| ======================================================= |
| Building a JIT: Starting out with KaleidoscopeJIT |
| ======================================================= |
| |
| .. contents:: |
| :local: |
| |
| Chapter 1 Introduction |
| ====================== |
| |
| **Warning: This text is currently out of date due to ORC API updates.** |
| |
| **The example code has been updated and can be used. The text will be updated |
| once the API churn dies down.** |
| |
| Welcome to Chapter 1 of the "Building an ORC-based JIT in LLVM" tutorial. This |
| tutorial runs through the implementation of a JIT compiler using LLVM's |
| On-Request-Compilation (ORC) APIs. It begins with a simplified version of the |
| KaleidoscopeJIT class used in the |
| `Implementing a language with LLVM <LangImpl01.html>`_ tutorials and then |
| introduces new features like optimization, lazy compilation and remote |
| execution. |
| |
| The goal of this tutorial is to introduce you to LLVM's ORC JIT APIs, show how |
| these APIs interact with other parts of LLVM, and to teach you how to recombine |
| them to build a custom JIT that is suited to your use-case. |
| |
| The structure of the tutorial is: |
| |
| - Chapter #1: Investigate the simple KaleidoscopeJIT class. This will |
| introduce some of the basic concepts of the ORC JIT APIs, including the |
| idea of an ORC *Layer*. |
| |
| - `Chapter #2 <BuildingAJIT2.html>`_: Extend the basic KaleidoscopeJIT by adding |
| a new layer that will optimize IR and generated code. |
| |
| - `Chapter #3 <BuildingAJIT3.html>`_: Further extend the JIT by adding a |
| Compile-On-Demand layer to lazily compile IR. |
| |
| - `Chapter #4 <BuildingAJIT4.html>`_: Improve the laziness of our JIT by |
| replacing the Compile-On-Demand layer with a custom layer that uses the ORC |
| Compile Callbacks API directly to defer IR-generation until functions are |
| called. |
| |
| - `Chapter #5 <BuildingAJIT5.html>`_: Add process isolation by JITing code into |
| a remote process with reduced privileges using the JIT Remote APIs. |
| |
| To provide input for our JIT we will use the Kaleidoscope REPL from |
| `Chapter 7 <LangImpl07.html>`_ of the "Implementing a language in LLVM tutorial", |
| with one minor modification: We will remove the FunctionPassManager from the |
| code for that chapter and replace it with optimization support in our JIT class |
| in Chapter #2. |
| |
| Finally, a word on API generations: ORC is the 3rd generation of LLVM JIT API. |
| It was preceded by MCJIT, and before that by the (now deleted) legacy JIT. |
| These tutorials don't assume any experience with these earlier APIs, but |
| readers acquainted with them will see many familiar elements. Where appropriate |
| we will make this connection with the earlier APIs explicit to help people who |
| are transitioning from them to ORC. |
| |
| JIT API Basics |
| ============== |
| |
| The purpose of a JIT compiler is to compile code "on-the-fly" as it is needed, |
| rather than compiling whole programs to disk ahead of time as a traditional |
| compiler does. To support that aim our initial, bare-bones JIT API will be: |
| |
| 1. Handle addModule(Module &M) -- Make the given IR module available for |
| execution. |
| 2. JITSymbol findSymbol(const std::string &Name) -- Search for pointers to |
| symbols (functions or variables) that have been added to the JIT. |
| 3. void removeModule(Handle H) -- Remove a module from the JIT, releasing any |
| memory that had been used for the compiled code. |
| |
| A basic use-case for this API, executing the 'main' function from a module, |
| will look like: |
| |
| .. code-block:: c++ |
| |
| std::unique_ptr<Module> M = buildModule(); |
| JIT J; |
| Handle H = J.addModule(*M); |
| int (*Main)(int, char*[]) = (int(*)(int, char*[]))J.getSymbolAddress("main"); |
| int Result = Main(); |
| J.removeModule(H); |
| |
| The APIs that we build in these tutorials will all be variations on this simple |
| theme. Behind the API we will refine the implementation of the JIT to add |
| support for optimization and lazy compilation. Eventually we will extend the |
| API itself to allow higher-level program representations (e.g. ASTs) to be |
| added to the JIT. |
| |
| KaleidoscopeJIT |
| =============== |
| |
| In the previous section we described our API, now we examine a simple |
| implementation of it: The KaleidoscopeJIT class [1]_ that was used in the |
| `Implementing a language with LLVM <LangImpl01.html>`_ tutorials. We will use |
| the REPL code from `Chapter 7 <LangImpl07.html>`_ of that tutorial to supply the |
| input for our JIT: Each time the user enters an expression the REPL will add a |
| new IR module containing the code for that expression to the JIT. If the |
| expression is a top-level expression like '1+1' or 'sin(x)', the REPL will also |
| use the findSymbol method of our JIT class find and execute the code for the |
| expression, and then use the removeModule method to remove the code again |
| (since there's no way to re-invoke an anonymous expression). In later chapters |
| of this tutorial we'll modify the REPL to enable new interactions with our JIT |
| class, but for now we will take this setup for granted and focus our attention on |
| the implementation of our JIT itself. |
| |
| Our KaleidoscopeJIT class is defined in the KaleidoscopeJIT.h header. After the |
| usual include guards and #includes [2]_, we get to the definition of our class: |
| |
| .. code-block:: c++ |
| |
| #ifndef LLVM_EXECUTIONENGINE_ORC_KALEIDOSCOPEJIT_H |
| #define LLVM_EXECUTIONENGINE_ORC_KALEIDOSCOPEJIT_H |
| |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ExecutionEngine/ExecutionEngine.h" |
| #include "llvm/ExecutionEngine/JITSymbol.h" |
| #include "llvm/ExecutionEngine/RTDyldMemoryManager.h" |
| #include "llvm/ExecutionEngine/SectionMemoryManager.h" |
| #include "llvm/ExecutionEngine/Orc/CompileUtils.h" |
| #include "llvm/ExecutionEngine/Orc/IRCompileLayer.h" |
| #include "llvm/ExecutionEngine/Orc/LambdaResolver.h" |
| #include "llvm/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/Mangler.h" |
| #include "llvm/Support/DynamicLibrary.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include <algorithm> |
| #include <memory> |
| #include <string> |
| #include <vector> |
| |
| namespace llvm { |
| namespace orc { |
| |
| class KaleidoscopeJIT { |
| private: |
| std::unique_ptr<TargetMachine> TM; |
| const DataLayout DL; |
| RTDyldObjectLinkingLayer ObjectLayer; |
| IRCompileLayer<decltype(ObjectLayer), SimpleCompiler> CompileLayer; |
| |
| public: |
| using ModuleHandle = decltype(CompileLayer)::ModuleHandleT; |
| |
| Our class begins with four members: A TargetMachine, TM, which will be used to |
| build our LLVM compiler instance; A DataLayout, DL, which will be used for |
| symbol mangling (more on that later), and two ORC *layers*: an |
| RTDyldObjectLinkingLayer and a CompileLayer. We'll be talking more about layers |
| in the next chapter, but for now you can think of them as analogous to LLVM |
| Passes: they wrap up useful JIT utilities behind an easy to compose interface. |
| The first layer, ObjectLayer, is the foundation of our JIT: it takes in-memory |
| object files produced by a compiler and links them on the fly to make them |
| executable. This JIT-on-top-of-a-linker design was introduced in MCJIT, however |
| the linker was hidden inside the MCJIT class. In ORC we expose the linker so |
| that clients can access and configure it directly if they need to. In this |
| tutorial our ObjectLayer will just be used to support the next layer in our |
| stack: the CompileLayer, which will be responsible for taking LLVM IR, compiling |
| it, and passing the resulting in-memory object files down to the object linking |
| layer below. |
| |
| That's it for member variables, after that we have a single typedef: |
| ModuleHandle. This is the handle type that will be returned from our JIT's |
| addModule method, and can be passed to the removeModule method to remove a |
| module. The IRCompileLayer class already provides a convenient handle type |
| (IRCompileLayer::ModuleHandleT), so we just alias our ModuleHandle to this. |
| |
| .. code-block:: c++ |
| |
| KaleidoscopeJIT() |
| : TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()), |
| ObjectLayer([]() { return std::make_shared<SectionMemoryManager>(); }), |
| CompileLayer(ObjectLayer, SimpleCompiler(*TM)) { |
| llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr); |
| } |
| |
| TargetMachine &getTargetMachine() { return *TM; } |
| |
| Next up we have our class constructor. We begin by initializing TM using the |
| EngineBuilder::selectTarget helper method which constructs a TargetMachine for |
| the current process. Then we use our newly created TargetMachine to initialize |
| DL, our DataLayout. After that we need to initialize our ObjectLayer. The |
| ObjectLayer requires a function object that will build a JIT memory manager for |
| each module that is added (a JIT memory manager manages memory allocations, |
| memory permissions, and registration of exception handlers for JIT'd code). For |
| this we use a lambda that returns a SectionMemoryManager, an off-the-shelf |
| utility that provides all the basic memory management functionality required for |
| this chapter. Next we initialize our CompileLayer. The CompileLayer needs two |
| things: (1) A reference to our object layer, and (2) a compiler instance to use |
| to perform the actual compilation from IR to object files. We use the |
| off-the-shelf SimpleCompiler instance for now. Finally, in the body of the |
| constructor, we call the DynamicLibrary::LoadLibraryPermanently method with a |
| nullptr argument. Normally the LoadLibraryPermanently method is called with the |
| path of a dynamic library to load, but when passed a null pointer it will 'load' |
| the host process itself, making its exported symbols available for execution. |
| |
| .. code-block:: c++ |
| |
| ModuleHandle addModule(std::unique_ptr<Module> M) { |
| // Build our symbol resolver: |
| // Lambda 1: Look back into the JIT itself to find symbols that are part of |
| // the same "logical dylib". |
| // Lambda 2: Search for external symbols in the host process. |
| auto Resolver = createLambdaResolver( |
| [&](const std::string &Name) { |
| if (auto Sym = CompileLayer.findSymbol(Name, false)) |
| return Sym; |
| return JITSymbol(nullptr); |
| }, |
| [](const std::string &Name) { |
| if (auto SymAddr = |
| RTDyldMemoryManager::getSymbolAddressInProcess(Name)) |
| return JITSymbol(SymAddr, JITSymbolFlags::Exported); |
| return JITSymbol(nullptr); |
| }); |
| |
| // Add the set to the JIT with the resolver we created above and a newly |
| // created SectionMemoryManager. |
| return cantFail(CompileLayer.addModule(std::move(M), |
| std::move(Resolver))); |
| } |
| |
| Now we come to the first of our JIT API methods: addModule. This method is |
| responsible for adding IR to the JIT and making it available for execution. In |
| this initial implementation of our JIT we will make our modules "available for |
| execution" by adding them straight to the CompileLayer, which will immediately |
| compile them. In later chapters we will teach our JIT to defer compilation |
| of individual functions until they're actually called. |
| |
| To add our module to the CompileLayer we need to supply both the module and a |
| symbol resolver. The symbol resolver is responsible for supplying the JIT with |
| an address for each *external symbol* in the module we are adding. External |
| symbols are any symbol not defined within the module itself, including calls to |
| functions outside the JIT and calls to functions defined in other modules that |
| have already been added to the JIT. (It may seem as though modules added to the |
| JIT should know about one another by default, but since we would still have to |
| supply a symbol resolver for references to code outside the JIT it turns out to |
| be easier to re-use this one mechanism for all symbol resolution.) This has the |
| added benefit that the user has full control over the symbol resolution |
| process. Should we search for definitions within the JIT first, then fall back |
| on external definitions? Or should we prefer external definitions where |
| available and only JIT code if we don't already have an available |
| implementation? By using a single symbol resolution scheme we are free to choose |
| whatever makes the most sense for any given use case. |
| |
| Building a symbol resolver is made especially easy by the *createLambdaResolver* |
| function. This function takes two lambdas [3]_ and returns a JITSymbolResolver |
| instance. The first lambda is used as the implementation of the resolver's |
| findSymbolInLogicalDylib method, which searches for symbol definitions that |
| should be thought of as being part of the same "logical" dynamic library as this |
| Module. If you are familiar with static linking: this means that |
| findSymbolInLogicalDylib should expose symbols with common linkage and hidden |
| visibility. If all this sounds foreign you can ignore the details and just |
| remember that this is the first method that the linker will use to try to find a |
| symbol definition. If the findSymbolInLogicalDylib method returns a null result |
| then the linker will call the second symbol resolver method, called findSymbol, |
| which searches for symbols that should be thought of as external to (but |
| visibile from) the module and its logical dylib. In this tutorial we will adopt |
| the following simple scheme: All modules added to the JIT will behave as if they |
| were linked into a single, ever-growing logical dylib. To implement this our |
| first lambda (the one defining findSymbolInLogicalDylib) will just search for |
| JIT'd code by calling the CompileLayer's findSymbol method. If we don't find a |
| symbol in the JIT itself we'll fall back to our second lambda, which implements |
| findSymbol. This will use the RTDyldMemoryManager::getSymbolAddressInProcess |
| method to search for the symbol within the program itself. If we can't find a |
| symbol definition via either of these paths, the JIT will refuse to accept our |
| module, returning a "symbol not found" error. |
| |
| Now that we've built our symbol resolver, we're ready to add our module to the |
| JIT. We do this by calling the CompileLayer's addModule method. The addModule |
| method returns an ``Expected<CompileLayer::ModuleHandle>``, since in more |
| advanced JIT configurations it could fail. In our basic configuration we know |
| that it will always succeed so we use the cantFail utility to assert that no |
| error occurred, and extract the handle value. Since we have already typedef'd |
| our ModuleHandle type to be the same as the CompileLayer's handle type, we can |
| return the unwrapped handle directly. |
| |
| .. code-block:: c++ |
| |
| JITSymbol findSymbol(const std::string Name) { |
| std::string MangledName; |
| raw_string_ostream MangledNameStream(MangledName); |
| Mangler::getNameWithPrefix(MangledNameStream, Name, DL); |
| return CompileLayer.findSymbol(MangledNameStream.str(), true); |
| } |
| |
| JITTargetAddress getSymbolAddress(const std::string Name) { |
| return cantFail(findSymbol(Name).getAddress()); |
| } |
| |
| void removeModule(ModuleHandle H) { |
| cantFail(CompileLayer.removeModule(H)); |
| } |
| |
| Now that we can add code to our JIT, we need a way to find the symbols we've |
| added to it. To do that we call the findSymbol method on our CompileLayer, but |
| with a twist: We have to *mangle* the name of the symbol we're searching for |
| first. The ORC JIT components use mangled symbols internally the same way a |
| static compiler and linker would, rather than using plain IR symbol names. This |
| allows JIT'd code to interoperate easily with precompiled code in the |
| application or shared libraries. The kind of mangling will depend on the |
| DataLayout, which in turn depends on the target platform. To allow us to remain |
| portable and search based on the un-mangled name, we just re-produce this |
| mangling ourselves. |
| |
| Next we have a convenience function, getSymbolAddress, which returns the address |
| of a given symbol. Like CompileLayer's addModule function, JITSymbol's getAddress |
| function is allowed to fail [4]_, however we know that it will not in our simple |
| example, so we wrap it in a call to cantFail. |
| |
| We now come to the last method in our JIT API: removeModule. This method is |
| responsible for destructing the MemoryManager and SymbolResolver that were |
| added with a given module, freeing any resources they were using in the |
| process. In our Kaleidoscope demo we rely on this method to remove the module |
| representing the most recent top-level expression, preventing it from being |
| treated as a duplicate definition when the next top-level expression is |
| entered. It is generally good to free any module that you know you won't need |
| to call further, just to free up the resources dedicated to it. However, you |
| don't strictly need to do this: All resources will be cleaned up when your |
| JIT class is destructed, if they haven't been freed before then. Like |
| ``CompileLayer::addModule`` and ``JITSymbol::getAddress``, removeModule may |
| fail in general but will never fail in our example, so we wrap it in a call to |
| cantFail. |
| |
| This brings us to the end of Chapter 1 of Building a JIT. You now have a basic |
| but fully functioning JIT stack that you can use to take LLVM IR and make it |
| executable within the context of your JIT process. In the next chapter we'll |
| look at how to extend this JIT to produce better quality code, and in the |
| process take a deeper look at the ORC layer concept. |
| |
| `Next: Extending the KaleidoscopeJIT <BuildingAJIT2.html>`_ |
| |
| Full Code Listing |
| ================= |
| |
| Here is the complete code listing for our running example. To build this |
| example, use: |
| |
| .. code-block:: bash |
| |
| # Compile |
| clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orcjit native` -O3 -o toy |
| # Run |
| ./toy |
| |
| Here is the code: |
| |
| .. literalinclude:: ../../examples/Kaleidoscope/BuildingAJIT/Chapter1/KaleidoscopeJIT.h |
| :language: c++ |
| |
| .. [1] Actually we use a cut-down version of KaleidoscopeJIT that makes a |
| simplifying assumption: symbols cannot be re-defined. This will make it |
| impossible to re-define symbols in the REPL, but will make our symbol |
| lookup logic simpler. Re-introducing support for symbol redefinition is |
| left as an exercise for the reader. (The KaleidoscopeJIT.h used in the |
| original tutorials will be a helpful reference). |
| |
| .. [2] +-----------------------------+-----------------------------------------------+ |
| | File | Reason for inclusion | |
| +=============================+===============================================+ |
| | STLExtras.h | LLVM utilities that are useful when working | |
| | | with the STL. | |
| +-----------------------------+-----------------------------------------------+ |
| | ExecutionEngine.h | Access to the EngineBuilder::selectTarget | |
| | | method. | |
| +-----------------------------+-----------------------------------------------+ |
| | | Access to the | |
| | RTDyldMemoryManager.h | RTDyldMemoryManager::getSymbolAddressInProcess| |
| | | method. | |
| +-----------------------------+-----------------------------------------------+ |
| | CompileUtils.h | Provides the SimpleCompiler class. | |
| +-----------------------------+-----------------------------------------------+ |
| | IRCompileLayer.h | Provides the IRCompileLayer class. | |
| +-----------------------------+-----------------------------------------------+ |
| | | Access the createLambdaResolver function, | |
| | LambdaResolver.h | which provides easy construction of symbol | |
| | | resolvers. | |
| +-----------------------------+-----------------------------------------------+ |
| | RTDyldObjectLinkingLayer.h | Provides the RTDyldObjectLinkingLayer class. | |
| +-----------------------------+-----------------------------------------------+ |
| | Mangler.h | Provides the Mangler class for platform | |
| | | specific name-mangling. | |
| +-----------------------------+-----------------------------------------------+ |
| | DynamicLibrary.h | Provides the DynamicLibrary class, which | |
| | | makes symbols in the host process searchable. | |
| +-----------------------------+-----------------------------------------------+ |
| | | A fast output stream class. We use the | |
| | raw_ostream.h | raw_string_ostream subclass for symbol | |
| | | mangling | |
| +-----------------------------+-----------------------------------------------+ |
| | TargetMachine.h | LLVM target machine description class. | |
| +-----------------------------+-----------------------------------------------+ |
| |
| .. [3] Actually they don't have to be lambdas, any object with a call operator |
| will do, including plain old functions or std::functions. |
| |
| .. [4] ``JITSymbol::getAddress`` will force the JIT to compile the definition of |
| the symbol if it hasn't already been compiled, and since the compilation |
| process could fail getAddress must be able to return this failure. |
