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8057737: Avoid G1 write barriers on newly allocated objects

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Reviewed-by: mgerdin, kvn, iveresov
This commit is contained in:
Staffan Friberg 2014-10-09 13:06:15 -07:00 committed by Mikael Gerdin
parent 60f3ade82b
commit 8b424b422f
2 changed files with 187 additions and 0 deletions
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183
hotspot/src/share/vm/opto/graphKit.cpp
View File

@ -3826,6 +3826,110 @@ void GraphKit::write_barrier_post(Node* oop_store,
// 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 if 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();
// Make sure that we are looking at the same allocation site.
// The alloc variable is guaranteed to not be null here from earlier check.
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 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,
@ -3846,6 +3950,12 @@ void GraphKit::g1_write_barrier_pre(bool do_load,
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");
@ -3927,6 +4037,65 @@ void GraphKit::g1_write_barrier_pre(bool do_load,
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 if 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->in(MemNode::Control);
if (mem->is_Proj() && mem->in(0)->is_Initialize()) {
InitializeNode* st_init = mem->in(0)->as_Initialize();
AllocateNode* st_alloc = st_init->allocation();
// Make sure we are looking at the same allocation
if (alloc == st_alloc) {
return true;
}
}
return false;
}
//
// Update the card table and add card address to the queue
//
@ -3979,6 +4148,20 @@ void GraphKit::g1_write_barrier_post(Node* oop_store,
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;

4
hotspot/src/share/vm/opto/graphKit.hpp
View File

@ -771,6 +771,10 @@ class GraphKit : public Phase {
Node* index, Node* index_adr,
Node* buffer, const TypeFunc* tf);
bool g1_can_remove_pre_barrier(PhaseTransform* phase, Node* adr, BasicType bt, uint adr_idx);
bool g1_can_remove_post_barrier(PhaseTransform* phase, Node* store, Node* adr);
public:
// Helper function to round double arguments before a call
void round_double_arguments(ciMethod* dest_method);