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

[Sean Silva]

On Wed, Dec 4, 2013 at 12:54 AM, Mikael Lyngvig <mikael at lyngvig.org> wrote:

> Hi Chris,
>
> Thanks for the supporting words!  I'm pushing the document both for
> egoistic motives (like so many others, I'll learn a ton from this document)
> and for altruistic motives - the easier it is to implement a new language,
> the more interesting and highly well-thought out languages we will see in
> the future.  And I see it as my purpose, as a mostly black-box user of
> LLVM, to enhance the experience for newcomers so that they don't turn away
> and waste time on other projects just because it all seems rather
> overwhelming at first.
>
> I couldn't recall having heard of the "alloca trick", but a Google search
> revealed that this is described in the Kaleidoscope sample.  I will be more
> than happy to include it - that's precisely what the document is also for:
> Teaching people all the things that cannot easily be said in a Language
> Reference.  In a way, the name is already now becoming poorly chosen.
>  Because I begin to see a User's Guide forming in the horizon.  And that
> would go really well with the Language Reference; most products have both.
>

TBQH I'm pretty set on this being a guide for language frontends, rather
than a general "user's guide" for the IR. The IR has at least two very
different classes of users: optimization writers (which are mostly
transforming IR) and language frontend writers (which are mostly creating
IR). Almost everything in this document is geared at language frontend
writers (or more generally "people generating IR"), rather than
optimization writers (we already have pretty good docs for them).


>
> I've added all of your suggestions to my to-do list, which I'll write into
> the document later today, so that none of the suggestions get lost.  Yes,
> the unions I thought about at some point but forgot about them again.  I
> also feel that there needs to be good documentation of GEP and extractvalue
> - when to use one and when to use the other.  In fact, the whole
> structure/union aspect seems mostly overlooked because I got too
> preoccupied with the class stuff.
>

GEP is for forming addresses, and extractvalue/insertvalue is for
extracting/inserting fields from aggregate-typed SSA values.


>
> I am not at all opposed to working directly from llvm.org/docs, the only
> thing is that I do a lot of small commits with an occasional large commit
> here and there, and I wouldn't want to provoke a review whenever I change a
> single line here or there.  The reason I use GitHub is that it provides a
> nifty, colorized page (
> https://github.com/archfrog/llvm-doc/blob/master/MappingHighLevelConstructsToLLVMIR.rst)
> that people (including myself) can view without going through the trouble
> of installing and running Sphinx locally.  And also, it allows people to
> submit reviews by forking, creating an issue, or attaching a comment (all
> three of which have already been in use).  I think it is better that I do
> it in GitHub for the time being as I tend to make many small, stupid
> mistakes that I usually discover quite quickly and then fix.  Then when I
> feel I've got something interesting to show people, I can submit a commit
> and everybody can join in the review.
>

If it's easier for your workflow to iterate on github, that's fine,
although eventually we will want to move it into docs/. It definitely has a
bit of a "grab bag" feel; as the content solidifies, I'd like to see a
better organization.

There's some content though that might be easier to develop in-tree, like
how to hint the optimizers to get maximum performance. Especially alias
analysis (both TBAA, and the various function/parameter attributes) and
alignment, as most non-C languages can provide very strong aliasing and
alignment guarantees.

-- Sean Silva


>
>
> -- Mikael
>
>
>
> 2013/12/4 Chris Lattner <clattner at apple.com>
>
>>
>> On Nov 28, 2013, at 6:07 PM, Mikael Lyngvig <mikael at lyngvig.org> wrote:
>>
>> Hi,
>>
>> It will probably take a few weeks or a month before the "Mapping
>> High-Level Constructs to LLVM IR" document is ready for prime time.  Until
>> then, you can review and study it at this URL:
>>
>>
>> https://github.com/archfrog/llvm-doc/blob/master/MappingHighLevelConstructsToLLVMIR.rst
>>
>>
>> Please notice that I specifically do not advocate reviewing the document
>> for a week or two.  But feel free to give me any feedback, comments, and
>> criticism that you may have to share.
>>
>>
>> This looks really awesome!  Great idea starting this, and thank you for
>> pushing it forward.  Some thoughts:
>>
>> - In "local variables", it would be great to talk about how the "alloca
>> trick" avoids forcing your frontend to build SSA.  You could even include
>> an example.
>>
>> - In the "constants" section, it is probably best to say that "constants
>> that allocate memory" are just global variables in LLVM IR, marked with the
>> 'constant' keyword.  It would also be great to mention constant exprs here,
>> since they are a point of confusion (and you introduce them in sizeof).
>>
>> - Having something that talks about lowering C-style unions to llvm IR
>> would be great :-)
>>
>> - A nice new top-level section would be "interoperating with a runtime
>> library", pointing out that not everything needs to be emitted as inline
>> LLVM IR: a frontend can either just call into a runtime library, or it can
>> even emit a call to a runtime library (whose body is also available as IR)
>> and then have the optimizer inline it if run.
>>
>>
>> Once the document has been finalized and comitted to LLVM, I'll delete
>> the repository at Github - or, perhaps even better, simply make a small
>> page that refers to the official copy in LLVM.
>>
>>
>> Are you interested in just building it in llvm.org/docs?  Unless your
>> workflow is better on github, it seems easier to do it on llvm.org - it
>> would make it easier for other people to chip in.
>>
>> -Chris
>>
>
>
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