Want to help DMD bugfixing? Write a simple utility.

[Don]

Jonathan M Davis wrote:
> On Saturday 19 March 2011 18:04:57 Don wrote:
>> Jonathan M Davis wrote:
>>> On Saturday 19 March 2011 17:11:56 Don wrote:
>>>> Here's the task:
>>>> Given a .d source file, strip out all of the unittest {} blocks,
>>>> including everything inside them.
>>>> Strip out all comments as well.
>>>> Print out the resulting file.
>>>>
>>>> Motivation: Bug reports frequently come with very large test cases.
>>>> Even ones which look small often import from Phobos.
>>>> Reducing the test case is the first step in fixing the bug, and it's
>>>> frequently ~30% of the total time required. Stripping out the unit tests
>>>> is the most time-consuming and error-prone part of reducing the test
>>>> case.
>>>>
>>>> This should be a good task if you're relatively new to D but would like
>>>> to do something really useful.
>>> Unfortunately, to do that 100% correctly, you need to actually have a
>>> working D lexer (and possibly parser). You might be able to get
>>> something close enough to work in most cases, but it doesn't take all
>>> that much to throw off a basic implementation of this sort of thing if
>>> you don't lex/parse it with something which properly understands D.
>>>
>>> - Jonathan M Davis
>> I didn't say it needs 100% accuracy. You can assume, for example, that
>> "unittest" always occurs at the start of a line. The only other things
>> you need to lex are {}, string literals, and comments.
>>
>> BTW, the immediate motivation for this is std.datetime in Phobos. The
>> sheer number of unittests in there is an absolute catastrophe for
>> tracking down bugs. It makes a tool like this MANDATORY.
> 
> I tried to create a similar tool before and gave up because I couldn't make it 
> 100% accurate and was running into problems with it. If someone wants to take a 
> shot at it though, that's fine.
> 
> As for the unit tests in std.datetime making it hard to track down bugs, that 
> only makes sense to me if you're trying to look at the whole thing at once and 
> track down a compiler bug which happens _somewhere_ in the code, but you don't 
> know where. Other than a problem like that, I don't really see how the unit 
> tests get in the way of tracking down bugs. Is it that you need to compile in a 
> version of std.datetime which doesn't have any unit tests compiled in but you 
> still need to compile with -unittest for other stuff?

No. All you know there's a bug that's being triggered somewhere in 
Phobos (with -unittest). It's probably not in std.datetime.
But Phobos is a horrible ball of mud where everything imports everything 
else, and std.datetime is near the centre of that ball. What you have to 
do is reduce the amount of code, and especially the number of modules, 
as rapidly as possible; this means getting rid of imports.

To do this, you need to remove large chunks of code from the files. This 
is pretty simple; comment out half of the file, if it still works, then 
delete it. Normally this works well because typically only about a dozen 
lines are actually being used. After doing this about three or four 
times it's small enough that you can usually get rid of most of the imports.
Unittests foul this up because they use functions/classes from inside 
the file.

In the case of std.datetime it's even worse because the signal-to-noise 
ratio is so incredibly poor; it's really difficult to find the few lines 
of code that are actually being used by other Phobos modules.

My experience (obviously only over the last month or so) has been that 
if the reduction of a bug is non-obvious, more than 10% of the total 
time taken to fix that bug is the time taken to cut down std.datetime.