Testing Truffle DSL Specializations

This document is about tools for testing Truffle DSL specializations.

Slow path specializations only compilation mode #

Let us start with a small motivating example:

abstract class PowNode extends Node {
  public abstract double execute(double a, int exp);

  @Specialization(guards = "exp==1")
  double doOne(double a, int exp) {
    return a;

  @Specialization(replaces = "doOne")
  int doGeneric(double a, int exp) {
    double res = 1;
    for (int i = 0; i < exp; i++)
      res *= a;
    return res;

In order to test that doGeneric produces correct result for argument exp == 1, we first need to execute this node with a different value, for example, exp == 2, to activate the doGeneric specialization and only then with 1, which will now be handled by the doGeneric specialization instead of the doOne specialization. With a real-world code, writing a test that covers specializations that replace other specializations can be much more complicated and it leads to fragile tests – changes in the production code may cause the test to suddenly cover different specializations and this can easily come unnoticed.

Truffle DSL provides a mode where the “fast-path” specializations (those that are “replaced” by some other specialization, doOne in our example) are ignored. This allows us to simply increase test coverage by running the same tests, which now may cover different code-paths.

When building your languages with mx pass additional option:

$ mx build -c -A-Atruffle.dsl.GenerateSlowPathOnly=true

After the rebuild the generated code will call only “slow-path” specializations. Make sure that your IDE does not recompile the sources automatically in the meantime. Note that if you compile your dependencies (e.g., Truffle) from source as part of your build, this option will apply to the code of those dependencies as well. You may choose to apply this option only to some classes by using a filter:

$ mx build -c -A-Atruffle.dsl.GenerateSlowPathOnly=true -A-Atruffle.dsl.GenerateSlowPathOnlyFilter=org.my.truffle.language.package