Note: this proposal has been withdrawn. It has been superceded by JDK-8243208

Overview

This is a design document for JDK-8236988 - VM flags: replace ALL_FLAGS macro with a modular designModular 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" (see also gc/z/z_globals.flags.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)

...

The main goal of the inline functions is to avoid duplicating the information between the HPP and CPP files, so the CPP files contains just boilerplate code:just boilerplate code.

• c2_globals.cpp

DEFN_PRODUCT_FLAG(StressLCM);
DEFN_PRODUCT_FLAG(StressGCM);
DEFN_PRODUCT_FLAG(MaxLoopPad); DEFN_PRODUCT_RANGE(MaxLoopPad);
DEFN_PRODUCT_FLAG_PD(InteriorEntryAlignment); DEFN_PRODUCT_CONSTRAINT(InteriorEntryAlignment);

Macros in the CPP files are expanded to

// Definition of the flag itself
FLAG_TYPE_StressLCM StressLCM = FLAG_DEFVAL_StressLCM();

// Definition of the flag's meta information
ProductFlag<FLAG_TYPE_StressLCM> FLAG_StressLCM(
    FLAG_TYPE_NAME_StressLCM(),
    "StressLCM",
    (FLAG_ATTR_StressLCM() | JVMFlag::C2),
    &StressLCM, FLAG_DOCS_StressLCM());

<... the expansion of some macros are omitted for clarity>

// Range check is implemented like this
JVMFlagRange<FLAG_TYPE_MaxLoopPad> FLAG_RANGE_MaxLoopPad(
    &FLAG_MaxLoopPad, FLAG_MIN_MaxLoopPad(), FLAG_MAX_MaxLoopPad());


// Inside the constructor of FLAG_RANGE_MaxLoopPad, it essentially registers
// itself as the range spec for FLAG_MaxLoopPad, 
FLAG_MaxLoopPad._range = &FLAG_RANGE_MaxLoopPad;

...

However, it's possible to forget to put DEFN_PRODUCT_RANGE(MaxLoopPad) in c2_globals.cpp. I can't think of a way to statically check for that, so I added a runtime check (debug builds only). See JVMFlag::validate_flags().

...