DOKK Library
Optimization Coaching for JavaScript
Vincent St-Amour1 and Shu-yu Guo2

1       PLT @ Northeastern University
        Boston, Massachusetts, USA
2       Mozilla Research
        San Francisco, California, USA

The performance of dynamic object-oriented programming languages such as JavaScript depends
heavily on highly optimizing just-in-time compilers. Such compilers, like all compilers, can silently
fall back to generating conservative, low-performance code during optimization. As a result, pro-
grammers may inadvertently cause performance issues on users’ systems by making seemingly
inoffensive changes to programs. This paper shows how to solve the problem of silent opti-
mization failures. It specifically explains how to create a so-called optimization coach for an
object-oriented just-in-time-compiled programming language. The development and evaluation
build on the SpiderMonkey JavaScript engine, but the results should generalize to a variety of
similar platforms.

1998 ACM Subject Classification D.2.3 [Software Engineering]: Coding Tools and Techniques;
D.3.4 [Programming Languages]: Processors—Compilers

Keywords and phrases Optimization Coaching, JavaScript, Performance Tools

Digital Object Identifier 10.4230/LIPIcs.ECOOP.2015.999

    1    Optimization Coaching for the Modern World
An optimization coach [22] opens a dialog between optimizing compilers and programmers.
It thus allows the latter to take full advantage of the optimization process. Specifically,
coaches provide programmers with actionable recommendations of changes to their programs
to trigger additional optimizations. Notably, the changes may not preserve the semantics of
the program.
    Our experiences with a prototype optimization coach for Racket show promising results.
This prototype exploits Racket’s [9] simple ahead-of-time byte-compiler,1 which performs
basic optimizations. The general idea of optimization coaching ought to apply beyond
languages with functional cores and simple compilers.
    Unsurprisingly, scaling coaching to object-oriented languages with advanced compilers
presents challenges. An object-oriented programming style gives rise to non-local optimization
failures, that is, the compiler may fail to optimize an operation in one part of the program
because of properties of a different part of the program. Advanced just-in-time (JIT) compilers
introduce a temporal dimension to the compilation and optimization process, that is, the
compiler may compile the same piece of code multiple times, potentially performing different
optimizations each time. Advanced compilers may also apply optimization tactics when

    Racket also includes a just-in-time code generator that does not perform many optimizations.
            © Vincent St-Amour and Shu-yu Guo;
            licensed under Creative Commons License CC-BY
29th European Conference on Object-Oriented Programming (ECOOP’15).
Editor: John Tang Boyland; pp. 999–1023
                   Leibniz International Proceedings in Informatics
                   Schloss Dagstuhl – Leibniz-Zentrum für Informatik, Dagstuhl Publishing, Germany
1000   Optimization Coaching for JavaScript

       optimizing programs, that is, they use batteries of related and complementary optimizations
       when compiling some operations.
          This paper presents new ideas on optimization coaching that allow it to scale to dynamic
       object-oriented languages with state-of-the-art JIT compilers. Our prototype optimization
       coach works with the SpiderMonkey2 JavaScript [7] engine, which is included in the Firefox3
       web browser.
          In this paper, we
          describe optimization coaching techniques designed for object-oriented languages with
          state-of-the-art compilers
          present an evaluation of the recommendations provided by our optimization coach for

          The rest of the paper is organized as follows. Sections 2 and 3 provide background on
       optimization coaching and on the SpiderMonkey JavaScript engine. Section 4 describes the
       optimizations that our prototype supports. Section 5 sketches out its architecture. Section 6
       outlines the challenges of coaching in an object-oriented setting and describes our solutions,
       and section 7 does likewise for the challenges posed by JIT compilation and optimization
       tactics. Section 8 presents coaching techniques that ultimately were unsuccessful. We then
       present evaluation results in section 9, compare our approach to related work and conclude.

       Prototype Our prototype optimization coach is available in source form.4 It depends on
       an instrumented version of SpiderMonkey whose source is also available.5

           2   Background: Optimization Coaching
       Because modern programming languages heavily rely on compiler optimizations for perfor-
       mance, failure to apply certain key optimizations is often the source of performance issues.
       To diagnose these performance issues, programmers need insight about what happens during
       the optimization process.
          This section first discusses an instance of an optimization failure causing a hard-to-diagnose
       performance issue. The rest of the section then provides background on how optimization
       coaching provides assistance in these situations, and introduces some key technical concepts
       from previous work on coaching.

       2.1     A Tale from the Trenches
       The Shumway project,6 an open-source implementation of Adobe Flash in JavaScript, provides
       an implementation of ActionScript’s parametrically polymorphic Vector API,7 which includes
       a forEach method. This method takes a unary kernel function f as its argument and calls
       it once for each element in the vector, passing that element as the argument to f. Initially,
       the Shumway implementors wrote a single implementation of the forEach method and used
       it for all typed variants of Vector.

V. St-Amour and S. Guo                                                                                1001

   Figure 1 Excerpt from the coaching report for a splay tree implementation

    This initial implementation performed poorly. Unbeknownst to Shumway’s implementors,
this implementation strategy triggers optimization failures inside JavaScript engines. If
the compiler observes code to be polymorphic, it may not apply crucial optimizations. For
instance, the compiler may be unable to determine a monomorphic context for the element
accesses and the calling of the kernel function, prohibiting optimizations such as inlining.
    Eventually, the Shumway engineers reverse engineered the JIT’s opaque optimization
decisions and could then diagnose the problem. They determined that performance could be
recouped by cloning forEach’s implementation for variants that needed high performance
(e.g., vectors of integers), as the JIT would then observe monomorphic accesses and call sites.
While the compiler lacked the necessary context to make the appropriate tradeoff decision,
the Shumway engineers were able to, once they understood the optimization decisions.

2.2    Optimization Coaching in a Nutshell
Failures such as those experienced by the first Shumway implementation are hard to diagnose
and solve for two main reasons. First, optimizers fail silently; programmers are never informed
that an optimization failed. Second, getting to the root causes of these failures requires skills
and knowledge that are out of reach for most programmers. Those skills include auditing
the compiler’s output, reverse engineering the optimizer’s decisions, etc.
    Optimization coaches help programmers get more out of their optimizers without requiring
such knowledge and with a minimum of effort. They achieve this feat by reporting optimization
near misses. Near misses are optimizations that the compiler did not apply to their program—
either due to a lack of information, or because doing so may be unsound in some cases—but
could apply safely if the source program were changed in a certain way.
    For example, consider the excerpt from a splay tree implementation in figure 2. The
isEmpty method may find the root_ property either on SplayTree instances (if the insert
method has been called) or on the SplayTree prototype (otherwise). Hence, the JavaScript
engine cannot specialize the property access to either of these cases and instead generates
code that can handle both of them. The generated code is thus much slower than necessary.

1002   Optimization Coaching for JavaScript

        // constructor
        function SplayTree() {};
        // default value on the prototype
        SplayTree.prototype.root_ = null;

        SplayTree.prototype.insert = function(key, value) {
           // regular value on instances
           ... this.root_ = new SplayTree.Node(key, value); ...

        SplayTree.prototype.isEmpty = function() {
           // property may be either on the instance or on the prototype
           return !this.root_;

          Figure 2 Splay tree implementation with an optimization near miss

        function SplayTree() {
            // default value on instances
            this.root_ = null;

          Figure 3 Improved splay tree constructor, without near miss

           Coaches supplement near miss reports with concrete recommendations of program changes
       that programmers can apply. These modifications may make the compiler’s analysis easier or
       may rule out corner cases, with an end result of the compiler succeeding to apply previously
       missed optimizations. Figure 1 shows the coach’s diagnosis and recommendations of program
       changes that may resolve the near miss.
           These recommendations are not required to preserve programs’ exact semantics. In
       other words, coaches may recommend changes that would be beyond the reach of optimizing
       compilers, which are limited to semantics-preserving transformations. Programmers remain
       in control and are free to veto any recommendation that would lead to semantic, or structural,
       changes that they deem unreasonable.
           In our splay tree example, the compiler cannot move root_’s default value to instances;
       this would change the behavior of programs that depend on the property being on the
       prototype. Programmers, on the other hand, are free to do so and may rewrite the program
       to the version from figure 3, which consistently stores the property on instances, and does
       not suffer from the previous near miss.

       2.3    Optimization Coaching Concepts
       To provide the necessary background to describe this paper’s technical contributions, we now
       provide an overview of existing optimization coaching concepts.
          At a high level, an optimization coach operates in four phases.
          First, instrumentation code inside the optimizer logs optimization decisions during
          compilation. This instrumentation distinguishes between optimization successes, i.e.,
V. St-Amour and S. Guo                                                                              1003

   optimizations that the compiler applies to the program, and optimization failures, i.e.,
   optimizations that it cannot apply. These logs include enough information to reconstruct
   the optimizer’s reasoning post facto. Section 4 describes the information recorded by our
   prototype’s instrumentation, and section 7.1 explains our approach to instrumentation in
   a JIT context.
   Second, after compilation, an offline analysis processes these logs. The analysis phase
   is responsible for producing high-level, human-digestible near miss reports from the
   low-level optimization failure events recorded in the logs. It uses a combination of
   optimization-agnostic techniques and optimization-specific heuristics. We describe some
   of these techniques below.
   Third, from the near miss reports, the coach generates recommendations of program
   changes that are likely to turn these near misses into optimization successes. These
   recommendations are generated from the causes of individual failures as determined
   during compilation and from metrics computed during the analysis phase.
   Finally, the coach shows reports and recommendations to programmers. The interface
   should leverage optimization analysis metrics to visualize the coach’s rankings of near
   misses or display high-estimated-impact recommendations only.

    To avoid overwhelming programmers with large numbers of low-level reports, an optimiza-
tion coach must carefully curate and summarize its output. In particular, it must restrict
its recommendations to those that are both likely to enable further optimizations and likely
to be accepted by the programmer. A coach uses three main classes of techniques for that
purpose: pruning, ranking and merging.

Pruning Not all optimization failures are equally interesting to programmers. For example,
showing failures that do not come with an obvious source-level solution, or failures that are
likely due to intentional design choices, would be a waste of programmer time. Coaches
therefore use heuristics that decide to remove optimization failures from the coach’s reports.
Optimization failures that remain after pruning constitute near misses, and are further
refined via merging.
    Our previous work describes several pruning techniques, such as irrelevant failure pruning,
which we discuss in section 7.2.1. Section 7.1.2 introduces a new form of pruning based on
profiling information.

Ranking Some optimization failures have a larger impact on program performance than
others. A coach must rank its reports based on their expected performance impact to allow
programmers to prioritize their responses. In order to do so, the coach computes a badness
metric for each near miss, which estimates its impact on performance.
   Our previous work introduces static heuristics to compute badness. Section 7.1.2 intro-
duces the new dynamic heuristic that our prototype uses.

Merging To provide a high-level summary of optimization issues affecting a program, a
coach should consolidate sets of related reports into single summary reports. Different
merging techniques use different notions of relatedness.
    These summary reports have a higher density of information than individual near miss
reports because they avoid repeating common information, which may include cause of failure,
solution, etc. depending on the notion of relatedness. They are also more efficient in terms of
programmer time. For example, merging reports with similar solutions or the same program
location, allows programmers to solve multiple issues at the same time.

1004   Optimization Coaching for JavaScript

           When merging reports, a coach must respect preservation of badness which, for summary
       reports, is the sum of that of the merged reports. The sum of their expected performance
       impacts is a good estimation of the estimated impact of the summary report. The increased
       badness value of summary reports causes them to rank higher than their constituents would
       separately. Because these reports have a higher impact-to-effort ratio, having them high in
       the rankings increases the actionability of the tool’s output.
           Our previous work introduces two merging techniques: causality merging and locality
       merging. This work introduces three additional kinds of merging, by-solution merging
       (section 6.3), by-constructor merging (section 6.4) and temporal merging (section 7.1.3).

           3     Background: The SpiderMonkey JavaScript Engine
       This section surveys the aspects of SpiderMonkey that are relevant to this work.

       3.1       Compiler Architecture
       Like other modern JavaScript engines,8 9 10 SpiderMonkey is a multi-tiered engine that
       uses type inference [13], type feedback [1], and optimizing just-in-time compilation [2] based
       on the SSA form [5], a formula proven to be well suited for JavaScript’s dynamic nature.
       Specifically, it has three tiers: the interpreter, the baseline JIT compiler, and the IonMonkey
       (Ion) optimizing JIT compiler.
           In the interpreter, methods are executed without being compiled to native code or
       optimized. Upon reaching a certain number of executions,11 the baseline JIT compiles
       methods to native code. Once methods become hotter still and reach a second threshold,12
       Ion compiles them. The engine’s gambit is that most methods are short-lived and relatively
       cold, especially for web workloads. By reserving heavyweight optimization for the hottest
       methods, it strikes a balance between responsiveness and performance.

       3.2       Optimizations in Ion
       Because Ion performs the vast majority of SpiderMonkey’s optimizations, our work focuses
       on coaching those. Ion is an optimistic optimizing compiler, meaning it assumes types and
       other observed information gathered during baseline execution to hold for future executions,
       and it uses these assumptions to drive the optimization process.

       Types and layout For optimization, the information SpiderMonkey observes mostly revolves
       around type profiling and object layout inference. In cases where inferring types would
       require a heavyweight analysis, such as heap accesses and function calls, SpiderMonkey uses
       type profiling instead. During execution, baseline-generated code stores the result types for
       heap acccesses and function calls for consumption by Ion.
           At the same time, the runtime system also gathers information to infer the layouts of
       objects, i.e., mappings of property names to offsets inside objects. These layouts are referred
       to as “hidden classes” in the literature. This information enables Ion to generate code for




            At the time of this writing, 10.
            At the time of this writing, 1000.
V. St-Amour and S. Guo                                                                                 1005

property accesses on objects with known layout as simple memory loads instead of hash table
    The applicability of Ion’s optimizations is thus limited by the information it observes.
The observed information is also used to seed a number of time-efficient static analyses, such
as intra-function type inference.

Bailouts To guard against changes in the observed profile information, Ion inserts dynamic
checks [15]. For instance, if a single callee is observed at a call site, Ion may optimistically
inline that callee, while inserting a check to ensure that no mutation changes the binding
referencing the inlinee. Should such a dynamic check fail, execution aborts from Ion-generated
code and resumes in the non-optimized—and therefore safe—code generated by the baseline
JIT compiler.

Optimization tactics As a highly optimizing compiler, Ion has a large repertoire of opti-
mizations at its disposal when compiling key operations, such as property accesses. These
optimizations are organized into optimization tactics. When compiling an operation, the
compiler attempts each known optimization strategy for that kind of operation in order—from
most to least profitable—until one of them applies.
    A tactic’s first few strategies are typically highly specialized optimizations that generate
extremely efficient code, but apply only in limited circumstances, e.g., accessing a property
of a known constant object. As compilation gets further into a tactic, strategies become more
and more general and less and less efficient, e.g., polymorphic inline caches, until it reaches
fallback strategies that can handle any possible situation but carry a significant performance
cost, e.g., calling into the VM.

 4     Optimization Corpus

Conventional wisdom among JavaScript compiler engineers points to property and element
accesses as the most important operations to be optimized. For this reason, our prototype
focuses on these two classes of operations.
    For both, the instrumentation code records similar kinds of information. The information
uniquely identifies each operation affected by optimization decisions, i.e., source location, type
of operation and parameters. Additionally, it records information necessary to reconstruct
optimization decisions themselves, i.e., the sets of inferred types for each operand, the
sequence of optimization strategies attempted, which attempts were successful, which were
not and why. This information is then used by the optimization analysis phase to produce
and process near miss reports.
    The rest of this section describes the relevant optimizations with an eye towards opti-
mization coaching.

4.1    Property Access and Assignment
Conceptually, JavaScript objects are open-ended maps from strings to values. In the most
general case, access to an object property is at best a hash table lookup, which, despite being
amortized constant time, is too slow in practice. Ion therefore applies optimization tactics
when compiling these operations so that it can optimize cases that do not require the full
generality of maps. We describe some of the most important options below.

1006   Optimization Coaching for JavaScript

       Definite slot Consider a property access o.x. In the best case, the engine observes o to be
       monomorphic and with a fixed layout. Ion then emits a simple memory load or store for
       the slot where x is stored. This optimization’s prerequisites are quite restrictive. Not only
       must all objects that flow into o come from the same constructor, they must also share the
       same fixed layout. An object’s layout is easily perturbed, however, for example by adding
       properties in different orders.

       Polymorphic inline cache Failing that, if multiple types of plain JavaScript objects13 are
       observed to flow to o, Ion can emit a polymorphic inline cache (PIC) [14]. The PIC is a
       self-patching structure in JIT code that dispatches on the type and layout of o. Initially, the
       PIC is empty. Each time a new type and layout of o flows into the PIC during execution,
       an optimized stub is generated that inlines the logic needed to access the property x for
       that particular layout of o. PICs embody the just-in-time philosophy of not paying for any
       expensive operation ahead of time. This optimization’s prerequisites are less restrictive than
       that of definite slots, and it applies for the majority of property accesses that do not interact
       with the DOM.

       VM call In the worst case, if o’s type is unknown to the compiler, either because the
       operation is in cold code and has no profiling information, or because o is observed to be an
       exotic object, then Ion can emit only a slow path call to a general-purpose runtime function
       to access the property.
           Such slow paths are algorithmically expensive because they must be able to deal with
       any aberration: o may be of a primitive type, in which case execution must throw an error;
       x may be loaded or stored via a native DOM accessor somewhere on o’s prototype chain;
       o may be from an embedded frame within the web page and require a security check; etc.
       Furthermore, execution must leave JIT code and return to the C++ VM. Emitting a VM
       call is a last resort; it succeeds unconditionally, requires no prior knowledge, and is capable
       of handling all cases.

       4.2       Element Access and Assignment
       JavaScript’s element access and assignment operations are polymorphic and operate on
       various types of indexable data, such as arrays, strings and TypedArrays. This polymorphism
       restricts the applicability of optimizations; most of them can apply only when the type of
       the indexed data is known in advance.
           Even when values are known to be arrays, JavaScript semantics invalidate common
       optimizations in the general case. For example, JavaScript does not require arrays in the
       C sense, that is, it does not require contiguous chunks of memory addressable by offset.
       Semantically, JavaScript arrays are plain objects that map indices—string representations of
       unsigned integers—to values. Element accesses into such arrays, then, are semantically (and
       perhaps surprisingly) equivalent to property lookups and are subject to the same set of rules,
       such as prototype lookups.
           As with inferring object layout, SpiderMonkey attempts to infer when JavaScript arrays
       are used as if they were dense, C-like arrays, and optimize accordingly. Despite new APIs such

            The restriction on plain JavaScript objects is necessary because properties may be accessed from a
            variety of exotic object-like values, such as DOM nodes and proxies. Those objects encapsulate their
            own logic for accessing properties that is free to deviate from the logic perscribed for plain objects by
            the ECMAScript standard.
V. St-Amour and S. Guo                                                                                        1007

  SpiderMonkey                                        Coach
                          logs                                         misses    Recom-      reports
           Optimizer                                    Optimization
                                 Profiler                                        mendation              UI
        Instrumentation                                   Analysis
                                             profile                             Generation

     Figure 4 Our prototype's architecture

as TypedArrays offering C-like arrays directly, SpiderMonkey’s dense array optimizations
remain crucial to the performance of the web.
   To manage all possible modes of use of element accesses and the optimizations that
apply in each of them, Ion relies on optimization tactics. We describe the most important
optimization strategy—dense array access—below. The PIC and VM call cases are similar to
the corresponding cases for property access. Other, specialized strategies heavily depend on
SpiderMonkey’s data representation and are beyond the scope of this paper, but are handled
by the prototype.

Dense array access Consider an element access o[i]. In the best case, if o is determined
to be used as a dense array and i an integer, Ion can emit a memory load or a store for
offset i plus bounds checking. For this choice to be valid, all types that flow into o must be
plain JavaScript objects that have dense indexed properties. An object with few indexed
properties spread far apart would be considered sparse, e.g., if only o[0] and o[2048] were
set, o would not be considered dense. Note that an object may be missing indexed properties
and still be considered dense. SpiderMonkey further distinguishes dense arrays—those with
allocated dense storage—from packed arrays—dense arrays with no holes between indexed
properties. Ion is able to elide checking whether an element is a hole, or a missing property,
for packed arrays. Furthermore, the object o must not have been observed to have prototypes
with indexed properties, as otherwise accessing a missing indexed property j on o would,
per specification, trigger a full prototype walk to search for j when accessing o[j].

 5      Architecture

As section 2.3 explains, our optimization coach operates in four phases. Figure 4 illustrates
how these phases interact. In the first phase, instrumentation inside IonMonkey’s optimizer
logs optimization successes and failures and sends that information to the SpiderMonkey
profiler (section 7.1.1). Next, the optimization analysis phase applies pruning heuristics
(sections 7.2.1 and 7.2.2), determines solution sites (section 6.1), computes badness scores
(section 7.1.2), and finally merges reports (sections 6.3, 6.4, and 7.1.3). Its end result is a list
of ranked near misses.
    The third phase, recommendation generation, fills out textual recommendation templates—
selected based on failure causes—with inferred solution sites, failure causes, type information,
and source information. Finally, the tool’s user interface presents the five highest-ranked
recommendations to programmers.

1008   Optimization Coaching for JavaScript

        6     Coaching for Object-Oriented Languages

       Dispatch optimizations for property operations fundamentally depend on non-local informa-
       tion. For example, the optimizer must know the layout of objects that flow to a property
       access site to determine whether it can be optimized to a direct dereference. That information
       is encoded in the constructor of these objects, which can be arbitrarily far away in source
       text from the property access considered for optimization.
           In turn, this gap causes optimization failures to be non-local; a failure at one program
       location—the property access site—can be resolved by program changes at a different
       location—the constructor. To provide actionable feedback to programmers, a coach must
       connect the two sites and link its reports to the solution site.
           Not all failures, however, are non-local in this manner. For example, failing to specialize
       a property access that receives multiple different types of objects is a purely local failure;
       it fails because the operation itself is polymorphic, which can only be solved by changing
       the operation itself. An optimization coach must therefore distinguish between local and
       non-local failures and target its reports accordingly. Our prototype accomplishes this using
       solution site inference.
           In addition, a coach should also merge near misses that have the same, or similar, solutions
       and report them together. Our prototype uses by-solution merging and by-constructor merging
       for this purpose. It also reuses the notion of locality merging (see [22]).

       6.1    Solution Site Inference
       The goal of solution site inference is to determine, for a given near miss, whether it could be
       resolved by changes at the site of the failing optimization or whether changes to the receivers’
       constructors may be required. We refer to the former as operation near misses and to the
       latter as constructor near misses. To reach a decision, the coach follows heuristics based
       on the cause of the failure, as well as on the types that flow to the affected operation. We
       briefly describe two of these heuristics.

       Monomorphic operations If an optimization fails for an operation to which a single receiver
       type flows, then that failure must be due to a property of that type, not of the operation’s
       context. The coach infers these cases to be constructor near misses.

       Property addition When a property assignment operation for property p receives an object
       that lacks a property p, the operation instead adds the property to the object. If the same
       operation receives both objects with a property p and objects without, that operation cannot
       be specialized for either mode of use. This failure depends on the operation’s context, and
       the coach infers it to be an operation near miss.

       6.2    Same-Property Analysis
       The merging techniques we describe below both depend on grouping near misses that affect
       the same property. The obvious definitions of “same property,” however, do not lead to
       satisfactory groupings. If we consider two properties with the same name to be the same,
       the coach would produce spurious groupings of unrelated properties from different parts of
       the program, e.g., grouping canvas.draw with gun.draw. Using these spurious groupings
       for merging would lead to incoherent reports that conflate unrelated near misses.
V. St-Amour and S. Guo                                                                                1009

Scheduler.prototype.schedule = function () {
   // this.currentTcb is only ever a TaskControlBlock
   this.currentTcb =;
}; = function () {
   // this.task can be all four kinds of tasks
   return this.task. run (packet);

IdleTask.prototype. run   = function (packet) { ... };
DeviceTask.prototype. run    = function (packet) { ... };
WorkerTask.prototype. run    = function (packet) { ... };
HandlerTask.prototype. run    = function (packet) { ... };

   Figure 5 Two different logical properties with name run in the Richards benchmark, one
underlined and one boxed.

    In contrast, if we considered only properties with the same name and the same hidden
class, the coach would discriminate too much and miss some useful groupings. For example,
consider the run property of various kinds of tasks in the Richards benchmark from the
Octane14 benchmark suite, boxed in figure 5. These properties are set independently for
each kind of task and thus occur on different hidden classes, but they are often accessed from
the same locations and thus should be grouped by the coach. This kind of pattern occurs
frequently when using inheritance or when using structural typing for ad-hoc polymorphism.
    To avoid these problems, we introduce another notion of property equivalence, logical
properties, which our prototype uses to guide its near-miss merging. We define two concrete
properties p1 and p2, which appear on hidden classes t1 and t2 respectively, to belong to
the same logical property if they
      have the same name p, and
      co-occur in at least one operation, i.e., there exists an operation o.p or o.p = v that
      receives objects of both class t1 and class t2

    As figure 5 shows, the four concrete run properties for tasks co-occur at an operation in
the body of, and therefore belong to the same logical
property., on the other hand, never co-occurs with the
other run properties, and the analysis considers it separate; near misses that are related to
it are unrelated from those affecting tasks’ run properties and should not be merged.

6.3      By-Solution Merging
In addition to linking near-miss reports with the likely location of their solution, an optimiza-
tion coach should group near misses with related solutions. That is, it should merge near
misses that can be addressed either by same program change or by performing analogous
changes at multiple program locations.


1010   Optimization Coaching for JavaScript

           Detecting whether multiple near misses call for the same kind of corrective action is a
       simple matter of comparing the causes of the respective failures and their context, as well as
       ensuring that the affected properties belong to the same logical property. This mirrors the
       work of the recommendation generation phase, as described in section 2.3.
           Once the coach identifies sets of near misses with related solutions, it merges each set
       into a single summary report. This new report includes the locations of individual failures,
       as well as the common cause of failure, the common solution and a badness score that is the
       sum of those of the merged reports.

       6.4    By-Constructor Merging
       Multiple near misses can often be solved at the same time by changing a single constructor. For
       example, inconsistent property layout for objects from one constructor can cause optimization
       failures for multiple properties, yet all of those can be resolved by editing the constructor.
       Therefore, merging constructor near misses that share a constructor can result in improved
       coaching reports.
           To perform this merging, the coach must identify which logical properties co-occur within
       at least one hidden class. To do this, it reuses knowledge about which logical properties
       occur within each hidden class from same-property analysis.
           Because, in JavaScript, properties can be added to objects dynamically—i.e., not inside
       the object’s constructor—a property occuring within a given hidden class does not necessarily
       mean that it was added by the constructor associated with that class. This may lead to
       merging reports affecting properties added in a constructor with others added elsewhere. At
       first glance, this may appear to cause spurious mergings, but it is in fact beneficial. For
       example, moving property initialization from the outside of a constructor to the inside often
       helps keeping object layout consistent. Reporting these near misses along with those from
       properties from the constructor helps reinforce this connection. We discuss instances of this
       problem in section 9.3.

        7     Coaching for an Advanced Compiler
       Advanced compilers such as IonMonkey operate differently from simpler compilers, such
       as the ahead-of-time portion of the Racket compiler, which we studied previously. An
       optimization coach needs to adapt to these differences. This section presents the challenges
       posed by two specific features of Ion that are absent in a simple compiler—JIT compilation
       and optimization tactics—and describes our solutions.

       7.1    JIT Compilation
       From a coaching perspective, JIT compilation poses two main challenges absent in an ahead-
       of-time (AOT) setting. First, compilation and execution are interleaved in a JIT system;
       there is no clear separation between compile-time and run-time, as there is in an AOT
       system. The latter’s separation makes it trivial for a coach’s instrumentation to not affect the
       program’s execution; instrumentation, being localized to the optimizer, does not cause any
       runtime overhead and emitting the optimization logs does not interfere with the program’s
       I/O proper. In a JIT setting, however, instrumentation may affect program execution, and a
       coach must take care when emitting optimization information.
           Second, whereas an AOT compiler compiles a given piece of code once, a JIT compiler
       may compile it multiple times as it gathers more information and possibly revises previous
V. St-Amour and S. Guo                                                                             1011

assumptions. In turn, a JIT compiler may apply—and fail to apply—different optimizations
each time. Hence, the near misses that affect a given piece of code may evolve over time, as
opposed to being fixed, as in the case of an AOT compiler. Near misses therefore need to be
ranked and merged along this new, temporal axis.
    Our prototype coach addresses both challenges via the use of a novel, profiler-driven
instrumentation strategy and by applying temporal merging, an extension of the locality
merging technique we presented in previous work.

7.1.1    Profiler-Driven Instrumentation
Our prototype coach uses SpiderMonkey’s profiling subsystem as the basis for its intrumenta-
tion. The SpiderMonkey profiler, as many profilers, provides an “event” API in addition to
its main sampling-based API. The former allows the engine to report various kinds of one-off
events that may be of interest to programmers: Ion compiling a specific method, garbage
collection, the execution bailing out of optimized code, etc.
    This event API provides a natural communication channel between the coach’s instrumen-
tation inside Ion’s optimizer and the outside world. As with an AOT coach, our prototype
records optimization decisions and context information as the optimizer processes code.
Where an AOT coach would emit that information on the fly, our prototype instead gathers
all the information pertaining to a given invocation of the compiler, encodes it as a profiler
event and emits it all at once. Our prototype’s instrumentation executes only when the
profiler is active; its overhead is therefore almost entirely pay-as-you-go.
    In addition to recording optimization information, the instrumentation code assigns a
unique identifier to the compiled code resulting from each Ion invocation. This identifier is
included alongside the optimization information in the profiling event. Object code that is
instrumented for profiling carries meta-information (e.g. method name and source location)
that allows the profiler to map the samples it gathers back to source code locations. We
include the compilation identifier as part of this meta-information, which allows the coach to
correlate profiler samples with optimization information, which in turn enables heuristics
based on profiling information as discussed below. This additional piece of meta-information
has negligible overhead and is present only when the profiler is active.

7.1.2    Profiling-Based Badness Metric
One of the key advantages of an optimization coach over raw optimization logs is the pruning
and ranking of near misses that a coach provides based on expected performance impact.
An AOT coach uses a number of static heuristics to estimate this impact.
    Our prototype incorporates profiling-based heuristics, which has two main advantages.
First, even in an AOT setting, actionable prioritization of near misses benefits from knowing
where programs spend their time; near misses in hot methods are likely to have a larger
impact on performance than those in cold code.
    Second, state-of-the-art JIT compilers may compile the same code multiple times—
producing different compiled versions of that code—potentially with different near misses
each time. A coach needs to know which of these compiled versions execute for a long time
and which are short-lived. Near misses from compiled versions that execute only for a short
time cannot have a significant impact on performance across the whole execution, regardless
of the number or severity of near misses, or how hot the affected method is overall. Because
profiler samples include compilation identifiers, our prototype associates each sample not

1012   Optimization Coaching for JavaScript

       only with particular methods, but with particular compiled versions of methods. It then
       enables the required distinctions discussed above.
           Concretely, our prototype uses the profiling weight of the compiled version of the function
       that surrounds a near miss as its badness score. We define the profiling weight of a compiled
       version to be the fraction of the total execution time that is spent executing it. Combined
       with temporal merging, this design ensures that near misses from hot compiled versions rise
       to the top of the rankings.
           To avoid overwhelming programmers with large numbers of potentially low-impact
       recommendations, our prototype prunes reports based on badness and shows only the five
       reports with the highest badness scores. This threshold has been effective in practice but is
       subject to adjustment.

       7.1.3        Temporal Merging
       Even though a JIT compiler may optimize methods differently each time they get compiled,
       this is not always the case. It is entirely possible to have an operation be optimized identically
       across multiple versions or even all of them. It happens, for instance, when recompilation is
       due to the optimizer’s assumptions not holding for a different part of the method or as a
       result of object code being garbage collected.15
           Identical near misses that originate from different invocations of the compiler necessarily
       have the same solution; they are symptoms of the same underlying issue. To reduce
       redundancy in the coach’s reports, we extend the notion of locality merging—which merges
       reports that affect the same operation—to operate across compiled version boundaries. The
       resulting technique, temporal merging, combines near misses that affect the same operation or
       constructor, originate from the same kind of failure and have the same causes across multiple
       compiled versions.

       7.2       Optimization Tactics
       When faced with an array of optimization options, Ion relies on optimization tactics to
       organize them. While we could consider each individual element of a tactic as a separate
       optimization and report near misses accordingly, all of a tactic’s elements are linked. Because
       the entire tactic returns as soon as one element succeeds, its options are mutually exclusive;
       only the successful option applies. To avoid overwhelming programmers with multiple reports
       about the same operation and provide more actionable results, a coach should consider a
       tactic’s options together.

       7.2.1        Irrelevant Failure Pruning
       Ion’s tactics often include strategies that only apply in narrow cases—e.g. indexing into
       values that are known to be strings, property accesses on objects that are known to be
       constant, etc. Because of their limited applicability, failure to apply these optimizations is
       not usually symptomatic of performance issues; these optimizations are expected to fail most
       of the time.

            In SpiderMonkey, object code is collected during major collections to avoid holding on to object code
            for methods that may not be executed anymore. While such collections may trigger more recompilation
            than strictly necessary, this tradeoff is reasonable in the context of a browser, where most scripts are
V. St-Amour and S. Guo                                                                              1013

    In these cases, we reuse the Racket coach’s irrelevant failure pruning technique. Failures
to apply optimizations that are expected to fail do not provide any actionable information to
programmers, and thus we consider them irrelevant. The coach prunes such failures from the
logs and does not show them in its reports.

7.2.2    Partial Success Shortcircuiting
While some elements of a given tactic may be more efficient than others, it is not always
reasonable to expect that all code be compiled with the best tactic elements. For example,
polymorphic call sites cannot be optimized as well as monomorphic call sites; polymorphism
notably prevents fixed-slot lookup. Polymorphism, however, is often desirable in a program.
Recommending that programmers eliminate it altogether in their programs is preposterous
and would lead to programmers ignoring the tool. Clearly, considering all polymorphic
operations to suffer from near misses is not effective.
    We partition a tactic’s elements according to source-level concepts—e.g., elements for
monomorphic operations vs polymorphic operations, elements that apply to array inputs
vs string inputs vs typed array inputs, etc.—and consider picking the best element from a
group to be an optimization success, so long as the operation’s context matches that group.
   For example, the coach considers picking the best possible element that is applicable to
polymorphic operations to be a success, as long as we can infer from the context that the
operation being compiled is actually used polymorphically. Any previous failures to apply
monomorphic-only elements to this operation would be ignored.
   With this approach, the coach reports polymorphic operations that do not use the best
possible polymorphic element as near misses, while considering those that do to be successes.
In addition, because the coach considers only uses of the best polymorphic elements to be
successes if operations are actually polymorphic according to their context, monomorphic
operations that end up triggering them are reported as near misses—as they should be.
   In addition to polymorphic property operations, our prototype applies partial success
shortcircuiting to array operations that operate on typed arrays and other indexable datatypes.
For example, Ion cannot apply dense-array access for operations that receive strings, but
multiple tactic elements can still apply in the presence of strings, some more performant
than others.

 8      Dead Ends

The previous sections describe successful coaching techniques, which result in actionable
reports. Along the way, we also implemented other techniques that ultimately did not prove
to be useful and which we removed from our prototype. These techniques either produced
reports that did not lead programmers to solutions or pointed out optimization failures that
did not actually impact performance.
    In the interest of saving other researchers from traveling down the same dead ends,
this section describes two kinds of optimization failures that we studied without success:
regressions and flip-flops. Both are instances of temporal patterns, that is, attempts by
the coach to find optimization patterns across time. None of our attempts at finding such
patterns yielded actionable reports, but there may be other kinds of temporal patterns that
we overlooked that would.

1014   Optimization Coaching for JavaScript

       8.1    Regression Reports
       The coach would report a regression when an operation that was optimized well during a
       compilation failed to be optimized as well during a subsequent one. This pattern occurred
       only rarely in the programs we studied, and when it did, it either was inevitable (e.g. a call
       site becoming polymorphic as a result of observing a sentinel value in addition to its usual
       receiver type) or did not point to potential improvements.

       8.2    Flip-Flop Reports
       As mentioned, SpiderMonkey discards object code and all type information during major
       collections. When this happens, the engine must start gathering type information and
       compiling methods from scratch. In some cases, this new set of type information may lead
       the engine to be optimistic in a way that was previously invalidated, then forgotten during
       garbage collection, leading to excessive recompilation. Engine developers refer to this process
       of oscillating back and forth between optimistic and conservative versions as flip-flopping.
           For example, consider a method that almost always receives integers as arguments, but
       sometimes receives strings as well. Ion may first optimize it under the first assumption,
       then have to back out of this decision after receiving strings. After garbage collection, type
       information is thrown away and this process starts anew. As a result, the method may end
       up being recompiled multiple times between each major collection.
           Engine developers believe that this behavior can cause significant performance issues,
       mostly because of the excessive recompilation. While we observed instances of flip-flopping
       in practice, modifying the affected programs to eliminate these recompilations often required
       significant reengineering and did not yield observable speedups.

        9     Evaluation
       For an optimization coach to be useful, it must provide actionable recommendations that
       improve the performance of a spectrum of programs. This section shows the results of
       evaluating our prototype along two axes: performance improvements and programmer effort.

       9.1    Experimental Protocol
       For our evaluation, we chose a subset of the widely-used Octane benchmark suite. We ran these
       programs using our prototype and modified them by following the coach’s recommendations.
       For each program, we applied all of the five top-rated recommendations, so long as the advice
       was directly actionable. That is, we rejected reports that did not suggest a clear course of
       action, as a programmer using the tool would do.
           To simulate a programmer looking for “low-hanging fruit,” we ran the coach only once
       on each program. Re-running the coach on a modified program may cause the coach to
       provide different recommendations. Therefore, it would in principle be possible to apply
       recommendations up to some fixpoint.
           For each program and recommendation, we measured a number of attributes to assess
       three dimensions of optimization coaching:

       Performance Impact Our primary goal is to assess the effect of recommended changes on
       program performance. Because a web page’s JavaScript code is likely to be executed by
       multiple engines, we used three of the major JavaScript engines: SpiderMonkey, Chrome’s
       V8 and Webkit’s JavaScriptCore.
V. St-Amour and S. Guo                                                                                        1015

   The Octane suite measures performance in terms of an Octane Score which, for the
benchmarks we discuss here, is inversely proportional to execution time.16 Our plots show
scores normalized to the pre-coaching version of each program with error bars marking 95%
confidence intervals. All our results represent the mean score of 30 executions on a 6-core
64-bit x86 Debian GNU/Linux system with 12GB of RAM. To eliminate confounding factors
due to interference from other browser components, we ran our experiments in standalone
JavaScript shells.

Programmer Effort As a proxy for programmer effort, we measured the number of lines
changed in each program while following recommendations. We also recorded qualitative
information about the nature of these changes.

Recommendation Usefulness To evaluate the usefulness of individual recommendations,
we classified them into four categories:
      positive recommendations led to an increase in performance,
      negative recommendations led to a decrease in performance,
      neutral recommendations did not lead to an observable change in performance, and
      non-actionable reports did not suggest a clear course of action.

For this aspect of the evaluation, we measured the impact of individual recommendations
under SpiderMonkey alone.
   Ideally, a coach should give only positive recommendations. Negative recommendations
require additional work on the part of the programmer to identify and reject. Reacting to
neutral recommendations is also a waste of programmer time, and thus their number should
be low, but because they do not harm performance, they need not be explicitly rejected
by programmers. Non-actionable recommendations decrease the signal-to-noise ratio of
the tool, but they can individually be dismissed pretty quickly by programmers. A small
number of non-actionable recommendations therefore does not contribute significantly to the
programmer’s workload. Large numbers of non-actionable recommendations, however, would
be cause for concern.

9.2       Program Selection
Our subset of the Octane suite focuses on benchmarks that use property and array operations
in a significant manner. It excludes, for example, the Regexp benchmark because it exercises
nothing but an engine’s regular expression subsystems. Coaching these programs would not
yield any recommendations with our current prototype. It also excludes machine-generated
programs from consideration. The output of, say, the Emscripten C/C++ to JavaScript
compiler17 is not intended to be read or edited by humans; it is therefore not suitable for
coaching.18 In total, the set consists of eight programs: Richards, DeltaBlue, RayTrace,
Splay, NavierStokes, PdfJS, Crypto and Box2D.

     The Octane suite also includes benchmarks whose scores are related to latency instead of execution
     time, but we did not use those for our experiments.
     It would, however, be possible to use coaching to improve the code generation of Emscripten or other
     compilers that target JavaScript, such as Shumway. This is a direction for future work.

1016   Optimization Coaching for JavaScript



                             score (higher
                                   (higher   is
                                    (higher is
                                            is  better)
                                             is better)


                         zed score




                                                                 Richards DeltaBlue
                                                                 Richards DeltaBlue
                                                                          DeltaBlue RayTrace
                                                                          DeltaBlue RayTrace
                                                                                    RayTrace   Splay
                                                                                               Splay Navi
                                                                                               Splay Navi erStokes
                                                                                                          erStokes PdfJS
                                                                                                     Navi erStokes PdfJS
                                                                                                                   PdfJS   Crypto
                                                                                                                           Crypto   Box2D

             Figure 6 Benchmarking results on SpiderMonkey

       9.3               Results and Discussion
       As figure 6 shows, following the coach’s recommendations leads to significant19 speedups on
       six of our eight benchmarks when run on SpiderMonkey. These speedups range from 1.02ˆ
       to 1.17ˆ. For the other two benchmarks, we observe no significant change; in no case do we
       observe a slowdown.
           The results are similar for the other engines, see figure 7. On both V8 and JavaScriptCore,
       we observe significant speedups on two and three benchmarks, respectively, ranging from
       1.02ˆ to 1.20ˆ. These speedups differ from those observed using SpiderMonkey, but are
       of similar magnitude. Only in the case of the DeltaBlue benchmark on JavaScriptCore is
       there a significant slowdown. These results provide evidence that, even though coaching
       recommendations are derived from the optimization process of a single engine, they can lead
       to cross-engine speedups.
           Keeping in mind that JavaScript engines are tuned to perform well on those benchmark
       programs,20 we consider these results quite promising. We conjecture that our prototype (or
       an extension of it) could yield even larger speedups on other, regular programs for which the
       engine is not specifically tuned.
           Figure 8 presents our results for the effort and usefulness dimensions. For all programs,
       the total number of lines changed is at most 42. Most of these changes are also fairly
       mechanical in nature—moving code, search and replace, local restructuring. Together, these
       amount to modest efforts on the programmer’s part.

            We consider speedups to be significant when the confidence intervals of the baseline and coached versions
            do not overlap.
V. St-Amour and S. Guo                                                                                                                                                                                                                                                                                                                          1017

                                          1.2                                                                                                                                     1.2

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                    Figure 7 Benchmarking results on V8 and JavaScriptCore

    We classified 17 out of 35 reports as positive, and only one as negative. We classified
12 reports as non-actionable, which we consider acceptably low. As discussed above, those
reports can be dismissed quickly and do not impose a burden. The remainder of the section
presents the coach’s recommendations for individual benchmarks.

Richards The coach provides three reports. Two of those point out an inconsistency in the
layout of TaskControlBlock objects. Figure 9 shows one of them. The state property
is initialized in two different locations, which causes layout inference to fail and prevents
optimizations when retrieving the property. Combining these two assignments into one, as
figure 10 shows, solves the issue and leads to a speedup of 1.03ˆ on SpiderMonkey. The
third report points to an operation that is polymorphic by design; it is not actionable.

DeltaBlue Two of the five reports have a modest positive impact. The first involves
replacing a singleton object’s properties with global variables to avoid dispatch; it is shown
in figure 11. The second recommends duplicating a superclass’s method in its subclasses,
making them monomorphic in the process.
    These changes may hinder modularity and maintainability in some cases. They clearly
illustrate the tradeoffs between performance and software engineering concerns, which
coaching tends to bring up. Which of those is more important depends on context, and the
decision of whether to follow a recommendation must remain in the programmer’s hands.
With a coach, programmers at least know where these tradeoffs may pay off by enabling
additional optimization.
    One of the recommendations (avoiding a prototype chain walk) yields a modest slowdown
of about 1%. This report has the lowest badness score of the five. We expect programmers
tuning their programs to try out these kinds of negative recommendations and revert them
after observing slowdowns.

1018   Optimization Coaching for JavaScript

            Benchmark       Size               Lines changed                   Recommendation impact
                          (SLOC)                  (SLOC)                        (# recommendations)
                                     Added       Deleted   Edited   Positive    Negative   Neutral   Non-act.

           Richards            538        1         5          0       2            0        0          1
          DeltaBlue            881        12        6          24      2            1        1          1
           RayTrace            903        10        11          0      5            0        0          0
               Splay           422         3        3           0      2            0        1          2
        NavierStokes           415         0         0          4      0            0        1          0
               PdfJS        33,053         2        1           0      0            0        1          4
             Crypto          1,698         2        0           1      4            0        0          1
              Box2D         10,970        8         0          0       2            0        0          3

            Figure 8 Summary of changes following recommendations

             badness: 24067
             for object type: TaskControlBlock:richards.js:255

             affected properties:
                 state (badness: 24067)

             This property is not guaranteed to always be in the same location.

             Are properties initialized in different orders in different places?
               If so, try to stick to the same order.
             Is this property initialized in multiple places?
               If so, try initializing it always in the same place.
             Is it sometimes on instances and sometimes on the prototype?
               If so, try using it consistently.

            Figure 9 Report of inconsistent property order in the Richards benchmark

        // before coaching
        if (queue == null) {
          this.state = STATE_SUSPENDED;
        } else {
          this.state = STATE_SUSPENDED_RUNNABLE;

        // after coaching
        this.state = queue == null ? STATE_SUSPENDED : STATE_SUSPENDED_RUNNABLE;

            Figure 10 Making object layout consistent in the Richards benchmark
V. St-Amour and S. Guo                                                                                             1019

        badness: 5422
        for object type: singleton
        affected properties:
            WEAKEST (badness: 2148)
            REQUIRED (badness: 1640)
            STRONG_DEFAULT (badness: 743)
            PREFERRED (badness: 743)
            NORMAL (badness: 147)

        This object is a singleton.
        Singletons are not guaranteed to have properties in a fixed slot.
        Try making the object's properties globals.

      Figure 11 Recommendation to eliminate a singleton object in the DeltaBlue benchmark

RayTrace All five of the coach’s reports yield performance improvements, for a total of
1.17ˆ on SpiderMonkey, 1.09ˆ on V8 and 1.20ˆ on JavaScriptCore. The proposed changes
include reordering property assignments to avoid inconsistent layouts, as well as replacing a
use of prototype.js’s class system with built-in JavaScript objects for a key data structure.
All these changes are mechanical in nature because they mostly involve moving code around.

Splay This program is the same as the example in section 2.2. Of the five reports, three
recommend moving properties from a prototype to its instances. These properties are using
a default value on the prototype and are sometimes left unset on instances, occasionally
triggering prototype chain walks. The fix is to change the constructor to assign the default
value to instances explicitly. While this may cause additional space usage by making instances
larger, the time/space tradeoff is worthwhile and leads to speedups on all three engines. Two
of the three changes yield speedups, with the third one not having a noticeable effect.

NavierStokes The coach provides a single recommendation for this program. It points
out that some array accesses are not guaranteed to receive integers as keys. Enforcing this
guarantee by bitwise or’ing the index with 0, as is often done in asm.js codebases, solves this
issue but does not yield noticeable performance improvements. It turns out that the code
involved only accounts for only a small portion of total execution time.

PdfJS One of the coach’s reports recommends initializing two properties in the constructor,
instead of waiting for a subsequent method call to assign them, because the latter arrangement
results in inconsistent object layouts. As with the recommendation for the NavierStokes
benchmark, this one concerns cold code21 and does not lead to noticeable speedups.
    We were not able to make changes based on the other four recommendations, which may
have been due to our lack of familiarity with this large codebase. Programmers more familiar
with PdfJS’s internals may find these reports more actionable.

Crypto Four of the five reports are actionable and lead to speedups. Three of the four
concern operations that sometimes add a property to an object and sometimes assign an
existing one, meaning that they therefore cannot be specialized for either use. Initializing

     PdfJS’s profile is quite flat in general, suggesting that most low-hanging fruit has already been picked,
     which is to be expected from such a high-profile production application.

1020   Optimization Coaching for JavaScript

       those properties in the constructor makes the above operations operate as assignments
       consistently, which solves the problem. The last positive recommendation concerns array
       accesses; it is similar to the one discussed in conjunction with the NavierStokes benchmark,
       with the exception that this one yields speedups.

       Box2D Two of the reports recommend consistently initializing properties, as with the PdfJS
       benchmark. Applying those changes yields a speedup of 1.07ˆ on SpiderMonkey. The other
       three recommendations are not actionable due to our cursory knowledge of this codebase.
       As with PdfJS, programmers knowledgeable about Box2D’s architecture may fare better.
           For reference, the Octane benchmark suite uses a minified version of this program. As
       discussed above, minified programs are not suitable for coaching so we used a non-minified,
       but otherwise identical, version of the program.

        10     Related Work
       This work is not the only attempt at helping programmers take advantage of their compilers’
       optimizers. This section discusses tools with similar goals and compares them with our work.

       10.1     Optimization Logging
       From an implementation perspective, the simplest way to inform programmers about the
       optimizer’s behavior on their programs is to provide them with logs recording its optimization
       decisions. This is the approach taken by tools such as JIT inspector [12] and IRHydra [8],
       both of which report optimization successes and failures, as well as other optimization-related
       events such as dynamic deoptimizations. JIT inspector reports optimizations performed by
       IonMonkey, while IRHydra operates with the V8 and Dart compilers.
           Similar facilities also exist outside of the JavaScript world. For instance, Common Lisp
       compilers such as SBCL [23] and LispWorks [18] report both optimization successes and
       optimization failures, such as failures to specialize generic operations or to allocate objects on
       the stack. The Cray XMT C and C++ compilers [4] report both successful optimimizations
       and parallelization failures. The Open Dylan IDE [6, chapter 10] reports optimizations such
       as inlining and dispatch optimizations using highlights in the IDE’s workspace.
           These tools provide reports equivalent to the raw output of our prototype’s instrumentation
       without any subsequent analysis, interpretation or recommendations. Expert programmers
       knowledgeable about compiler internals may find this information actionable and use it as a
       starting point for their tuning efforts. In constrast, our prototype coach targets programmers
       who may not have the necessary knowledge and expertise to digest such raw information,
       and it does so by providing recommendations that only require source-level knowledge.

       10.2     Rule-Based Performance Bug Detection
       Some performance tools use rule-based approaches to detect code patterns that may be
       symptomatic of performance bugs.
           JITProf [10] is a dynamic analysis tool for JavaScript that detects code patterns that
       JavaScript JIT compilers usually do not optimize well. The tool looks for six dynamic patterns
       during program execution, such as inconsistent object layouts and arithmetic operations on
       the undefined value, and reports instances of these patterns to programmers.
           The JITProf analysis operates independently from the host engine’s optimizer; its patterns
       essentially constitute a model of a typical JavaScript JIT compiler. As a result, JITProf
V. St-Amour and S. Guo                                                                                    1021

does not impose any maintenance burden on engine developers, unlike a coach whose
instrumentation must live within the engine itself. Then again, this separation may cause
the tool’s model to be inconsistent with the actual behavior of engines, either because the
model does not perfectly match an engine’s heuristics, or because engines may change their
optimization strategies as their development continues. In contrast, an optimization coach
reports ground truth by virtue of getting its optimization information from the engine itself.
    By not being tied to a specific engine, JITProf’s reports are not biased by the imple-
mentation details of that particular engine. Section 9 shows, however, that engines behave
similarly enough in practice that a coach’s recommendations, despite originating from a
specific engine, usually lead to cross-engine performance improvements.
    Jin et al. [16] distill performance bugs found in existing applications to source-level
patterns which can then be used to detect similar latent bugs in other applications. Their
tool suggests fixes for these new bugs based on those used to resolve the original bugs. Their
work focuses on API usage and algorithms, and is complementary to optimization coaching.
    Chen et al. [3] present a tool that uses static analysis to detect performance anti-patterns
that result from the use of object-relational mapping in database-backed applications. The
tool detects these anti-patterns using rules that the authors synthesized from observing
existing database-related performance bugs. To cope with the large number of reports, the
tool estimates the performance impact of each anti-pattern, and uses that information to
prioritize reports. This is similar to the use of ranking by optimization coaches.

10.3     Profilers
When they encounter performance issues, programmers often reach for a profiler [11, 19, 21,
24]. Unlike an optimization coach, a profiler does not point out optimization failures directly.
Instead, it identifies portions of the program where most of its execution time is spent, some
of which may be symptomatic of optimization failures. That inference, however, is left to
    Profilers also cannot distinguish between code that naturally runs for a long time from code
that runs for an abnormally long time. Again, the programmer is called upon to make this
distinction. In contrast, coaches distinguish between optimization failures that are expected
from those that are not. In addition, coaches aim to provide actionable recommendations to
programmers, whereas profilers report data without pointing towards potential solutions.
    Note, though, that profilers can point to a broader range of performance issues than
optimization coaches. For example, a profiler would report code that runs for a long time
due to an inefficient algorithm, which an optimization coach could not detect. To summarize,
the two classes of tools cover different use cases and are complementary.

10.4     Assisted Optimization
A number of performance tools are aimed at helping programmers optimize specific aspects
of program performance. This section discusses the ones most closely related to this work.
    Larsen et al. [17] present an interactive tool that helps programmers parallelize their
programs. Like an optimization coach, their tool relies on compiler instrumentation to
reconstruct the optimization process—specifically automatic parallelization—and discover the
causes of parallelization failures. Larsen et al.’s tool is specifically designed for parallelization
and is thus complementary to optimization coaching.
    Precimonious [20] is a tool that helps programmers balance precision and performance
in floating-point computations. It uses dynamic program analysis to discover floating-point

1022   Optimization Coaching for JavaScript

       variables that can be converted to use lower-precision representations without affecting
       the overall precision of the program’s results. The tool then recommends assignments of
       precisions to variables that programmers can apply. This workflow is similar to that of an
       optimization coach, but applied to a different domain.
          Xu et al. [25] present a tool that detects data structures that are expensive to compute,
       but that the program either does not use, or only uses a small portion of. Based on the
       tool’s reports, programmers can replace the problematic structures with more lightweight
       equivalents that only store the necessary data. The tool relies on a novel program slicing
       technique to detect those low-utility data structures. This tool is also complementary to
       optimization coaches.

        11     Conclusion
       In this paper, we present an adaptation of optimization coaching to the world of dynamic
       object-oriented languages with advanced JIT compilers. The additional constraints imposed
       by these languages and their compilers require novel coaching techniques such as profiler-based
       instrumentation and solution-site inference.
           We additionally provide evidence, in the form of case studies using well-known benchmark
       programs, that optimization coaching is an effective means of improving the performance of
       JavaScript programs. The evaluation also shows that its usage is well within the reach of
       JavaScript programmers.

       Acknowledgments We would like to thank Niko Matsakis, Dave Herman, and Michael
       Bebenita for discussions and suggestions about the tool’s design and development. Kannan
       Vijayan, Luke Wagner, and Nicolas Pierron helped with the design of the profiler-driven
       instrumentation. Finally, we thank Matthias Felleisen, Sam Tobin-Hochstadt and Jan Vitek
       for their comments on previous drafts.
           This work was partially supported by Darpa, NSF SHF grants 1421412, 1421652, and

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Optimization Coaching for JavaScript

Authors Shu-yu Guo Vincent St-Amour

License CC-BY-3.0