[LLVMdev] "Mapping High-Level Constructs to LLVM IR"

Mon Nov 25 02:15:15 PST 2013

Wow: that's a lot of text to have written in a short time. I'll try to have
a detailed look later, but one borderline thing is you mention there are 6
different casts. It might be worth pointing out there's a 7th, the
addrspacecast which is beyond the scope of the document but is used with
address spaces describing devices which (may) have different address spaces
which are used for different things (either for performance or security
reasons).

http://llvm.org/docs/LangRef.html#i-addrspacecast

Cheers,
Dave

On Mon, Nov 25, 2013 at 9:46 AM, Mikael Lyngvig <mikael at lyngvig.org> wrote:

> Hi David,
>
> I'm glad you like the idea :-)
>
> I've been busy and so far only lack about three or four things in the C++
> area of features: Proper exception handling (which I need to understand
> myself first), closures (which I don't think I've ever used), and
> generators (which I always wondered how they were implemented).  I think
> you are absolutely right - some day, this document will be routinely be
> extended with new and wonderful language constructs and this will help
> these features to propagate much faster into existing and new languages as
> people will quickly come to realize how simple it is to do this or that
> feature.  Besides, I think it is an awesome way of teaching people how to
> use LLVM IR: They can use their knowledge of familiar structures and
> constructs to see how it can be done in LLVM IR.
>
> So far, it hasn't been too bad with GEPs and loads.  The most advanced
> example uses like two or three GEPs/loads in a row and I have already
> explained at the top of the document that the user should not worry - most
> commonly two or three LLVM IR instructions will be coalesced into a single
> instruction after optimization.
>
> I'll let your idea simmer a bit and see what comes up.  For now, I have
> had the joy of coding up quite a few snippets in LLVM IR.  I am learning to
> swim as I am sinking into all the intricacies of LLVM IR.  So far is has
> only been pleasant working with LLVM IR, albeit I am a tad tired of typing
> types because LLVM IR demands explicit types on most expressions and
> arguments.  But I do understand that LLVM IR is meant to be automatically
> generated by a compiler, not hand-crafted by a tech writer.
>
> I've attached my current draft.  If you are busy, don't care, or prefer to
> wait for the final result, please don't waste time on looking at it.
>  There's still a quite a bit to do.  Comments are more than welcome,
> though.  And feel free to suggest new language features and so on: Even if
> I cannot document them, I can always ask the list for help and together we
> can make up a great document, which I personally suspect will one day be
> almost as popular as the Language Reference.
>
>
> -- Mikael
>
>
> 2013/11/25 David Tweed <david.tweed at gmail.com>
>
>> Hi, documentation is always good and this is a great idea. It'll be
>> particularly useful as a place where additional examples of constructs from
>> non-C-family languages could be added (since most compiler tutorials
>> inevitably focus on languages that are a lot like C/C++/Obj-C).
>>
>> I'd imagine you've already thought of this, but it might be something
>> where using pseudo-LLVM-IR is of the most pedagogical use. (For example,
>> writing code that accesses complex structured memory using multiple levels
>> of GEP's and then loads is quite tricky, but for a lot of the constructs
>> it's only a detail so you could probably express those bits using some
>> pseudo instruction (going another step in to fully explicit LLVM-IR if
>> necessary.)
>>
>> Cheers,
>> Dave
>>
>>
>> On Sat, Nov 23, 2013 at 6:18 AM, Mikael Lyngvig <mikael at lyngvig.org>wrote:
>>
>>> Thanks, you have a lot of valid points there.  I have myself long ago
>>> abandoned the path of using C as a backend language due to the very factors
>>> you mention.
>>>
>>> However, as I said, the document was put together in 30 minutes.  Not
>>> exactly ready for prime time :-)
>>>
>>> I do agree that all of the things you mention should be described,
>>> including Lambdas, closures, and generators, but I must admit up front that
>>> I don't know how to implement half of them.  But I suppose I could do a lot
>>> of research and perhaps occasionally ask you guys for specifics.
>>>
>>> We are not going to find much common ground on the issue of "calling
>>> propagated return values for exception handling", I think :-)  See
>>> https://www.lyngvig.org/Teknik/A-Proposal-for-Exception-Handling-in-Cfor the details.
>>>
>>> I started out with C++ as the example language because a lot of people
>>> know that language - and most certainly the majority of the LLVM user base.
>>>  Obviously, you'd have to add source code from other languages than C++
>>> when C++ does not provide features to illustrate the process.
>>>
>>> I now agree that the lowering into C is not such a good idea after all.
>>>  So I'll go straight from source language to LLVM IR, which is not that
>>> difficult after all, and won't be very different for the reader.  In fact,
>>> I think it will be much better than my original approach.
>>>
>>> Thanks again for your valid objections.
>>>
>>>
>>> -- Mikael
>>>
>>>
>>>
>>>
>>> 2013/11/23 Joshua Cranmer 🐧 <Pidgeot18 at gmail.com>
>>>
>>>> On 11/22/2013 9:25 PM, Mikael Lyngvig wrote:
>>>>
>>>>> Hi guys,
>>>>>
>>>>> I have begun writing on a new document, named "Mapping High-Level
>>>>> Constructs to LLVM IR", in which I hope to eventually explain how to map
>>>>> pretty much every contemporary high-level imperative and/or OOP language
>>>>> construct to LLVM IR.
>>>>>
>>>>> I write it for two reasons:
>>>>>
>>>>> 1. I need to know this stuff myself to be able to continue on my own
>>>>> language project.
>>>>> 2. I feel that this needs to be documented once and for all, to save
>>>>> tons of time for everybody out there, especially for the language inventors
>>>>> who just want to use LLVM as a backend.
>>>>>
>>>>> So my plan is to write this document and continue to revise and
>>>>> enhance it as I understand more and helpful people on the list and
>>>>> elsewhere explain to me how these things are done.
>>>>>
>>>>> Basically, I just want to know if there is any interest in such a
>>>>> document or if I should put it on my own website.  If you know of any books
>>>>> or articles that already do this, then please let me know about them.
>>>>>
>>>>> I've attached the result of 30 minutes work, just so that you can see
>>>>> what I mean.  Please don't review the document as it is still in its very
>>>>> early infancy.
>>>>>
>>>>
>>>> There is a strong bias towards C++ in the document, which isn't a
>>>> particularly strong slice of higher-level constructs. For example, C++'s
>>>> RTTI constructs serve three distinct purposes: exception handling, dynamic
>>>> casts, and reflection (although C++'s reflection capabilities are extremely
>>>> weak). You'll need to talk about inheritance in the three cases: single,
>>>> multiple, and virtual (to use C++'s terminology) (note that Java's
>>>> interfaces can be implemented as virtual inheritance). Boxing is another
>>>> important topic. Lambdas, closures, and generators (yield keyword) are
>>>> becoming increasingly common in modern programming languages, and should
>>>> not be ignored.
>>>>
>>>> Finally, calling propagated return values "exception handling" does an
>>>> extreme disservice to your readers. LLVM IR explicitly models exception
>>>> handling, and attempting to describe it lowered as return values is not how
>>>> anyone should implement it. If you badly want to describe it in C terms,
>>>> you could at least use C's setjmp/longjmp to describe it; the truth is,
>>>> this is a feature which doesn't exist cleanly in C.
>>>>
>>>> Trying to describe mapping higher-level languages to C and then C to IR
>>>> is a poor idea. C is in some ways an extremely limited language (no native
>>>> exception handling constructs, e.g.). If you want to be a guide to how to
>>>> lower languages to LLVM IR, you need to also explain how to take advantage
>>>> of features in the IR to optimize code better (e.g., TBAA). Cfront-like C++
>>>> compilers are extremely rare-to-nonexistent (in part because it is
>>>> difficult to map some features, most notably exception handling, cleanly
>>>> and efficiently into C); if your guide is describing such an approach, it
>>>> reads like an implicit endorsement. It is possible to describe some aspects
>>>> of the IR in C, but if the goal is to lower to IR, then the description
>>>> should be lowering to IR, not lowering to C.
>>>>
>>>> --
>>>> Joshua Cranmer
>>>> Thunderbird and DXR developer
>>>> Source code archæologist
>>>>
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>>>>
>>>
>>>
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>>>
>>>
>>
>>
>> --
>> cheers, dave tweed__________________________
>> high-performance computing and machine vision expert:
>> david.tweed at gmail.com
>> "while having code so boring anyone can maintain it, use Python." --
>> attempted insult seen on slashdot
>>
>>
>
>

-- 
cheers, dave tweed__________________________
high-performance computing and machine vision expert: david.tweed at gmail.com
"while having code so boring anyone can maintain it, use Python." --
attempted insult seen on slashdot
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