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

Mikael Lyngvig mikael at lyngvig.org
Fri Nov 22 22:18:22 PST 2013


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-C for
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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