How to use LLVM backend

Prerequisites

You will need LLVM development headers and libraries. Current version of the backend is tested to work against 01b595c0cb78764384be769236af2e23cc11ab52 of llvm-project built and installed globally.

Building

We are assuming that you have functional Dart SDK checkout, which can be obtained by doing fetch dart and you checked out llvm-experiment branch.

The following targets need to built:

• vm_platform - provides Kernel binary with Dart SDK libraries;
• gen_snapshot - provides Dart AOT compiler;
• dart_precompiled_runtime - provides Dart AOT runtime.

$ tools/build.py -m product -a x64 vm_platform gen_snapshot dart_precompiled_runtime

Note: specifying product creates a product build which is considered the most minimal runtime environment, excluding features like vmservice, debugging and profiling support. You can also use normal release and debug builds for debugging.

Additionally you will need to build LLVM backend, which is currently not integrated into GN build:

$ mkdir out/llvm-codegen
$ cd out/llvm-codegen
$ cmake -DTARGET_ARCH=X64 -G Ninja ../../llvm-codegen
$ ninja

Testing

Currently LLVM backend supports lowering of a subset of IL instructions which Dart VM AOT compiler can generate. Thus is can't yet compile any application in its entirety. Instead functions that you would like to compile through LLVM backend need to be annotated with @pragma('vm:llvm').

Consider the following input file /tmp/hello.dart:

@pragma('vm:llvm')          // compile this function through LLVM
@pragma('vm:never-inline')  // prevent inlining of this function into main
int sum(int start, int end) {
  final list = <int>[];
  for (var i = start; i < end; i++) list.add(i * i);

  var sum = 0;
  for (var elem in list) {
    sum += elem;
  }
  return sum;
}

void main(List<String> args) {
  final result = sum(int.parse(args[0]), int.parse(args[1]));
  print('sum: ${result}');
}

This file can be compiled using pkg/vm/tool/release_llvm_precompiler like so:

$ DART_CONFIGURATION=ProductX64 pkg/vm/tool/release_llvm_precompiler /tmp/hello
# This would produce /tmp/hello.so which can be run using precompiled runtime
$ out/ProductX64/dart_precompiled_runtime --llvm-mode /tmp/hello.so

Note: .dart is removed.

You can pass -v or --verbose to see the steps that the script is executing.

Script generates the following interpediate files next to the input file:

• hello.il the serialized IL;
• hello.llvm the LLVM IR in text format;
• hello.s the corresponding assembly;
• hello.S the assembly from the Dart AOT pipeline;
• hello.dill Kernel binary (AOT optimized);
• hello.so library that contains all necessary snapshots and code