Table of Contents

Goal

Improve start-up time by storing pre-generated LambdaForm handlers in an application-specific CDS archive.

Background

In JDK-8086045 (Improve the java.lang.invoke first initialization costs), the JDK can reduce the number of dynamically generated LambdaForm  LambdaForm classes. Here's an example:

# Do this in your JDK build directory.
# The exploded JDK build:
$ jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ 
[0.142s][info][class,load] java.lang.invoke.LambdaForm$MH/0x0000000800066c40 source: __JVM_LookupDefineClass__
[0.143s][info][class,load] java.lang.invoke.LambdaForm$DMH/0x0000000800066040 source: __JVM_LookupDefineClass__
[0.143s][info][class,load] java.lang.invoke.LambdaForm$MH/0x0000000800066440 source: __JVM_LookupDefineClass__
[snip]

$ jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ | wc
    102     408   11400


# The final JDK image: 
$ images/jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ |  wc
     34     136    3800

...

The input of generateDirectMethodHandleHolderClassBytes comes from the defaultthe default_jli_trace.txt file that is generated in GenerateLinkOptData.gmk:

...

$ grep invokeSpecialIFC ./support/link_opt/default_jli_trace.txt
[LF_RESOLVE] java.lang.invoke.DirectMethodHandle$Holder invokeSpecialIFC L3I_I (fail)
[LF_RESOLVE] java.lang.invoke.DirectMethodHandle$Holder invokeSpecialIFC LLI_I (fail)

$ images/jdk/bin/javap  'java.lang.invoke.DirectMethodHandle$Holder' | grep invokeSpecialIFC
  static int invokeSpecialIFC(java.lang.Object, java.lang.Object, java.lang.Object, int);
  static int invokeSpecialIFC(java.lang.Object, java.lang.Object, int);

The methods in DirectMethodHandle$Holder in DirectMethodHandle$Holder are used in InvokerBytecodeGenerator.java when compiling for LambdaForms:

    private static MemberName lookupPregenerated(LambdaForm form, MethodType invokerType) {
        if (form.customized != null) {
            // No pre-generated version for customized LF
            return null;
        }
        String name = form.kind.methodName; 
        switch (form.kind) {
            [......]
            case DIRECT_INVOKE_INTERFACE:   // fall-through
            case DIRECT_INVOKE_SPECIAL:     // fall-through
            case DIRECT_INVOKE_SPECIAL_IFC: // fall-through
            case DIRECT_INVOKE_STATIC:      // fall-through
            case DIRECT_INVOKE_STATIC_INIT: // fall-through
            case DIRECT_INVOKE_VIRTUAL:     return resolveFrom(name, invokerType, DirectMethodHandle.Holder.class);
        }
        return null;

Estimated Benefits

The expected benefits will be application specific. Here's an experiment with Javac (I modified the JDK makefile to invoke com.sun.tools.javac.Main while generating default_jli_trace.txt).

Results of " perf stat -r 50 bin/javac -J-Xshare:on -J-XX:SharedArchiveFile=javac2.jsa Bench_HelloWorld.java "
   1:   2239149872  2203293903 (-35855969)      ----     375.580   367.190 ( -8.390)      ----  
   2:   2245304165  2198900711 (-46403454)      -----    375.350   366.910 ( -8.440)      ----  
   3:   2242558895  2205167765 (-37391130)      ----     376.720   366.480 (-10.240)      ----- 
   4:   2246851428  2198990987 (-47860441)      -----    375.119   367.010 ( -8.109)      ----  
   5:   2238549008  2202480450 (-36068558)      ----     374.100   367.600 ( -6.500)      ---   
   6:   2240816859  2200725384 (-40091475)      ----     375.743   366.880 ( -8.863)      ----  
   7:   2243272464  2199639926 (-43632538)      -----    374.296   365.990 ( -8.306)      ----  
   8:   2239650882  2203211291 (-36439591)      ----     375.315   365.520 ( -9.795)      ----- 
   9:   2242434935  2202380291 (-40054644)      ----     376.052   367.305 ( -8.747)      ----  
  10:   2238725993  2201342398 (-37383595)      ----     375.478   366.743 ( -8.735)      ----  
============================================================
        2241729810  2201612439 (-40117371)      ----     375.375   366.762 ( -8.612)      ----  
instr delta =    -40117371    -1.7896%
time  delta =       -8.612 ms -2.2943%

Why do it in CDS

• We cannot store all possible LambdaForm handlers in the standard JDK image – we cannot enumerate all possible forms that may be used by all possible apps.
• We could also do this as part of jlink, when creating a custom JDK image. However, some users may not want to create a new JDK image.
  • Also, there's currently no work-flow to include profiling data when building a custom JDK image (although this can be changed, probably as part of Project Leyden)
• CDS dynamic dump can be used without generating profiling data in a separate step, so the usability is better.

CDS Design

Overally, we want to modify lookupPregenerated() to something like:

    case xxxxxx: return resolveFrom(name, invokerType, directMethodHandleHolders);


Class[] directMethodHandleHolders = {
    DirectMethodHandle.Holder.class, // the one in jdk/lib/modules
    Class.forName("java/lang/invoke/DirectMethodHandle$CDSHolder1"), // if available, from CDS static archive
    Class.forName("java/lang/invoke/DirectMethodHandle$CDSHolder2"), // if available, from CDS static archive
};

During CDS dump (static or dynamic), we will generate the CDSHolder1 and CDSHolder2 classes according to the program's execution profile.

Static Dump

We regenerate the XXX$Holder classes to contain all necessary methods used by the application.

Static Dump

This has been implemented in JDK-8247536 (Support for pre-generated java.lang.invoke classes in CDS static archive - integrated into JDK 16).

During the trial run (with -XX:DumpLoadedClassList=classlist), we can save the list of LambdaForm handlers using a special syntax like:

# regular class specifier
java/lang/Object
# LambdaForm specifier (syntax TBD, but probably following the output of
# -Djava.lang.invoke.MethodHandle.TRACE_RESOLVE=true

@lambda-form-invoker [LF_RESOLVE] java.lang.invoke.DirectMethodHandle$Holder invokeSpecialIFC L3I_I
[LF@lambda-form-invoker [SPECIES_RESOLVE] java.lang.invoke.SimpleMethodHandle

We collect all the @lambda-form-invoker lines when parsing the classlist. At the end of the static dump:

• We invoke generateDirectMethodHandleHolderClassBytes to generate a customized version of DirectMethodHandle$Holder (as well as other holder classes that are currently generated by GenerateJLIClassesPlugin.java.)
  • This DirectMethodHandle$Holder class has all the methods needed for resolving the MethodHandle LambdaForms during the trial run.
  • Store the InstanceKlass of the customized version of DirectMethodHandle$Holder into the CDS archive.

...

Preliminary webrev:

• http://cr.openjdk.java.net/~minqi/2020/8247536/webrev-01/
• The generation of the customized DirectMethodHandle$Holder is done in MetaspaceShared::regenerate_holder_classes().

Dynamic Dump

• See JDK-8255493 (Support for pre-generated java.lang.invoke classes in CDS dynamic archive)
• Collect a @lambda-form-invoker list (in memory) when -XX:ArchiveClassesAtExit is specified.
• When the dynamic archive is dumped, use this list to create customized Holder classes.
• TBD

Dynamic Dump

...