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218 lines
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218 lines
13 KiB
Markdown
---
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layout: developer-doc
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title: Runtime Guide
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category: summary
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tags: [contributing, guide, graal, truffle]
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order: 7
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---
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# Runtime Guide
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## GraalVM and Truffle
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### Papers
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1. [One VM To Rule Them All](http://lafo.ssw.uni-linz.ac.at/papers/2013_Onward_OneVMToRuleThemAll.pdf)
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a high-level overview of what GraalVM is and how it works.
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2. [Practical Partial Evaluation for High-Performance Dynamic Language Runtimes](https://chrisseaton.com/rubytruffle/pldi17-truffle/pldi17-truffle.pdf)
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an introduction to basic Truffle concepts, including Polymorphic Inline
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Caches and other frequently used techniques.
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3. [Fast, Flexible, Polyglot Instrumentation Support for Debuggers and other Tools](https://arxiv.org/pdf/1803.10201.pdf)
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an introduction to Truffle instrumentation framework – this is what Enso's
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runtime server for the IDE uses.
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4. [Cross-Language Interoperability in a Multi-Language Runtime](https://chrisseaton.com/truffleruby/cross-language-interop.pdf)
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an introduction to how Truffle cross-language interop works.
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5. [The whole list of publications](https://www.graalvm.org/community/publications/)
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because something may be useful at some point.
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### Tutorials
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1. [The list of Truffle docs on specific topics](https://github.com/oracle/graal/tree/master/truffle/docs)
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Certain more advanced topics are covered in these, use as needed.
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1. [Optimizing Tutorial](https://github.com/oracle/graal/blob/master/truffle/docs/Optimizing.md)
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you'll want to read this one for sure.
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2. [TruffleLibrary Tutorial](https://github.com/oracle/graal/blob/master/truffle/docs/TruffleLibraries.md)
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this is an important architectural concept for building Truffle
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interpreters. We wish we knew about this sooner and we recommend you
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structure the interpreter around this in the future.
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2. [A tutorial on building a LISP in Truffle](https://cesquivias.github.io/blog/2015/01/15/writing-a-language-in-truffle-part-4-adding-features-the-truffle-way/)
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It's a 4-part tutorial, linked is part 4, start with part 2 (part 1 is not
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about Truffle). This one is important, even though it is old and uses stale
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APIs – it will still highlight the most important concepts, in particular the
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way Enso deals with lexical scopes and Tail Call Optimization.
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3. [Simple Language](https://github.com/graalvm/simplelanguage) this is an
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implementation of a very simple toy language. Read it for basic understanding
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of simple Truffle concepts.
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### Tips and Tricks
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1. Familiarize yourself with
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[IGV](https://www.graalvm.org/graalvm-as-a-platform/language-implementation-framework/Profiling/).
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It's a horrible tool. It's clunky, ugly, and painful to use. It has also
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saved us more times than we can count, definitely worth investing the time to
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understand it. Download
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[Enso Language Support for IGV](../tools/enso4igv/README.md). Use
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[this tutorial](https://shopify.engineering/understanding-programs-using-graphs)
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(and
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[the follow up post](https://chrisseaton.com/truffleruby/basic-truffle-graphs/))
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to familiarize yourself with the representation.
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2. Use our sbt
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[`withDebug`](https://github.com/enso-org/enso/blob/develop/project/WithDebugCommand.scala)
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utility. Familiarize yourself with the different otpions. It is a useful
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helper for running your programs and microbenchmarks with different Truffle
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debugging options.
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3. Use [hsdis](https://github.com/liuzhengyang/hsdis/) for printing the
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generated assembly – you can often spot obvious problems with compilations.
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That being said, IGV (with
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[Enso Language Support](../tools/enso4igv/README.md)) is usually the better
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tool, if you take a look at the later compilation stages.
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4. Pay attention to making things `final` and `@CompilationFinal`. This is the
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most important way Graal does constant-folding. Whenever a loop bound can be
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compilation final, take advantage (and use `@ExplodeLoop`).
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5. Read the generated code for the nodes generated by the DSL. Learning the DSL
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is quite difficult and the documentation is sorely lacking. It is best to
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experiment with different kinds of `@Specialization` and read the generated
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code. Without this understanding, it's way too easy to introduce very subtle
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bugs to the language semantics.
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6. Join the [GraalVM Slack server](https://www.graalvm.org/slack-invitation/).
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All the authors are there and they will happily help and answer any
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questions.
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7. Be aware that Truffle Instrumentation is more constrained than it could be,
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because it wants to be language agnostic. The Enso runtime server is
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Enso-specific and therefore you may be better served in the future by rolling
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your own instrumentation. Read the instrumentation sources, it will help you
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understand how non-magical it actually is.
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8. Clone the sources of Truffle and TruffleRuby. Set them up as projects in your
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IDE. Read the code when in doubt. Truffle documentation is really lacking
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sometimes, even though it is improving.
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9. Understand the boundary between the language-side APIs (see e.g.
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`InteropLibrary`) and embedder side (see `Value`). You want to make sure you
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use the proper APIs in the proper places in the codebase. As a rule of thumb:
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all code in the `runtime` project is language/instrumentation-side. All code
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elsewhere is embedder-side. In particular, the only Graal dependency in
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embedder code should be `graal-sdk`. If you find yourself pulling things like
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`truffle-api`, you've done something wrong. Similarly, if you ever import
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anything from `org.graalvm.polyglot` in the language code, you're doing
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something wrong.
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10. Avoid
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[deoptimizations](https://www.graalvm.org/22.2/graalvm-as-a-platform/language-implementation-framework/Optimizing/#debugging-deoptimizations).
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Understanding IGV graphs can be a very time-consuming and complex process
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(even with the help of [Enso tooling for IGV](../tools/enso4igv/README.md)).
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Sometimes it is sufficient to only look at the compilation traces to
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discover repeated or unnecessary deoptimizations which can significantly
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affect overall performance of your program. You can tell runner to generate
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compilation traces via additional options:
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```
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JAVA_OPTS="-Dpolygot.engine.TracePerformanceWarnings=all -Dpolyglot.engine.TraceTransferToInterpreter=true -Dpolyglot.engine.TraceDeoptimizeFrame=true -Dpolyglot.engine.TraceCompilation=true -Dpolyglot.engine.TraceCompilationDetails=true"
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```
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Make sure you print trace logs by using `--log-level TRACE`.
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11. Occasionally a piece of code runs slower than we anticipated. Analyzing
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Truffle inlining traces may reveal locations that one thought would be
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inlined but Truffle decided otherwise. Rewriting such locations to builtin
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methods or more inliner-friendly representation can significantly improve
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the performance. You can tell runner to generate inlining traces via
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additional options:
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```
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JAVA_OPTS="-Dpolyglot.engine.TraceInlining=true -Dpolyglot.engine.TraceInliningDetails=true"
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```
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Make sure you print trace logs by using `--log-level TRACE`. See
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[documentation](https://www.graalvm.org/22.2/graalvm-as-a-platform/language-implementation-framework/Inlining/#call-tree-states)
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for the explanation of inlining decisions.
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## Code & Internal Documentation Map
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Other than the subsections here, go through the
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[existing documentation](https://github.com/enso-org/enso/tree/develop/docs).
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### Entry Points
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1. See `Main` in `engine-runner` and `Language` in `runtime`. The former is the
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embedder-side entry point, the latter the language-side one. They do a bit of
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ping-pong through the polyglot APIs. That is unfortunate, as this API is
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stringly typed. Therefore, chase the usages of method-name constants to jump
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between the language-side implementations and the embedder-side calls.
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Alternatively, step through the flow in a debugger.
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2. Look at the `MainModule` in `language-server` and `RuntimeServerInstrument`
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in `runtime`. This is the entry point for IDE, with language/embedder
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boundary as usual, but with a server-like message exchange instead of
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polyglot API use.
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### Compiler
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Look at `Compiler` in `runtime`. It is the main compiler class and the flow
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should be straightforward. A high level overview is: the compiler alternates
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between running module-local passes (currently in 3 groups) and global join
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points, where information flows between modules.
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### Interpreter
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There are a few very convoluted spots in the interpreter, with non-trivial
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design choices. Here's a list with some explanations:
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1. **Function Call Flow**: It is quite difficult to efficiently call an Enso
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function. Enso allows passing arguments by name, supports currying and
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eta-expansion, and defaulted argument values. It also has to deal with
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polyglot method calls. And it has to be instrumentable, to enable the "enter
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a method via call site" functionality of the IDE. Start reading from
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`ApplicationNode` and follow the execute methods (or `@Specialization`s).
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There's a lot of them, but don't get too scared. It is also outlined
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[here](https://github.com/enso-org/enso/blob/develop/docs/runtime/function-call-flow.md).
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2. **Polyglot Code**: While for some languages (Java, Ruby and Python) it is
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straightforward and very Truffle-like, for others (JS and R) it becomes
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tricky. The reason is that Truffle places strong limitations on threading in
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these languages and it is impossible to call JS and R from a multithreaded
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language context (like Enso's). For this reason, we have a special, internal
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sub-language, running on 2 separate Truffle contexts, exposing the single
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threaded languages in a safe way (through a GIL). The language is called EPB
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(Enso Polyglot Bridge) and lives in
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[this subtree](https://github.com/enso-org/enso/tree/develop/engine/runtime/src/main/java/org/enso/interpreter/epb).
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To really understand it, you'll need to familiarize yourself with what a
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[TruffleContext](https://www.graalvm.org/truffle/javadoc/com/oracle/truffle/api/TruffleContext.html)
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is and how it relates to polyglot and language contexts (oh, and also get
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ready to work with about 7 different meanings of the word `Context`...).
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3. **Threading & Safepoints**: Enso has its own safepointing system and a thread
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manager. The job of the thread manager is to halt all the executing threads
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when needed. Safepoints are polled during normal code execution (usually at
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the start of every non-inlined method call and at each iteration of a TCO
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loop). See
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[the source](https://github.com/enso-org/enso/blob/develop/engine/runtime/src/main/java/org/enso/interpreter/runtime/ThreadManager.java).
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4. **Resource Finalization**: Enso exposes a system for automatic resource
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finalization. This is non-trivial on the JVM and is handled in the
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[ResourceManager](https://github.com/enso-org/enso/blob/develop/engine/runtime/src/main/java/org/enso/interpreter/runtime/ResourceManager.java).
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5. **Builtin Definitions**: Certain basic functions and types are exposed
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directly from the interpreter. They currently are all bundled in a virtual
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module called `Standard.Builtins`. See
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[the Builtins class](https://github.com/enso-org/enso/blob/develop/engine/runtime/src/main/java/org/enso/interpreter/runtime/builtin/Builtins.java)
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to see how that module is constructed. There's also a java-side
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annotation-driven DSL for automatic generation of builtin method boilerplate.
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See nodes in
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[this tree](https://github.com/enso-org/enso/tree/develop/engine/runtime/src/main/java/org/enso/interpreter/runtime/builtin)
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to get an idea of how it works. Also
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[read the doc](https://github.com/enso-org/enso/blob/develop/docs/runtime/builtin-base-methods.md)
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6. **Standard Library Sources**: These are very non-magical – just plain old
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Enso projects that get shipped with every compiler release. They live
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[in this tree](https://github.com/enso-org/enso/tree/develop/distribution/lib/Standard).
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And are tested through
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[these projects](https://github.com/enso-org/enso/tree/develop/test). It also
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makes heavy use of host interop. The Java methods used by the standard
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library are located in
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[this directory](https://github.com/enso-org/enso/tree/develop/std-bits).
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7. **Microbenchmarks**: There are some microbenchmarks for tiny Enso programs
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for basic language constructs. They are located in
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[this directory](https://github.com/enso-org/enso/tree/develop/engine/runtime/src/bench).
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They can be run through `sbt runtime/bench`. Each run will generate (or
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append to) the `bench-report.xml` file. It will also fail the benchmark suite
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if any benchmark is more than 20% slower than the fastest recorded run. Don't
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use your computer when running these. It is also worth noting that these can
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be run through the `withDebug` utility, which allows you to test truffle
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compilations (and e.g. watch the graphs in IGV with
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[Enso Language Support](../tools/enso4igv/README.md)).
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8. **Tests**: There are scalatests that comprehensively test all of the language
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semantics and compiler passes. These are run with `sbt runtime/test`. For
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newer functionalities, we prefer adding tests to the `Tests` project in the
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standard library test. At this point, Enso is mature enough to self-test.
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### Language Server
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Talk to Dmitry! He's the main maintainer of this part.
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