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8348411: C2: Remove the control input of LoadKlassNode and LoadNKlassNode #23274

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8348411: C2: Remove the control input of LoadKlassNode and LoadNKlassNode #23274

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79cf199
remove control input of LoadKlassNode
merykitty Jan 23, 2025
ff924de
remove always_see_exact_class
merykitty Jan 23, 2025
c676188
clearer intention, revert formatting, add assert
merykitty Jan 30, 2025
175232a
format
merykitty Jan 30, 2025
7c2b595
Merge branch 'master' into loadklassctrl
merykitty Feb 6, 2025
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61 changes: 27 additions & 34 deletions src/hotspot/share/opto/parseHelper.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -164,42 +164,35 @@ void Parse::array_store_check() {
// cast array_klass to EXACT array and uncommon-trap if the cast fails.
// Make constant out of the inexact array klass, but use it only if the cast
// succeeds.
bool always_see_exact_class = false;
if (MonomorphicArrayCheck
&& !too_many_traps(Deoptimization::Reason_array_check)
&& !tak->klass_is_exact()
if (MonomorphicArrayCheck && !too_many_traps(Deoptimization::Reason_array_check) && !tak->klass_is_exact()
&& tak != TypeInstKlassPtr::OBJECT) {
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I'd also turn tak != TypeInstKlassPtr::OBJECT into tak->isa_aryklassptr() to stress the intention.
(Please, keep the original formatting here.)

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Done, I assume you meant the formatting of the comment below, right?

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No, I referred to the original shape of the condition (1 check per line). IMO it is easier to follow.

// Regarding the fourth condition in the if-statement from above:
//
// If the compiler has determined that the type of array 'ary' (represented
// by 'array_klass') is java/lang/Object, the compiler must not assume that
// the array 'ary' is monomorphic.
//
// If 'ary' were of type java/lang/Object, this arraystore would have to fail,
// because it is not possible to perform a arraystore into an object that is not
// a "proper" array.
//
// Therefore, let's obtain at runtime the type of 'ary' and check if we can still
// successfully perform the store.
//
// The implementation reasons for the condition are the following:
//
// java/lang/Object is the superclass of all arrays, but it is represented by the VM
// as an InstanceKlass. The checks generated by gen_checkcast() (see below) expect
// 'array_klass' to be ObjArrayKlass, which can result in invalid memory accesses.
//
// See issue JDK-8057622 for details.

always_see_exact_class = true;
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Why is it ok to remove this?
If this branch is not taken, it used to be false, and would lead to something different below...

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The only use of this is to decide if we need to attach a control input to the LoadKlass. As the control input is not needed, this can be removed.

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Got it, thanks!

// (If no MDO at all, hope for the best, until a trap actually occurs.)

// Make a constant out of the inexact array klass
const TypeKlassPtr *extak = tak->cast_to_exactness(true);

// Regarding the fourth condition in the if-statement from above:
//
// If the compiler has determined that the type of array 'ary' (represented
// by 'array_klass') is java/lang/Object, the compiler must not assume that
// the array 'ary' is monomorphic.
//
// If 'ary' were of type java/lang/Object, this arraystore would have to fail,
// because it is not possible to perform a arraystore into an object that is not
// a "proper" array.
//
// Therefore, let's obtain at runtime the type of 'ary' and check if we can still
// successfully perform the store.
//
// The implementation reasons for the condition are the following:
//
// java/lang/Object is the superclass of all arrays, but it is represented by the VM
// as an InstanceKlass. The checks generated by gen_checkcast() (see below) expect
// 'array_klass' to be ObjArrayKlass, which can result in invalid memory accesses.
//
// See issue JDK-8057622 for details.

// Make a constant out of the exact array klass
const TypeAryKlassPtr* extak = tak->cast_to_exactness(true)->is_aryklassptr();
if (extak->exact_klass(true) != nullptr) {
Node* con = makecon(extak);
Node* cmp = _gvn.transform(new CmpPNode( array_klass, con ));
Node* bol = _gvn.transform(new BoolNode( cmp, BoolTest::eq ));
Node* cmp = _gvn.transform(new CmpPNode(array_klass, con));
Node* bol = _gvn.transform(new BoolNode(cmp, BoolTest::eq));
Node* ctrl= control();
{ BuildCutout unless(this, bol, PROB_MAX);
uncommon_trap(Deoptimization::Reason_array_check,
Expand All @@ -210,7 +203,7 @@ void Parse::array_store_check() {
set_control(ctrl); // Then Don't Do It, just fall into the normal checking
} else { // Cast array klass to exactness:
// Use the exact constant value we know it is.
replace_in_map(array_klass,con);
replace_in_map(array_klass, con);
CompileLog* log = C->log();
if (log != nullptr) {
log->elem("cast_up reason='monomorphic_array' from='%d' to='(exact)'",
Expand Down