[LLVMdev] technical debt

Fri Jun 15 02:34:35 PDT 2012

Hello,

I am new to this alias but still wanted to pitch in. When I was learning tablegen syntax I found tablegen tests to be a lot more educative than examples in the "tablegen fundamentals".
I'd hope that newer documentation found adopt some of those tests as examples. I found writing backends quite easy but modifying front end rather hard. I wanted to modify the parser to allow #NAME# notation in multiclasses to be used in the right side of definitions but could not find an easy way to inject such change.

Thanks,

Ivan

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Sean Silva
Sent: 05 June, 2012 7:49
To: reed kotler
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] technical debt

FWIW, I'm putting together (hopefully to be done by the end of this weekend) a substantial refactoring of the TableGen backend API along with shiny new documentation (reStructuredText with sphinx) of all of TableGen, including documentation about how to write backends and---depending on how adventurous I get---a more detailed coverage of the syntax.

Also, Reed, in your TableGen talk, IIRC, you guessed that there are maybe ~10KLOC of TableGen, and said something to the tune that it wasn't too late to move over to something new and better. There are over 100KLOC in TableGen files, so unfortunately a "flag day" transition to another language is out of the question.

Feel free to get in contact with me if you would like to discuss TableGen or related topics.

--Sean Silva

On Mon, Jun 4, 2012 at 5:42 PM, reed kotler <rkotler at mips.com<mailto:rkotler at mips.com>> wrote:
On 06/04/2012 05:17 PM, Daniel Berlin wrote:
> Can we get back to the substantive discussion about your ideas for
> lessening the technical debt?
The lessening requires enlisting people that are willing to do this as
opposed to doing fun science like cool optimization. I,for example, find
the documentaiton, cleanup and refactoring to be interesting so I don't
feel cheated to work on it as opposed to implementing some new fangled
register allocator.

For example, there is almost no documentation on all the application
specific plugins for tablegen.
There are some tablegen files and some small comments here and there and
you can guess
some of it from just knowing about compilers but it's nothing close to
what could be called
documentation.

I've started on my own to try and further document tablegen. I gave a
talk/tutorial at LLVM
Europe on the general tablegen language and it was well received. Even
people that had worked
with it for a while said they took away things they never understood
about it.

It was clear when I studied tablegen that there are many serious
problems with it from a language
point of view and from a tool point view. Those things would all need to
be cleared up before
some bigger form of it that could go beyond just laying out data
structures could be
developed.

If there is sufficient interest, I think that maybe a separate
discussion list to deal with technical
debt would make sense. I think for a lot of people it would be
uninteresting to get all those
extra posts.

It's a question of enlisting people that want to work on it and
convincing people that are not interested to work on it that it's
something important to do and to welcome the help and
not obstruct the effort.

So far I have created some google code projects for various things I'm
interested to work on.
I've created separate google code projects because I don't have the
bandwidth to work on this
if there is resistance to it. So in my google code areas, I can do what
I want without a big
discussion on every step. So maybe only my team will use it and then it
can just sit in google code
forever.

So there is a cutting edge of the llvm/clang project which will never
want to wait for all the technical debt to get paid. This is a natural
thing. You can't more forward trying to make everything be A+ quality;
you can only do the A+ work after some reflection and experience with a
given problem and rewriting and refactoring it many times. But at the
same time, the technical debt needs to be settled or it will get out of
hand and unpayable in the future.

Reed

> On Mon, Jun 4, 2012 at 8:05 PM, reed kotler<rkotler at mips.com<mailto:rkotler at mips.com>>  wrote:
>> Well, differences of opinion is what makes horse races.
>>
>> Reed
>>
>>
>> On 06/04/2012 04:57 PM, Daniel Berlin wrote:
>>> On Mon, Jun 4, 2012 at 7:53 PM, reed kotler<rkotler at mips.com<mailto:rkotler at mips.com>>    wrote:
>>>> On 06/04/2012 03:25 PM, Daniel Berlin wrote:
>>>>> I'm pretty sure neither llvm nor clang have any technical debt at all.
>>>>>
>>>>> On Mon, Jun 4, 2012 at 5:18 PM, reed kotler<rkotler at mips.com<mailto:rkotler at mips.com>>      wrote:
>>>>>> something to think about as llvm and clang grows.
>>>>>>
>>>>>> http://en.wikipedia.org/wiki/Technical_debt
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>         http://llvm.cs.uiuc.edu
>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>> I hope you are joking.
>>>>
>>> Why would I be joking?
>>>
>>>> It's not meant as a criticism of llvm or clang but there is already an
>>>> enormous amount
>>>> of technical debt.
>>> I don't see that.
>>>
>>>> It's something to try and get a handle on before it gets out of hand.
>>> The consequences will never be the same
>>>> Documentation is one area where it is accumulating fast but there are
>>>> others.
>>> I think LLVM is incredibly well documented
>>>> Testing is another area.
>>> It also has at least 10-15 tests.
>>>> Tablegen alone has huge technical debt.
>>> I'm sorry you feel that way.
>>>> To me, there should be a cap placed on the number of lines of code in
>>>> llvm.
>>> Will there be a credit offset system?
>>>> Like a budget. We should try and rewrite and refactor to keep the number
>>>> of
>>>> lines from growing
>>>> without bound.
>>>>
>>>> At this point lots of patterns should be developing where other tools
>>>> (like
>>>> tablegen) could be
>>>> written to reduce the amount of code and make things more understandable.
>>> I agree. We should macroize most of the passes so they aren't so wordy.
>>>
>>>> Reed
>>>>

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