[llvm-dev] The future of LLVM's C APIs: Notes and BoF.

[Rodney M. Bates via llvm-dev]

On 10/17/2015 01:20 PM, Justin Bogner via llvm-dev wrote:
> (Moving this to llvm-dev)
>
> On Friday, October 16, 2015, Justin Bogner <mail at justinbogner.com <mailto:mail at justinbogner.com>> wrote:
>
>     Some users of llvm-c want stable API interfaces into various parts of
>     the LLVM infrasture, others want further ABI guarantees about this
>     usage, and still others simply want a way to bind to LLVM through their
>     language frontend’s existing FFI support for C.
>
>     If we want to improve the situation for any of these users, we need to
>     properly understand how these APIs are being used (or abused)
>     today. Juergen and I will be hosting a BoF at the dev meeting where we
>     can discuss what the requirements of a sustainable C API are, and how we
>     can organize things in LLVM to support this.
>
>     There's been a fair amount of discussion about this lately, and it's
>     pretty clear that the "hopefully stable bindings to whatever APIs
>     somebody needed" approach isn't really good enough for anybody. There
>     are a couple of points that I think are fairly non-controversial, but
>     will more or less drive the discussion at the BoF:
>
>     1. It isn't practical to keep a bindings API stable, unless the
>         underlying API is also stable.
>
>     2. Handrolling bindings as they're needed tends to leave conspicuous
>         gaps where some API is inaccessible for no good reason.
>
>     So based on (1), we'll really want to create some purpose built APIs
>     that we can keep stable for various tasks. What's needed here? People
>     want to do things like building a pass manager, setting up a canned JIT
>     config, and to some degree even emit IR. We'll discuss what's practical
>     and what people want, and hopefully strike a good balance.
>
>     Similarly, (2) implies that if we really need a *full* bindings API
>     we'll want to automate it. But what is a full bindings API? Who uses it,
>     and what do they want from it? If it's automated, should installing LLVM
>     install this API, or should we simply provide an easy way to generate
>     it?
>
>     In the end, I hope to have a good idea of what people are actually using
>     these APIs for, and both to support stable API users less haphazardly
>     and to make unstable API more thorough and/or easier to create.
>

One actual use:

A couple of us in the Modula3 support community are working on splicing a
llvm back end onto our Modula3 compiler front end.  We have zero-thickness
bindings written in Modula3 which match C bindings like core.h.  In our front
end executable, we only build llvm IR in memory, then use LLVMPrintModuleToFile
and/or LLVMWriteBitcodeToFile.  We only link in the needed parts of the llvm
infrastructure to build and write the llvm IR.  Then we run stock llc on the
IR code in the emitted file.

We have had to write an additional binding to DIBuilder for this purpose,
as well as to a few odd other C++ functions here and there.  The latest
version of this is for llvm 3.6.1, after the separation of metadata from
values.  It has had minimal testing, but is fairly complete.  Older bindings
to an older DIBuilder exist, and are at least partly working, but only
contain specifically needed functions.

This scheme is passing a majority of preexisting compiler tests of code
function, but there is not much testing of emitted debug info.  Speaking
for myself, better debugger support is one of the primary motivations for
using llvm.  Our older back ends use a really cobbled-up extension of
stabs+.  It's a mess, and lots of debugger function is difficult to
provide.

The DIBuilder binding includes .h and .cpp files similar to core, that provide
a C binding, with lots of [un]wrapping, etc.  Like core, it loses the type
hierarchy.  Modula3 has a type hierarchy, but machine-level representation
is undoubtedly not ABI compatible with any C++ compiler.  The current handling
of this is not thoroughly thought out, just using lots of unsafe casting of
pointers.  I would like to think of something better, but haven't had much time
for that, so far.

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-- 
Rodney Bates
rodney.m.bates at acm.org