fix(comments): v21 fields absent from vmStructs in all JDK 21-26 builds, not just some distributions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jbachorik

ddprof-lib/src/main/cpp/hotspot/vmStructs.cpp

@@ -290,7 +290,8 @@ void VMStructs::verify_offsets() {
 // Note: DECLARE_V21_TYPE_FIELD_DO is intentionally excluded here.
 // Continuation-related fields (_cont_entry_offset, _cont_return_barrier_addr,
 // _cont_entry_return_pc_addr, _cont_entry_parent_offset) are absent from
-// gHotSpotVMStructs in many JDK 21-26 distributions; walkVM degrades
+// gHotSpotVMStructs in all JDK 21-26 builds: ContinuationEntry was not
+// exported in the vmStructs table until JDK 27 (JDK-8378985). walkVM degrades
 // gracefully when they are missing.
 #define offset_value -1
 #define address_value nullptr

ddprof-lib/src/main/cpp/hotspot/vmStructs.h

@@ -132,11 +132,11 @@ inline T* cast_to(const void* ptr) {
     f(VMContinuationEntry,    MATCH_SYMBOLS("ContinuationEntry"))
 
 // Fields for JDK 21+ virtual-thread / continuation support.
-// These are NOT reliably exported in gHotSpotVMStructs before JDK 27
-// (added via JDK-8378985) and may be absent even on JDK 21-26 in some
-// distributions.  They are declared and populated the same way as regular
-// fields but are intentionally excluded from verify_offsets() assertions
-// so that a missing entry causes graceful degradation rather than SIGABRT.
+// ContinuationEntry was not exported in gHotSpotVMStructs until JDK 27
+// (JDK-8378985), so these fields are absent from the table in all JDK 21-26
+// builds and are populated via C++ mangled-symbol fallback instead.  They are
+// intentionally excluded from verify_offsets() so that a missing entry causes
+// graceful degradation rather than SIGABRT.
 #define DECLARE_V21_TYPE_FIELD_DO(type_begin, field, field_with_version, type_end) \
     type_begin(VMJavaThread, MATCH_SYMBOLS("JavaThread", "Thread"))                \
         field_with_version(_cont_entry_offset, offset, 21, MAX_VERSION, MATCH_SYMBOLS("_cont_entry")) \
@@ -870,6 +870,10 @@ DECLARE(VMThread)
         return VMJavaFrameAnchor::cast(at(_thread_anchor_offset));
     }
 
+    // Returns the innermost active ContinuationEntry for this thread, or nullptr
+    // if none exists or ContinuationEntry layout is unavailable (JDK 21-26 without
+    // vmStructs export).  Used by stackWalker to locate the enterSpecial frame
+    // when crossing the virtual-thread continuation boundary.
     VMContinuationEntry* contEntry() {
         if (_cont_entry_offset < 0 || VMContinuationEntry::type_size() == 0) return nullptr;
         void* ptr = SafeAccess::loadPtr((void**) at(_cont_entry_offset), nullptr);