Overview

This is a design document for JDK-8236988 - Modular Design for JVM Flags

• Lots of files have changed. Please read this document first before heading over to the webrev
• Preliminary webrev: http://cr.openjdk.java.net/~iklam/jdk15/vm_flags_overhaul.008/

Goals

• Improve the implementation of JVM command-line flags (such as -XX:+UseCompressedOops)

• Break up monolithic globals.hpp

  • No need to include all flags for GC, C1, C2, JVMCI, .....

  • Before:
    • #include "runtime/globals.hpp" + globals_shared.hpp  = declare all possible flags
  • After:
    • If you just need flags for ZGC:
      #include "gc/z/z_globals.hpp"
  • Future: subdivide globals.hpp into individual components:
    • (Not in this patch, but possible)
    • #include "memory/metaspaceShared_globals.hpp" // just flags related to CDS
• Remove complex macros for flags manipulation
  • Example: jvmFlags.cpp, globals_extension.hpp, jvmFlagRangeList.cpp
• Hotspot command line switches should have multiple type attributes
  
  • See JDK-7123237

    • E.g., one can have a "manageable" switch and an "experimental" switch, but not a "manageable_experimental" switch.

• Templatize duplicated flag processing code:
  • E.g., .jvmFlags.cpp
• Speed up range/constraint checking for flags
  • Avoid linear search for every -XX:NumericFlag=value argument in the command-line (jvmFlagRangeList.cpp)

Proposal

(Design contribution by Erik Österlund, Stefan Karlsson, Coleen Phillimore)

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