Question

it was comment as follow，but I can t find any usage of this flag

  // Flag to indicate if the compiler has recorded all dependencies. When the
  // can_redefine_classes capability is enabled in the OnLoad phase then the compiler
  // records all dependencies from startup. However if the capability is first
  // enabled some time later then the dependencies recorded by the compiler
  // are incomplete. This flag is used by RedefineClasses to know if the
  // dependency information is complete or not.
  static bool _all_dependencies_are_recorded;

搜查了jvm代码基,但没有发现任何东西。

Answer 1

如上述评论所写,国旗用于确定:RedefineClasses是否只需要根据重新界定的类别(如果_all_dependencies_are_document_= real)而汇编的方法,或对其他汇编的所有方法进行消化(如果并非所有的方法都记录在案)。

国旗在_RedefineClasses:flush_depend_code/a>:

  // This is the first redefinition, mark all the nmethods for deoptimization
  if (!JvmtiExport::all_dependencies_are_recorded()) {
    CodeCache::mark_all_nmethods_for_evol_deoptimization(&deopt_scope);
    log_debug(redefine, class, nmethod)("Marked all nmethods for deopt");
  } else {
    CodeCache::mark_dependents_for_evol_deoptimization(&deopt_scope);
    log_debug(redefine, class, nmethod)("Marked dependent nmethods for deopt");
  }

在 JDK-83241 上更详细地说明基本问题。

A recent fix for this issue adds the JVM flag -XX:+AlwaysRecordEvolDependencies (on by default) which forces the JVM to record all nmethod dependencies unconditionally from the very beginning. Once this new behavior is always enabled, there will be no need for the _all_dependencies_are_recorded flag.

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