[lworld] RFR: 8373608: [lworld] Remove ObjBufferAllocator

[Stefan Karlsson]

On Fri, 12 Dec 2025 15:28:27 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Please review this change to remove `ObjBufferAllocator` since the impementation now is identical to `ObjAllocator`. 
> 
> **Summary**
> This was not the case when ObjBufferAllocator was first introduced. Back then it skipped clearing the memory and just called oopDesc::set_klass_gap(mem, 0). This is no longer the case after [JDK-8350630](https://bugs.openjdk.org/browse/JDK-8350630) in which `ObjBufferAllocator` was changed to also clear the memory. The previous optimization is likely not needed and we should streamline the code to always use `ObjAllocator`.
> 
> **Testing**
> * mach5 tier1-3 (ongoing)

src/hotspot/share/prims/unsafe.cpp line 520:

> 518:   Handle vh(THREAD, v);
> 519:   InlineKlass* vk = InlineKlass::cast(v->klass());
> 520:   instanceOop new_value = vk->allocate_instance(CHECK_NULL);

This changes one instance in a function named Unsafe_MakePrivateBuffer. Is that also dead code now? I don't see any Java usage of that API:

$ grep -r akePrivateBuffer src/
src/hotspot/share/prims/unsafe.cpp:UNSAFE_ENTRY(jobject, Unsafe_MakePrivateBuffer(JNIEnv *env, jobject unsafe, jobject value)) {
src/hotspot/share/prims/unsafe.cpp:    {CC "makePrivateBuffer",     CC "(" OBJ ")" OBJ,      FN_PTR(Unsafe_MakePrivateBuffer)},
src/hotspot/share/classfile/vmIntrinsics.hpp:  do_name(makePrivateBuffer_name,"makePrivateBuffer")                                                                   \
src/hotspot/share/classfile/vmIntrinsics.hpp:  do_intrinsic(_makePrivateBuffer,  jdk_internal_misc_Unsafe,     makePrivateBuffer_name, object_object_signature, F_RN)   \
src/hotspot/share/classfile/vmIntrinsics.cpp:  case vmIntrinsics::_makePrivateBuffer:
src/hotspot/share/opto/doCall.cpp:  if (callee->intrinsic_id() == vmIntrinsics::_makePrivateBuffer || callee->intrinsic_id() == vmIntrinsics::_finishPrivateBuffer) {
src/hotspot/share/opto/doCall.cpp:    if (!rtype->is_void() && cg->method()->intrinsic_id() != vmIntrinsicID::_makePrivateBuffer) {
src/hotspot/share/opto/c2compiler.cpp:  case vmIntrinsics::_makePrivateBuffer:
src/hotspot/share/opto/graphKit.cpp:    // language, Unsafe::makePrivateBuffer returns an Object that is checkcast-ed to the concrete
src/hotspot/share/opto/library_call.cpp:  case vmIntrinsics::_makePrivateBuffer:        return inline_unsafe_make_private_buffer();
src/hotspot/share/opto/library_call.cpp:    C->record_method_not_compilable("value passed to Unsafe::makePrivateBuffer is not of a constant value type");
src/hotspot/share/opto/library_call.cpp:    C->record_method_not_compilable("value passed to Unsafe::finishPrivateBuffer must be allocated by Unsafe::makePrivateBuffer");
src/java.base/share/classes/jdk/internal/misc/Unsafe.java:    public native <V> V makePrivateBuffer(V value);

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1794#discussion_r2614654286