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.

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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.compiler.splitter.threshold=x

This will change the node weight that requires a subgraph of the IR to

be split into several classes in order not to run out of bytecode space.

The default value is 0x8000 (32768).

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>[,*]], 

  -Dnashorn.compiler.symbol.stacktrace=[<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. 

Several symbols names to watch can be specified by comma separation.

If no variable name is specified (and no equals sign), all symbols

will be watched

By using "stacktrace" instead of or together with "trace", stack

traces will be displayed upon symbol changes according to the same

semantics.

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.

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

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

and give the subsystem name as a constructor argument.

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.s

* 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

This is the first lowering pass.

Lower is a code generation pass that turns high level IR nodes into

lower level one, for example substituting comparisons to RuntimeNodes

and inlining finally blocks.

Lower is also responsible for determining control flow information

like end points.

* attr

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.

* finalize

This --log=finalize log option outputs information for type finalization,

the third tier of the compiler. This means things like placement of 

specialized scope nodes or explicit conversions. 

* 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)