state (e.g., a call).
+ break;
+ }
+
+ return false;
+}
+
//
// Update the card table and add card address to the queue
//
void GraphKit::g1_mark_card(IdealKit& ideal,
Node* card_adr,
@@ -3977,10 +4166,24 @@
assert(t == Type::TOP || t == TypePtr::NULL_PTR, "must be NULL");
// No post barrier if writing NULLx
return;
}
+ if (use_ReduceInitialCardMarks() && obj == just_allocated_object(control())) {
+ // We can skip marks on a freshly-allocated object in Eden.
+ // Keep this code in sync with new_store_pre_barrier() in runtime.cpp.
+ // That routine informs GC to take appropriate compensating steps,
+ // upon a slow-path allocation, so as to make this card-mark
+ // elision safe.
+ return;
+ }
+
+ if (use_ReduceInitialCardMarks() &&
+ g1_can_remove_post_barrier(&_gvn, oop_store, adr)) {
+ return;
+ }
+
if (!use_precise) {
// All card marks for a (non-array) instance are in one place:
adr = obj;
}
// (Else it's an array (or unknown), and we want more precise card marks.)