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 This document describes system properties that are used for internal

 debugging and instrumentation purposes, along with the system loggers,

 which are used for the same thing.

 This document is intended as a developer resource, and it is not

 needed as Nashorn documentation for normal usage. Flags and system

 properties described herein are subject to change without notice.

 =====================================

 1. System properties used internally

 =====================================

 This documentation of the system property flags assume that the

 default value of the flag is false, unless otherwise specified.

 SYSTEM PROPERTY: -Dnashorn.unstable.relink.threshold=x

 This property controls how many call site misses are allowed before a 

 callsite is relinked with "apply" semantics to never change again. 

 In the case of megamorphic callsites, this is necessary, or the 

 program would spend all its time swapping out callsite targets. Dynalink 

 has a default value (currently 8 relinks) for this property if it 

 is not explicitly set.

 SYSTEM PROPERTY: -Dnashorn.callsiteaccess.debug

 See the description of the access logger below. This flag is

 equivalent to enabling the access logger with "info" level.

 SYSTEM PROPERTY: -Dnashorn.compiler.ints.disable

 This flag prevents ints and longs (non double values) from being used

 for any primitive representation in the lowered IR. This is default

 false, i.e Lower will attempt to use integer variables as long as it

 can. For example, var x = 17 would try to use x as an integer, unless

 other operations occur later that require coercion to wider type, for

 example x *= 17.1;

 SYSTEM PROPERTY: -Dnashorn.compiler.intarithmetic 

 Arithmetic operations in Nashorn (except bitwise ones) typically

 coerce the operands to doubles (as per the JavaScript spec). To switch

 this off and remain in integer mode, for example for "var x = a&b; var

 y = c&d; var z = x*y;", use this flag. This will force the

 multiplication of variables that are ints to be done with the IMUL

 bytecode and the result "z" to become an int.

 WARNING: Note that is is experimental only to ensure that type support

 exists for all primitive types. The generated code is unsound. This

 will be the case until we do optimizations based on it. There is a CR

 in Nashorn to do better range analysis, and ensure that this is only

 done where the operation can't overflow into a wider type. Currently

 no overflow checking is done, so at the moment, until range analysis

 has been completed, this option is turned off.

 We've experimented by using int arithmetic for everything and putting

 overflow checks afterwards, which would recompute the operation with

 the correct precision, but have yet to find a configuration where this

 is faster than just using doubles directly, even if the int operation

 does not overflow. Getting access to a JVM intrinsic that does branch

 on overflow would probably alleviate this.

 There is also a problem with this optimistic approach if the symbol

 happens to reside in a local variable slot in the bytecode, as those

 are strongly typed. Then we would need to split large sections of

 control flow, so this is probably not the right way to go, while range

 analysis is. There is a large difference between integer bytecode

 without overflow checks and double bytecode. The former is

 significantly faster.

 SYSTEM PROPERTY: -Dnashorn.codegen.debug, -Dnashorn.codegen.debug.trace=<x>

 See the description of the codegen logger below.

 SYSTEM_PROPERTY: -Dnashorn.fields.debug

 See the description on the fields logger below.

 SYSTEM PROPERTY: -Dnashorn.fields.dual

 When this property is true, Nashorn will attempt to use primitive

 fields for AccessorProperties (currently just AccessorProperties, not

 spill properties). Memory footprint for script objects will increase,

 as we need to maintain both a primitive field (a long) as well as an

 Object field for the property value. Ints are represented as the 32

 low bits of the long fields. Doubles are represented as the

 doubleToLongBits of their value. This way a single field can be used

 for all primitive types. Packing and unpacking doubles to their bit

 representation is intrinsified by the JVM and extremely fast.

 While dual fields in theory runs significantly faster than Object

 fields due to reduction of boxing and memory allocation overhead,

 there is still work to be done to make this a general purpose

 solution. Research is ongoing.

 In the future, this might complement or be replaced by experimental

 feature sun.misc.TaggedArray, which has been discussed on the mlvm

 mailing list. TaggedArrays are basically a way to share data space

 between primitives and references, and have the GC understand this.

 As long as only primitive values are written to the fields and enough

 type information exists to make sure that any reads don't have to be

 uselessly boxed and unboxed, this is significantly faster than the

 standard "Objects only" approach that currently is the default. See

 test/examples/dual-fields-micro.js for an example that runs twice as

 fast with dual fields as without them. Here, the compiler, can

 determine that we are dealing with numbers only throughout the entire

 property life span of the properties involved.

 If a "real" object (not a boxed primitive) is written to a field that

 has a primitive representation, its callsite is relinked and an Object

 field is used forevermore for that particular field in that

 PropertyMap and its children, even if primitives are later assigned to

 it.

 As the amount of compile time type information is very small in a

 dynamic language like JavaScript, it is frequently the case that

 something has to be treated as an object, because we don't know any

 better. In reality though, it is often a boxed primitive is stored to

 an AccessorProperty. The fastest way to handle this soundly is to use

 a callsite typecheck and avoid blowing the field up to an Object. We

 never revert object fields to primitives. Ping-pong:ing back and forth

 between primitive representation and Object representation would cause

 fatal performance overhead, so this is not an option.

 For a general application the dual fields approach is still slower

 than objects only fields in some places, about the same in most cases,

 and significantly faster in very few. This is due the program using

 primitives, but we still can't prove it. For example "local_var a =

 call(); field = a;" may very well write a double to the field, but the

 compiler dare not guess a double type if field is a local variable,

 due to bytecode variables being strongly typed and later non

 interchangeable. To get around this, the entire method would have to

 be replaced and a continuation retained to restart from. We believe

 that the next steps we should go through are instead:

 1) Implement method specialization based on callsite, as it's quite

 frequently the case that numbers are passed around, but currently our

 function nodes just have object types visible to the compiler. For

 example "var b = 17; func(a,b,17)" is an example where two parameters

 can be specialized, but the main version of func might also be called

 from another callsite with func(x,y,"string").

 2) This requires lazy jitting as the functions have to be specialized

 per callsite.

 Even though "function square(x) { return x*x }" might look like a

 trivial function that can always only take doubles, this is not

 true. Someone might have overridden the valueOf for x so that the

 toNumber coercion has side effects. To fulfil JavaScript semantics,

 the coercion has to run twice for both terms of the multiplication

 even if they are the same object. This means that call site

 specialization is necessary, not parameter specialization on the form

 "function square(x) { var xd = (double)x; return xd*xd; }", as one

 might first think.

 Generating a method specialization for any variant of a function that

 we can determine by types at compile time is a combinatorial explosion

 of byte code (try it e.g. on all the variants of am3 in the Octane

 benchmark crypto.js). Thus, this needs to be lazy

 3) Possibly optimistic callsite writes, something on the form

 x = y; //x is a field known to be a primitive. y is only an object as

 far as we can tell

 turns into

 try {

   x = (int)y;

 } catch (X is not an integer field right now | ClassCastException e) {

   x = y;

 }

 Mini POC shows that this is the key to a lot of dual field performance

 in seemingly trivial micros where one unknown object, in reality

 actually a primitive, foils it for us. Very common pattern. Once we

 are "all primitives", dual fields runs a lot faster than Object fields

 only.

 We still have to deal with objects vs primitives for local bytecode

 slots, possibly through code copying and versioning.

 SYSTEM PROPERTY: -Dnashorn.compiler.symbol.trace=<x>

 When this property is set, creation and manipulation of any symbol

 named "x" will show information about when the compiler changes its

 type assumption, bytecode local variable slot assignment and other

 data. This is useful if, for example, a symbol shows up as an Object,

 when you believe it should be a primitive. Usually there is an

 explanation for this, for example that it exists in the global scope

 and type analysis has to be more conservative. In that case, the stack

 trace upon type change to object will usually tell us why.

 SYSTEM PROPERTY: nashorn.lexer.xmlliterals

 If this property it set, it means that the Lexer should attempt to

 parse XML literals, which would otherwise generate syntax

 errors. Warning: there are currently no unit tests for this

 functionality.

 XML literals, when this is enabled, end up as standard LiteralNodes in

 the IR.

 SYSTEM_PROPERTY: nashorn.debug

 If this property is set to true, Nashorn runs in Debug mode. Debug

 mode is slightly slower, as for example statistics counters are enabled

 during the run. Debug mode makes available a NativeDebug instance

 called "Debug" in the global space that can be used to print property

 maps and layout for script objects, as well as a "dumpCounters" method

 that will print the current values of the previously mentioned stats

 counters.

 These functions currently exists for Debug:

 "map" - print(Debug.map(x)) will dump the PropertyMap for object x to

 stdout (currently there also exist functions called "embedX", where X

 is a value from 0 to 3, that will dump the contents of the embed pool

 for the first spill properties in any script object and "spill", that

 will dump the contents of the growing spill pool of spill properties

 in any script object. This is of course subject to change without

 notice, should we change the script object layout.

 "methodHandle" - this method returns the method handle that is used

 for invoking a particular script function.

 "identical" - this method compares two script objects for reference

 equality. It is a == Java comparison

 "dumpCounters" - will dump the debug counters' current values to

 stdout.

 Currently we count number of ScriptObjects in the system, number of

 Scope objects in the system, number of ScriptObject listeners added,

 removed and dead (without references).

 We also count number of ScriptFunctions, ScriptFunction invocations

 and ScriptFunction allocations.

 Furthermore we count PropertyMap statistics: how many property maps

 exist, how many times were property maps cloned, how many times did

 the property map history cache hit, prevent new allocations, how many

 prototype invalidations were done, how many time the property map

 proto cache hit.

 Finally we count callsite misses on a per callsite bases, which occur

 when a callsite has to be relinked, due to a previous assumption of

 object layout being invalidated.

 SYSTEM PROPERTY: nashorn.methodhandles.debug,

 nashorn.methodhandles.debug=create

 If this property is enabled, each MethodHandle related call that uses

 the java.lang.invoke package gets its MethodHandle intercepted and an

 instrumentation printout of arguments and return value appended to

 it. This shows exactly which method handles are executed and from

 where. (Also MethodTypes and SwitchPoints). This can be augmented with

 more information, for example, instance count, by subclassing or

 further extending the TraceMethodHandleFactory implementation in

 MethodHandleFactory.java.

 If the property is specialized with "=create" as its option,

 instrumentation will be shown for method handles upon creation time

 rather than at runtime usage.

 SYSTEM PROPERTY: nashorn.methodhandles.debug.stacktrace

 This does the same as nashorn.methodhandles.debug, but when enabled

 also dumps the stack trace for every instrumented method handle

 operation. Warning: This is enormously verbose, but provides a pretty

 decent "grep:able" picture of where the calls are coming from.

 See the description of the codegen logger below for a more verbose

 description of this option

 SYSTEM PROPERTY: nashorn.scriptfunction.specialization.disable

 There are several "fast path" implementations of constructors and

 functions in the NativeObject classes that, in their original form,

 take a variable amount of arguments. Said functions are also declared

 to take Object parameters in their original form, as this is what the

 JavaScript specification mandates.

 However, we often know quite a lot more at a callsite of one of these

 functions. For example, Math.min is called with a fixed number (2) of

 integer arguments. The overhead of boxing these ints to Objects and

 folding them into an Object array for the generic varargs Math.min

 function is an order of magnitude slower than calling a specialized

 implementation of Math.min that takes two integers. Specialized

 functions and constructors are identified by the tag

 @SpecializedFunction and @SpecializedConstructor in the Nashorn

 code. The linker will link in the most appropriate (narrowest types,

 right number of types and least number of arguments) specialization if

 specializations are available.

 Every ScriptFunction may carry specializations that the linker can

 choose from. This framework will likely be extended for user defined

 functions. The compiler can often infer enough parameter type info

 from callsites for in order to generate simpler versions with less

 generic Object types. This feature depends on future lazy jitting, as

 there tend to be many calls to user defined functions, some where the

 callsite can be specialized, some where we mostly see object

 parameters even at the callsite.

 If this system property is set to true, the linker will not attempt to

 use any specialized function or constructor for native objects, but

 just call the generic one.

 SYSTEM PROPERTY: nashorn.tcs.miss.samplePercent=<x>

 When running with the trace callsite option (-tcs), Nashorn will count

 and instrument any callsite misses that require relinking. As the

 number of relinks is large and usually produces a lot of output, this

 system property can be used to constrain the percentage of misses that

 should be logged. Typically this is set to 1 or 5 (percent). 1% is the

 default value.

 SYSTEM_PROPERTY: nashorn.profilefile=<filename>

 When running with the profile callsite options (-pcs), Nashorn will

 dump profiling data for all callsites to stderr as a shutdown hook. To

 instead redirect this to a file, specify the path to the file using

 this system property.

 ===============

 2. The loggers.

 ===============

 The Nashorn loggers can be used to print per-module or per-subsystem

 debug information with different levels of verbosity. The loggers for

 a given subsystem are available are enabled by using

 --log=<systemname>[:<level>]

 on the command line.

 Here <systemname> identifies the name of the subsystem to be logged

 and the optional colon and level argument is a standard

 java.util.logging.Level name (severe, warning, info, config, fine,

 finer, finest). If the level is left out for a particular subsystem,

 it defaults to "info". Any log message logged as the level or a level

 that is more important will be output to stderr by the logger.

 Several loggers can be enabled by a single command line option, by

 putting a comma after each subsystem/level tuple (or each subsystem if

 level is unspecified). The --log option can also be given multiple

 times on the same command line, with the same effect.

 For example: --log=codegen,fields:finest is equivalent to

 --log=codegen:info --log=fields:finest

 The subsystems that currently support logging are:

 * compiler

 The compiler is in charge of turning source code and function nodes

 into byte code, and installs the classes into a class loader

 controlled from the Context. Log messages are, for example, about

 things like new compile units being allocated. The compiler has global

 settings that all the tiers of codegen (e.g. Lower and CodeGenerator)

 use.

 * codegen

 The code generator is the emitter stage of the code pipeline, and

 turns the lowest tier of a FunctionNode into bytecode. Codegen logging

 shows byte codes as they are being emitted, line number information

 and jumps. It also shows the contents of the bytecode stack prior to

 each instruction being emitted. This is a good debugging aid. For

 example:

 [codegen] #41                       line:2 (f)_afc824e 

 [codegen] #42                           load symbol x slot=2 

 [codegen] #43  {1:O}                    load int 0 

 [codegen] #44  {2:I O}                  dynamic_runtime_call GT:ZOI_I args=2 returnType=boolean 

 [codegen] #45                              signature (Ljava/lang/Object;I)Z 

 [codegen] #46  {1:Z}                    ifeq  ternary_false_5402fe28 

 [codegen] #47                           load symbol x slot=2 

 [codegen] #48  {1:O}                    goto ternary_exit_107c1f2f 

 [codegen] #49                       ternary_false_5402fe28 

 [codegen] #50                           load symbol x slot=2 

 [codegen] #51  {1:O}                    convert object -> double 

 [codegen] #52  {1:D}                    neg 

 [codegen] #53  {1:D}                    convert double -> object 

 [codegen] #54  {1:O}                ternary_exit_107c1f2f 

 [codegen] #55  {1:O}                    return object 

 shows a ternary node being generated for the sequence "return x > 0 ?

 x : -x"

 The first number on the log line is a unique monotonically increasing

 emission id per bytecode. There is no guarantee this is the same id

 between runs.  depending on non deterministic code

 execution/compilation, but for small applications it usually is. If

 the system variable -Dnashorn.codegen.debug.trace=<x> is set, where x

 is a bytecode emission id, a stack trace will be shown as the

 particular bytecode is about to be emitted. This can be a quick way to

 determine where it comes from without attaching the debugger. "Who

 generated that neg?"

 The --log=codegen option is equivalent to setting the system variable

 "nashorn.codegen.debug" to true.

 * lower

 The lowering annotates a FunctionNode with symbols for each identifier

 and transforms high level constructs into lower level ones, that the

 CodeGenerator consumes.

 Lower logging typically outputs things like post pass actions,

 insertions of casts because symbol types have been changed and type

 specialization information. Currently very little info is generated by

 this logger. This will probably change.

 * access

 The --log=access option is equivalent to setting the system variable

 "nashorn.callsiteaccess.debug" to true. There are several levels of

 the access logger, usually the default level "info" is enough

 It is very simple to create your own logger. Use the DebugLogger class

 and give the subsystem name as a constructor argument.

 * fields

 The --log=fields option (at info level) is equivalent to setting the

 system variable "nashorn.fields.debug" to true. At the info level it

 will only show info about type assumptions that were invalidated. If

 the level is set to finest, it will also trace every AccessorProperty

 getter and setter in the program, show arguments, return values

 etc. It will also show the internal representation of respective field

 (Object in the normal case, unless running with the dual field

 representation)