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8371804: C2: Tighten up LoadNode::Value comments after JDK-8346184

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Reviewed-by: kvn, vlivanov
This commit is contained in:
Aleksey Shipilev 2025-11-14 15:31:28 +00:00
parent 466cb38314
commit 10f262a6ad
1 changed files with 11 additions and 12 deletions
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23
src/hotspot/share/opto/memnode.cpp
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@ -2012,11 +2012,7 @@ const Type* LoadNode::Value(PhaseGVN* phase) const {
assert(off != Type::OffsetTop, "case covered by TypePtr::empty");
Compile* C = phase->C;
// If we are loading from a freshly-allocated object, produce a zero,
// if the load is provably beyond the header of the object.
// (Also allow a variable load from a fresh array to produce zero.)
const TypeOopPtr* tinst = tp->isa_oopptr();
bool is_instance = (tinst != nullptr) && tinst->is_known_instance_field();
// If load can see a previous constant store, use that.
Node* value = can_see_stored_value(mem, phase);
if (value != nullptr && value->is_Con()) {
assert(value->bottom_type()->higher_equal(_type), "sanity");
@ -2227,13 +2223,16 @@ const Type* LoadNode::Value(PhaseGVN* phase) const {
}
}
bool is_vect = (_type->isa_vect() != nullptr);
if (is_instance && !is_vect) {
// If we have an instance type and our memory input is the
// programs's initial memory state, there is no matching store,
// so just return a zero of the appropriate type -
// except if it is vectorized - then we have no zero constant.
Node *mem = in(MemNode::Memory);
// If we are loading from a freshly-allocated object/array, produce a zero.
// Things to check:
// 1. Load is beyond the header: headers are not guaranteed to be zero
// 2. Load is not vectorized: vectors have no zero constant
// 3. Load has no matching store, i.e. the input is the initial memory state
const TypeOopPtr* tinst = tp->isa_oopptr();
bool is_not_header = (tinst != nullptr) && tinst->is_known_instance_field();
bool is_not_vect = (_type->isa_vect() == nullptr);
if (is_not_header && is_not_vect) {
Node* mem = in(MemNode::Memory);
if (mem->is_Parm() && mem->in(0)->is_Start()) {
assert(mem->as_Parm()->_con == TypeFunc::Memory, "must be memory Parm");
return Type::get_zero_type(_type->basic_type());