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TOOLCHAIN.md

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The GraalVM LLVM Toolchain

The toolchain is a set of tools and APIs for compiling native projects, such as C and C++, to bitcode that can be executed with the GraalVM LLVM runtime. It is aimed to simplify ahead-of-time compilation for users and language implementers who want to use the GraalVM LLVM runtime.

Use Cases

  1. Simplify compilation to bitcode GraalVM users who want to run native projects via the GraalVM LLVM runtime must first compile these projects to LLVM bitcode. Although it is possible to do this with the standard LLVM tools (clang, llvm-link, etc.), there are several additional considerations, such as optimizations and manual linking. The toolchain aims to simplify this process, by providing an out-of-the-box drop-in replacement for the compiler when building native projects targeting the GraalVM LLVM runtime.

  2. Compilation of native extensions GraalVM language implementers often use the GraalVM LLVM runtime to execute native extensions, and these extensions are commonly installed by a package manager. For example, packages in python are usually added via pip install, which means that the python implementation is required to be able to compile these native extensions on demand. The toolchain provides a Java API for languages to access the tools currently (optionally) bundled with GraalVM.

  3. Compiling to bitcode at build time GraalVM languages that integrate with the GraalVM LLVM runtime usually need to build bitcode libraries to integrate with the native pieces of their implementation. The toolchain can be used as a build-time dependency to achieve this in a standardized and compatible way.

File Format

To be compatible with existing build systems, by default, the toolchain will produce native executables with embedded bitcode (ELF files on Linux, Mach-O files on MacOS). See Compiling.md for more information.

Toolchain Identifier

The GraalVM LLVM runtime can be ran in different configurations, which can differ in how the bitcode is being compiled. Generally, users of the toolchain do not need to be concerned, as the GraalVM LLVM runtime knows the mode it is running and will always provide the right toolchain. However, if a language implementation wants to store the bitcode compilation for later use, it will need to be able to identify the toolchain and its configurations used to compile the bitcode. To do so, each toolchain has an identifier. Conventionally, the identifier denotes the compilation output directory. The internal GraalVM LLVM runtime library layout follows the same approach.

Java API

Language implementations can access the toolchain via the Toolchain service. The service provides three methods:

  • TruffleFile getToolPath(String tool) Returns the path to the executable for a given tool.
  • List<TruffleFile> getPaths(String pathName) Returns a list of directories for a given path name.
  • String getIdentifier() Returns the identifier for the toolchain.

Consult the Javadoc for more details.

The Toolchain lives in the SULONG_API distribution. The LLVM runtime will always provide a toolchain that matches its current execution mode. The service can be looked-up via the Env:

LanguageInfo llvmInfo = env.getInternalLanguages().get("llvm");
Toolchain toolchain = env.lookup(llvmInfo, Toolchain.class);
TruffleFile toolPath = toolchain.getToolPath("CC");
String toolchainId = toolchain.getIdentifier();

See the ToolchainExampleSnippet for a full example.

C API

There is also a C level API for accessing the Toolchain defined in graalvm/llvm/toolchain-api.h. It provides the following function which correspond to the Java API methods:

// Returns the path to the executable for a given tool.
void *toolchain_api_tool(const void *name);
// Returns a list of directories for a given path name.
void *toolchain_api_paths(const void *name);
// Returns the identifier for the toolchain.
void *toolchain_api_identifier(void);

The return values (and arguments) are polyglot values and need to be accessed via the polyglot_* API function:

#include <stdio.h>
#include <graalvm/llvm/polyglot.h>
#include <graalvm/llvm/toolchain-api.h>

#define BUFFER_SIZE 1024

int main() {
  char buffer[BUFFER_SIZE + 1];
  void *cc = toolchain_api_tool("CC");
  polyglot_as_string(cc, buffer, BUFFER_SIZE, "ascii");
  buffer[BUFFER_SIZE] = '\0'; // ensure zero terminated
  printf("CC=%s\n", buffer);
  return 0;
}

See the test case for a usage example.

Command Line API

It is also possible to access the Toolchain API from the command line via the lli launcher. The --print-toolchain-api-* arguments correspond to the C and Java APIs with the respective name. Example:

$ lli --print-toolchain-api-identifier
native
$ lli --print-toolchain-api-paths PATH
<path-to-llvm-runtime>/native/bin
$ lli --print-toolchain-api-tool CC
<path-to-llvm-runtime>/native/bin/graalvm-native-clang

Consult the Javadoc for more details.

mx integration

On the mx side, the toolchain can be accessed via the substitutions toolchainGetToolPath and toolchainGetIdentifier. Note that they expect a toolchain name as the first argument. See for example the following snippet from a suite.py file:

  "buildEnv" : {
    "CC": "<toolchainGetToolPath:native,CC>",
    "CXX": "<toolchainGetToolPath:native,CXX>",
    "PLATFORM": "<toolchainGetIdentifier:native>",
  },

GraalVM Deployment

On the implementation side, the toolchain consists of multiple ingredients:

  • The LLVM.org component is similar to a regular LLVM release (clang, lld, llvm-* tools). In GraalVM, the LLVM.org component is located in $GRAALVM/lib/llvm/ (or $GRAALVM/jre/lib/llvm/ in the Java 8 version). This component is considered as internal and should not be directly used. The LLVM.org component might not be installed by default. If that is the case, it can be installed via gu install llvm-toolchain.
  • The toolchain wrappers are GraalVM launchers that invoke the tools from the LLVM.org component with special flags to produce results that can be executed by the GraalVM LLVM runtime. The Java and mx APIs return paths to those wrappers. The command $GRAALVM/bin/lli --print-toolchain-path can be used to get their location. The wrappers are shipped with the GraalVM LLVM runtime and do not need to be installed separately. They are meant to be drop in replacements for the C/C++ compiler when compiling a native project. The goal is to produce a GraalVM LLVM runtime executable result by simply pointing any build system to those wrappers, for example via CC/CXX environment variables or by setting PATH.

Using a prebuilt GraalVM as a Bootstrapping Toolchain

To speed up toolchain compilation during development, the SULONG_BOOTSTRAP_GRAALVM environment variable can be set to a prebuilt GraalVM. Sulong comes with a configuration file that makes building a bootstrapping GraalVM easy:

$ mx --env toolchain-only build
$ export SULONG_BOOTSTRAP_GRAALVM=`mx --env toolchain-only graalvm-home`

WARNING: The bootstrapping GraalVM will not be rebuilt automatically. You are responsible for keeping it up-to-date.