Hacking on Perl 6 (Rakudo and Roast)

I found a bug in Perl 6 recently. Really I independently discovered one that was already reported.

Here’s how to trigger it:

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$ perl6 -e 'say <2147483648/3>'
===SORRY!===
Cannot find method 'compile_time_value'

Any numerator of 2³¹ or greater causes that error. Note that Perl 6 is perfectly happy to represent rationals of that size or larger:

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$ perl6 -e 'say Rat.new(2147483648, 3)'
715827882.666667

So the problem was clearly somewhere in the compiler.

Here’s a quick guide to how I fixed this.

First, I added a test to the Perl 6 test suite. Unlike many programming languages, the primary test suite for Perl 6 is not in the same repo as the compiler. Instead, it has its own repo, roast. Roast is The Official Perl 6 Test Suite, and any toolchain which can pass the test suite is a valid Perl 6.

In this particular case, I added a test to S32-num/rat.t. The test makes sure that a Rat value with a large numerator can round trip via the .perl method and EVAL:

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is(Rat.new(807412079564, 555).perl.EVAL, Rat.new(807412079564, 555), '807412079564/555 round trips without a compile_time_value error');

Running this with the latest Rakudo caused the test to blow up with the same error as my first example. Success! Well, failure, but success at failing the way I wanted it to.

Next I had to figure out where this error was happening. This can be a bit tricky with compiler errors like this. The best way to get a clue to the problem’s location is to pass --ll-exception as a CLI flag:

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 perl6 --ll-exception -e 'say <2147483648/3>'
annot find method 'compile_time_value'
  at gen/moar/m-Perl6-Actions.nqp:7230  (/home/autarch/.rakudobrew/moar-nom/install/share/nqp/lib/Perl6/Actions.moarvm:bare_rat_number:35)
from gen/moar/stage2/QRegex.nqp:1342  (/home/autarch/.rakudobrew/moar-nom/install/share/nqp/lib/QRegex.moarvm:!reduce:29)
from gen/moar/stage2/QRegex.nqp:1303  (/home/autarch/.rakudobrew/moar-nom/install/share/nqp/lib/QRegex.moarvm:!cursor_pass:47)
from :1  (/home/autarch/.rakudobrew/moar-nom/install/share/nqp/lib/Perl6/Grammar.moarvm:bare_rat_number:133)
from :1  (/home/autarch/.rakudobrew/moar-nom/install/share/nqp/lib/Perl6/Grammar.moarvm:rat_number:64)
... [many more lines]

If we look at the top of the trace we see references to Perl6/Actions.moarvm:bare_rat_number. Looking in the rakudo repo, we can find a src/Perl6/Actions.nqp file that contains method bare_rat_number($/) {...}. This seemed like a pretty good guess at where the error was coming from.

Here’s the method in full before my patch:

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method bare_rat_number($/) {
    my $nu := @($<nu>.ast)[0].compile_time_value;
    my $de := $<de>.ast;
    my $ast := $*W.add_constant('Rat', 'type_new', $nu, $de, :nocache(1));
    $ast.node($/);
    make $ast;
}

After doing some dumping of values in the AST with code like note($<nu>.dump), I realized that the numerator could end up being passed in as either a QAST::Want or QAST::WVal object. What are these and how do they differ? Why is there a break at 2³¹? I have no clue!

However, I could see that while a QAST::Want object could be treated as an array, a QAST::WVal could not. Fortunately, both objects support a compile_time_value method. Looking at this method’s implementation in QAST::Want I could see that it was getting the first array element from the object’s internals, while QAST::WVal implemented this differently. But since they both implement the same method why not just call it and be done with it?

Here’s the patched method:

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method bare_rat_number($/) {
    my $nu := $<nu>.ast.compile_time_value;
    my $de := $<de>.ast;
    my $ast := $*W.add_constant('Rat', 'type_new', $nu, $de, :nocache(1));
    $ast.node($/);
    make $ast;
}

All the tests passed, so I think I fixed it.

Overall, this wasn’t too hard. Because much of Perl 6 is either written in Perl 6 or in NQP (a subset of Perl 6), fixing the core can be much easier than with many other languages, especially most dynamic languages which are implemented in C.