dart-sdk/pkg/compiler/README.md
Michael Thomsen e84412af45 Remove unsupported APIs in dart:mirrors
Contributes to bug: https://github.com/dart-lang/sdk/issues/34233
Fixes https://github.com/dart-lang/sdk/issues/32507

Change-Id: Iee72e9f91fc1fc49761e6f5e6e9dba412493c8ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/248003
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Michael Thomsen <mit@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
2022-07-20 18:28:35 +00:00

624 lines
26 KiB
Markdown

# The dart2js compiler
Welcome to the sources of the dart2js compiler!
## Architecture
The compiler is structured to operate in several phases. By default these phases
are executed in sequence in a single process, but on some build systems, some of
these phases are split into separate processes. As such, there is plenty of
indirection and data representations used mostly for the purpose of serializing
intermediate results during compilation.
The current compiler phases are:
1. **common front-end**: Execute traditional front-end compilation phases.
Dart2js delegates to the common front-end (also used by DDC and the VM) to
do all front-end features, this includes:
* parsing Dart source code,
* type checking,
* inferring implicit user types, like locals with a `var` declaration,
* lowering or simplifying Dart features. For example, this is how many
syntactic features, like extension methods and list comprehensions, are
implemented.
* additional web-specific lowering or simplifications. For example,
expansion of JS-interop features and web specific implementation of
language features like late variables.
The result of this phase is a kernel AST which is serialized as a `.dill`
file.
2. **modular analysis**: Using kernel as input, compute data recording
properties about each method in the program, especially around dependencies
and features they may need. We call this "impact data" (i1).
When the compiler runs as a single process, this is done lazily/on-demand
during the tree-shaking phase (below). However, this data can also be
computed independently for individual methods, files, or packages in the
application. That makes it possible to run this modularly and in parallel.
The result of this phase can be emitted as files containing impact data in
a serialized format.
3. **tree-shake and create world**: Create a model to understand what parts of
the code are used by an application. This consists of:
* creating an intermediate representation called the "K model" that
wraps our kernel representation
* calculating which classes and methods are considered live in the
program. This is done by incrementally combining impact data (i1)
starting from `main`, then visiting reachable methods in the program
with an Rapid Type Analysis (RTA) algorithm to aggregate impacts
together.
The result of this phase is what we call a "closed world" (w1). The closed
world is also a datastructure that can answer interesting queries, such as:
Is this interface implemented by a single class? Is this method available
in any stubtype of some interface? The answers to these questions can help
the compiler generate higher quality JavaScript.
4. **global analysis**: Run a global analysis that assumes closed world
semantics (from w1) and propagates information across method boundaries to
further understand what values flow through the program. This phase is
very valuable in narrowing down possibilities that are ambiguous based
solely on type information written by developers. It often finds
oportunities that enable the compiler to devirtualize or inline method
calls, generate code specializations, or trigger performance optimizations.
The result of this phase is a "global result" (g).
5. **codegen model**: Create a JS or backend model of the program. This is an
intermediate representation of the entities in the program we referred to
as the "J model". It is very similar to the "K model", but it is tailored
to model JavaScript specific concepts (like the split of constructor bodies
as separate elements) and provide a mapping to the Dart model.
6. **codegen**: Generate code for each method that is deemed necessary. This
includes:
* build an SSA graph from kernel ASTs and global results (g)
* optimize the SSA representation
* compute impact (i2) from optimized code
* emit JS ASTs for the code
7. **link tree-shake**: Using the results of codegen, we perform a second
round of tree-shaking. This is important because code that was deemed
reachable in (w1) may be found unreachable after optimizations. The process
is very similar to the earlier phase: we combine incrementally the codegen
impact data (i2) and compute a codegen closed world (w2).
When dart2js runs as a single process the codegen phase is done lazily and
on-demand, together with the tree-shaking phase.
8. **emit JavaScript files**: The final step is to assemble and minify the
final program. This includes:
* Build a JavaScript program structure from the compiled pieces (w2)
* Use frequency namer to minify names.
* Emit js and source map files.
## Code organization
### Some terminology used in the compiler
* **world**: the compiler exploits closed-world assumptions to do
optimizations. The *world* encapsulates some of our knowledge of the
program, like what's reachable from `main`, which classes are instantiated,
etc.
* **universe**: rather than loading entire programs and removing unreachable
code, the compiler uses a tree-growing approach: it enqueues work based on
what it sees. While this is happening the *world* is considered to be
growing, in the past the term *universe* was used to describe this growing
world. While the term is not completely deleted from the codebase, a lot of
progress has been made to rename *universe* into *world builders*.
* **model**: there are many models in the compiler:
* **entity model**: this is an abstraction describing the elements seen in
Dart programs, like "libraries", "classes", "methods", etc. We currently
have two entity models, the "K model" (which is frontend centric and
usually maps 1:1 with kernel entities) and the "J model" (which is backend
centric).
* **emitter model**: this is a model just used for dumping out the structure
of the program in a .js text file. It doesn't have enough semantic meaning
to be a JS model for compilation, which is why there is a separate "J
model".
* **enqueuer**: a work-queue used to achieve tree-shaking (or more precisely
tree-growing): elements are added to the enqueuer as we recognize that they
are needed in a given application (as described by the impact data). Note
that we even track how elements are used, since some ways of using an
element require more code than others.
### Code layout
Here are some details of our current code layout and what's in each file. This
list also includes some action items (labeled AI below), which are mainly
cleanup tasks that we have been discussing for a while:
**bin folder**: some experimental command-line entrypoints, these need to be
revisited
* `bin/dart2js.dart`: is a dart2js entry point, not used today other than
locally for development, most of our tools launch dart2js from
`lib/src/dart2js.dart` instead.
AI: change how we build the SDK to launch dart2js from here, most logic might
remain inside `lib/src/dart2js.dart` for testing purposes.
**lib folder**: API to use dart2js as a library. This is used by our
command-line tool to launch dart2js, but also by pub to invoke dart2js as a
library during `pub-build` and `pub-serve`.
* `lib/compiler_api.dart`: the compiler API. This API is used by our
command-line tool to spawn the dart2js compiler. This API (and everything that
is transitively created from it) has no dependencies on `dart:io` so that the
compiler can be used in contexts where `dart:io` is not available (e.g.
running in a browser worker) or where `dart:io` is not used explicitly (e.g.
running as a pub transformer).
**lib/src folder**: most of the compiler lives here, as very little of its
functionality is publicly exposed.
* `lib/src/dart2js.dart`: the command-line script that runs dart2js. When
building the SDK, the dart2js snapshot is built using the main method on this
script. This file creates the parameters needed to invoke the API defined in
`lib/compiler.dart`. All dependencies on `dart:io` come from here. This is
also where we process options (although some of the logic is done in
`options.dart`).
* `lib/src/compiler.dart`: defines the core `Compiler` object, which contains
all the logic about what the compiler pipeline does and how data is organized
and communicated between different phases.
* `lib/src/closure.dart`: closures are compiled as classes, this file has the
logic to do this kind of conversion in the Dart element model. This includes
computing what needs to be boxed and creating fake element models to represent
closures as classes. We use the fake model approach because the compiler
currently uses the same element model for Dart and JS. Our goal with the
compiler rearchitecture described earlier is to have two models. The
Dart model will be able to encode closures directly, and we'll introduce their
corresponding classes when we create the corresponding JS model, removing the
need of the fake elements.
* `lib/src/colors.dart`: ANSI support for reporting error messages with colors.
AI: this file should move under a utilities folder.
* Handling of options: as mentioned earlier `lib/src/dart2js.dart` has some
handling of command-line options, the rest is divided into these files:
* `lib/src/commandline_options.dart`: defines the flags that dart2js accepts.
* `lib/src/options.dart`: defines first-class objects to represent options of
dart2js. This includes a parse function that can translate flags into their
corresponding objects. This was added recently to simplify how options were
propagated throughout the compiler.
AI: simplify further how we specify options. Ideally all changes can be done
in a single file (`options.dart`?), and unit-tests can specify options via an
options object rather than command-line flags.
* `lib/src/common.dart`: convenience file that reexports code used in many
places in the compiler.
AI: consider deleting this file.
* Constants: the compiler has a constant system that evaluates constant
expressions based on JS semantics.
* `lib/src/constants/value.dart`: this is the represented value of a constant
after it has been evaluated.
* `lib/src/constants/constant_system.dart`: implements evaluating constant
Dart expressions and produces values.
* Common elements: the compiler often refers to certain elements during
compilation either because they are first-class in the language or because
they are implicitly used to implement some features. These include:
* `lib/src/common/elements.dart`: provides an interface to lookup basic
elements like the class of `Object`, `int`, `List`, and their corresponding
interface types, constructors for symbols, annotations such as the `identical` function. These are normally restricted
to elements that are understood directly in Dart.
* `lib/src/js_backend/backend_helpers.dart`: provides a way to lookup internal
elements of the Javascript backend, like our internal
representation of JSInt31, JSArray, and other implementation-specific
elements.
* `lib/src/deferred_load/deferred_load.dart`: general analysis for deferred
loading. This is where we compute how to split the code in different JS chunks
or fragments. This is run after resolution, but at a time when no code is
generated yet, so the decisions made here are used later on by the emitter to
dump code into different files.
* `lib/src/dump_info.dart`: a special phase used to create a .info.json file.
This file contains lots of information computed by dart2js including decisions
about deferred loading, results of the global type-inference, and the actual
code generated for each function. The output is used by tools provided in the
`dart2js_info` package to analyze a program and better understand why
something wasn't optimized as you'd expect.
* Tree-shaking: The compiler does two phases of reducing the program size by
throwing away unreachable code. The first phase is done while resolving the
program (reachablity is basically based on dependencies that appear in the
code), the second phase is done as functions are optimized (which in turn can
delete branches of the code and make more code unreachable). Externally
we refer to it as tree-shaking, but it behaves more like a tree-growing
algorithm: elements are added as they are discovered or as they are used.
On some large apps we've seen 50% of code tree-shaken: 20% from the first
phase, and an additional 30% from the second phase.
* `lib/src/enqueue.dart`: this is the basic algorithm that adds things as they
are discovered during resolution.
* `lib/src/js_backend/enqueuer.dart`: this is the enqueuer used during code
generation.
* `lib/src/environment.dart`: simple interface for collecting environment values
(these are values passed via -D flags on the command line).
* `lib/src/filenames.dart`: basic support for converting between native and Uri
paths.
AI: move to utils
* `lib/src/id_generator.dart`: simple id generator
AI: move to utils
* `lib/src/library_loader.dart`: the loader of the dart2js frontend. Asks the
compiler to read and scan files, produce enough metadata to understand
import, export, and part directives and keep crawling. It also triggers the
patch parser to load patch files.
* Input/output: the compiler is designed to avoid all dependencies on dart:io.
Most data is consumed and emitted via provider APIs.
* `lib/src/compiler.dart`: defines the interface of these providers (see
`CompilerInput` and `CompilerOutput`).
* `lib/src/null_compiler_output.dart`: a `CompilerOutput` that discards all
data written to it (name derives from /dev/null).
* `lib/src/source_file_provider.dart`: _TODO: add details_.
* Parsing: most of the parsing logic is now in the `front_end` package,
currently under `pkg/front_end/lib/src/fasta/scanner` and
`pkg/front_end/lib/src/fasta/parser`. The `front_end` parser is AST agnostic
and uses listeners to create on the side what they want as the result of
parsing. The logic to create dart2js' ASTs is defined in listeners within the
compiler package:
* `lib/src/parser/element_listener.dart`: listener used to create the first
skeleton of the element model (used by the diet parser)
* `lib/src/parser/partial_elements.dart`: representation of elements in the
element model whose body is not parsed yet (e.g. a diet-parsed member).
* `lib/src/parser/node_listener.dart`: listener used to create the body of
methods.
* `lib/src/parser/member_listener.dart`: listener used to attach method bodies
to class members.
* `lib/src/parser/parser_task.dart`: Task to execute the full parser.
* `lib/src/parser/diet_parser_task.dart`: Task to execute diet parsing.
* `lib/src/patch_parser.dart`: additional support for parsing patch files. We
expect this will also move under `front_end` in the future.
* URI resolution: the compiler needs special logic to resolve `dart:*` URIs and
`package:*` URIs. These are specified in three parts:
* sdk library files are specified in a .platform file. This file has a special
.ini format which is parsed with `lib/src/platform_configuration.dart`.
* sdk patch files are hardcoded in the codebase in
`lib/src/js_backend/backend.dart` (see `_patchLocations`).
* `lib/src/resolved_uri_translator.dart`: has the logic to translate all these
URIs when they are encountered by the library loader.
AI: consider changing the .platform file format to yaml.
* `lib/src/typechecker.dart`: the type checker (spec mode semantics, no support
for strong mode here).
* World: _TODO: add details_
* `lib/src/world.dart`
* `lib/src/universe/call_structure.dart`
* `lib/src/universe/use.dart`
* `lib/src/universe/feature.dart`
* `lib/src/universe/world_impact.dart`
* `lib/src/universe/selector.dart`
* `lib/src/universe/side_effects.dart`
* `lib/src/universe/class_set.dart`
* `lib/src/universe/world_builder.dart`
* `lib/src/universe/function_set.dart`
* Testing, debugging, and what not: _TODO: add details_
* `lib/src/tracer.dart`
* `lib/src/use_unused_api.dart`
* SSA (`lib/src/ssa`): internal IR used to optimize functions before emitting
JavaScript. _TODO: add details_.
* `ssa.dart`
* `kernel_string_builder.dart`
* `codegen.dart`
* `variable_allocator.dart`
* `type_builder.dart`
* `value_set.dart`
* `types.dart`
* `jump_handler.dart`
* `codegen_helpers.dart`
* `switch_continue_analysis.dart`
* `types_propagation.dart`
* `nodes.dart`
* `kernel_ast_adapter.dart`
* `graph_builder.dart`
* `validate.dart`
* `builder.dart.rej`
* `interceptor_simplifier.dart`
* `builder_kernel.dart`
* `locals_handler.dart`
* `optimize.dart`
* `kernel_impact.dart`
* `invoke_dynamic_specializers.dart`
* `builder.dart`
* `ssa_branch_builder.dart`
* `value_range_analyzer.dart`
* `ssa_tracer.dart`
* `loop_handler.dart`
* `tool`: some helper scripts, some of these could be deleted
* `tool/perf.dart`: used by our benchmark runners to measure performance of
some frontend pieces of dart2js. We should be able to delete it in the near
future once the front end code is moved into `fasta`.
* `tool/perf_test.dart`: small test to ensure we don't break `perf.dart`.
* `tool/track_memory.dart`: a helper script to see memory usage of dart2js
while it's running. Used in the past to profile the global analysis phases
when run on very large apps.
* `tool/dart2js_stress.dart` and `tool/dart2js_profile_many.dart`: other
helper wrappers to make it easier to profile dart2js with Observatory.
* Source map tracking (`lib/src/io`): helpers used to track source information
and to build source map files. _TODO: add details_.
* `lib/src/io/code_output.dart`
* `lib/src/io/source_map_builder.dart`
* `lib/src/io/start_end_information.dart`
* `lib/src/io/position_information.dart`
* `lib/src/io/source_information.dart`
* `lib/src/io/source_file.dart`
* `lib/src/io/line_column_provider.dart`
* Kernel conversion (`lib/src/kernel`): temporary code to create kernel within
dart2js (previously known as `rasta`). Most of this code will be gone when we
are in the final architecture. _TODO: add details_.
* `lib/src/kernel/task.dart`
* `lib/src/kernel/kernel_visitor.dart`
* `lib/src/kernel/kernel_debug.dart`
* `lib/src/kernel/unresolved.dart`
* `lib/src/kernel/kernel.dart`
* `lib/src/kernel/unavailable.dart`
* `lib/src/kernel/accessors.dart`
* `lib/src/kernel/constant_visitor.dart`
* `lib/src/kernel/error.dart`
* Global whole-program analysis (a.k.a. type inference): We try to avoid the
term "type inference" to avoid confusion with strong-mode type inference.
However the code still uses the term inference for this global analysis. The
code is contained under `lib/src/inferrer`. _TODO: add details_.
* `lib/src/inferrer/type_graph_dump.dart`
* `lib/src/inferrer/node_tracer.dart`
* `lib/src/inferrer/list_tracer.dart`
* `lib/src/inferrer/closure_tracer.dart`
* `lib/src/inferrer/inferrer_engine.dart`
* `lib/src/inferrer/type_graph_inferrer.dart`
* `lib/src/inferrer/type_graph_nodes.dart`
* `lib/src/inferrer/type_system.dart`
* `lib/src/inferrer/debug.dart`
* `lib/src/inferrer/locals_handler.dart`
* `lib/src/inferrer/map_tracer.dart`
* `lib/src/inferrer/builder.dart`
---------
_TODO: complete the documentation for the following files_.
`lib/src/ordered_typeset.dart`
`lib/src/script.dart`
`lib/src/string_validator.dart`
`lib/src/native`
`lib/src/native/ssa.dart`
`lib/src/native/scanner.dart`
`lib/src/native/js.dart`
`lib/src/native/enqueue.dart`
`lib/src/native/behavior.dart`
`lib/src/native/native.dart`
`lib/src/js_emitter`
`lib/src/js_emitter/native_emitter.dart`
`lib/src/js_emitter/main_call_stub_generator.dart`
`lib/src/js_emitter/model.dart`
`lib/src/js_emitter/headers.dart`
`lib/src/js_emitter/native_generator.dart`
`lib/src/js_emitter/parameter_stub_generator.dart`
`lib/src/js_emitter/constant_ordering.dart`
`lib/src/js_emitter/program_builder`
`lib/src/js_emitter/program_builder/collector.dart`
`lib/src/js_emitter/program_builder/program_builder.dart`
`lib/src/js_emitter/program_builder/field_visitor.dart`
`lib/src/js_emitter/program_builder/registry.dart`
`lib/src/js_emitter/metadata_collector.dart`
`lib/src/js_emitter/code_emitter_task.dart.rej`
`lib/src/js_emitter/code_emitter_task.dart.orig`
`lib/src/js_emitter/code_emitter_task.dart`
`lib/src/js_emitter/interceptor_stub_generator.dart`
`lib/src/js_emitter/full_emitter`
`lib/src/js_emitter/full_emitter/class_builder.dart`
`lib/src/js_emitter/full_emitter/container_builder.dart`
`lib/src/js_emitter/full_emitter/deferred_output_unit_hash.dart`
`lib/src/js_emitter/full_emitter/class_emitter.dart`
`lib/src/js_emitter/full_emitter/interceptor_emitter.dart`
`lib/src/js_emitter/full_emitter/code_emitter_helper.dart`
`lib/src/js_emitter/full_emitter/emitter.dart`
`lib/src/js_emitter/full_emitter/setup_program_builder.dart`
`lib/src/js_emitter/full_emitter/declarations.dart`
`lib/src/js_emitter/full_emitter/nsm_emitter.dart`
`lib/src/js_emitter/type_test_registry.dart`
`lib/src/js_emitter/js_emitter.dart.rej`
`lib/src/js_emitter/class_stub_generator.dart`
`lib/src/js_emitter/startup_emitter`
`lib/src/js_emitter/startup_emitter/deferred_fragment_hash.dart`
`lib/src/js_emitter/startup_emitter/model_emitter.dart`
`lib/src/js_emitter/startup_emitter/emitter.dart`
`lib/src/js_emitter/startup_emitter/fragment_emitter.dart`
`lib/src/js_emitter/js_emitter.dart`
`lib/src/js_emitter/helpers.dart`
`lib/src/js_emitter/runtime_type_generator.dart`
`lib/src/js_emitter/js_emitter.dart.orig`
`lib/src/elements`
`lib/src/elements/modelx.dart`
`lib/src/elements/types.dart`
`lib/src/elements/resolution_types.dart`
`lib/src/elements/entities.dart`
`lib/src/elements/common.dart`
`lib/src/elements/names.dart`
`lib/src/elements/visitor.dart`
`lib/src/elements/elements.dart`
`lib/src/diagnostics`
`lib/src/diagnostics/invariant.dart`
`lib/src/diagnostics/messages.dart`
`lib/src/diagnostics/source_span.dart`
`lib/src/diagnostics/code_location.dart`
`lib/src/diagnostics/diagnostic_listener.dart`
`lib/src/diagnostics/spannable.dart`
`lib/src/common`
`lib/src/common/codegen.dart`
`lib/src/common/resolution.dart`
`lib/src/common/tasks.dart`
`lib/src/common/work.dart`
`lib/src/common/backend_api.dart`
`lib/src/common/names.dart`
`lib/src/tokens/token_map.dart`: unused
`lib/src/resolution`
`lib/src/resolution/typedefs.dart`
`lib/src/resolution/registry.dart.orig`
`lib/src/resolution/scope.dart`
`lib/src/resolution/members.dart`
`lib/src/resolution/label_scope.dart`
`lib/src/resolution/registry.dart.rej`
`lib/src/resolution/resolution.dart`
`lib/src/resolution/access_semantics.dart`
`lib/src/resolution/operators.dart`
`lib/src/resolution/member_impl.dart`
`lib/src/resolution/resolution_common.dart`
`lib/src/resolution/semantic_visitor.dart`
`lib/src/resolution/resolution_result.dart`
`lib/src/resolution/send_resolver.dart`
`lib/src/resolution/send_structure.dart`
`lib/src/resolution/variables.dart`
`lib/src/resolution/enum_creator.dart`
`lib/src/resolution/members.dart.orig`
`lib/src/resolution/type_resolver.dart`
`lib/src/resolution/class_members.dart`
`lib/src/resolution/constructors.dart`
`lib/src/resolution/secret_tree_element.dart`
`lib/src/resolution/registry.dart`
`lib/src/resolution/tree_elements.dart`
`lib/src/resolution/semantic_visitor_mixins.dart`
`lib/src/resolution/class_hierarchy.dart`
`lib/src/resolution/signatures.dart`
`lib/src/scanner`
`lib/src/scanner/scanner_task.dart`
`lib/src/js`
`lib/src/js/js.dart`
`lib/src/js/placeholder_safety.dart`
`lib/src/js/js_debug.dart`
`lib/src/js/js_source_mapping.dart`
`lib/src/js/rewrite_async.dart`
`lib/src/util`
`lib/src/util/uri_extras.dart`
`lib/src/util/indentation.dart`
`lib/src/util/enumset.dart`
`lib/src/util/link.dart`
`lib/src/util/util.dart`
`lib/src/util/maplet.dart`
`lib/src/util/setlet.dart`
`lib/src/util/characters.dart`
`lib/src/util/emptyset.dart`
`lib/src/util/link_implementation.dart`
`lib/src/util/util_implementation.dart`
`lib/src/util/command_line.dart`
`lib/src/js_backend`
`lib/src/js_backend/frequency_namer.dart`
`lib/src/js_backend/patch_resolver.dart`
`lib/src/js_backend/minify_namer.dart`
`lib/src/js_backend/mirrors_analysis.dart`
`lib/src/js_backend/js_backend.dart`
`lib/src/js_backend/field_naming_mixin.dart`
`lib/src/js_backend/native_data.dart`
`lib/src/js_backend/namer.dart`
`lib/src/js_backend/custom_elements_analysis.dart`
`lib/src/js_backend/type_variable_handler.dart`
`lib/src/js_backend/js_interop_analysis.dart`
`lib/src/js_backend/backend_impact.dart`
`lib/src/js_backend/constant_emitter.dart`
`lib/src/js_backend/namer_names.dart`
`lib/src/js_backend/runtime_types.dart`
`lib/src/js_backend/no_such_method_registry.dart`
`lib/src/js_backend/constant_system_javascript.dart`
`lib/src/js_backend/backend.dart`
`lib/src/js_backend/checked_mode_helpers.dart`
`lib/src/js_backend/constant_handler_javascript.dart`
`lib/src/tree`
`lib/src/tree/prettyprint.dart`
`lib/src/tree/tree.dart`
`lib/src/tree/nodes.dart`
`lib/src/tree/dartstring.dart`
`lib/src/tree/unparser.dart`
`lib/src/types`
`lib/src/types/abstract_value_domain.dart`
`lib/src/types/types.dart`
`lib/src/types/type_mask.dart`
`lib/src/types/dictionary_type_mask.dart`
`lib/src/types/map_type_mask.dart`
`lib/src/types/forwarding_type_mask.dart`
`lib/src/types/container_type_mask.dart`
`lib/src/types/constants.dart`
`lib/src/types/flat_type_mask.dart`
`lib/src/types/masks.dart`
`lib/src/types/value_type_mask.dart`
`lib/src/types/union_type_mask.dart`
`lib/src/hash`
`lib/src/hash/sha1.dart`