[LLVMdev] Lost in the documentation
gordonhenriksen at mac.com
Tue Apr 29 06:46:35 PDT 2008
On 2008-04-29, at 08:41, Hendrik Boom wrote:
> On Mon, 28 Apr 2008 17:54:31 -0400, Gordon Henriksen wrote:
>> On Apr 28, 2008, at 17:32, Hendrik Boom wrote:
>>> In http://llvm.org/docs/FAQ.html, when taking about writing a
>>> that uses LLVM (at least I think that's what the FAQ question is
>>> the FAQ recommends
>>>> # Call into the LLVM libraries code using your language's FFI
>>>> function interface).
>>>> * for: best tracks changes to the LLVM IR, .ll syntax, and .bc
>>>> * for: enables running LLVM optimization passes without a
>>>> emit/parse overhead
>>>> * for: adapts well to a JIT context
>>>> * against: lots of ugly glue code to write
>>> Now, which particular libraries would that be
>> With the exception of the 'util' and 'tools' directories, the entire
>> LLVM source tree consists of libraries.
> Indeed, quite a lot of them. Most of them appear to be internal.
> I'm trying to identify the ones that are intended for use by LLVM
include/llvm is all public (modulo some implementation details as
required by the nature of C++). Private includes are in lib. But
realize that not all users are front-end compilers. A back-end code
generator is also a user of the framework; as is an IR optimization or
analysis. The C++ interfaces support all of these clients equally.
VMCore and BitWriter are the libraries absolutely necessary for any
static compiler that outputs bitcode. You'll likely want Analysis for
the verifier; and Target for memory layout information. That's the
> I have to say I missed the crucial paragraph:
> : If you go with the first option, the C bindings in include/llvm-c
> : should help a lot, since most languages have strong support for
> : interfacing with C. The most common hurdle with calling C from
> : code is interfacing with the garbage collector. The C interface was
> : designed to require very little memory management, and so is
> : straightforward in this regard.
> Evidently I have to go look in include/llvm-c, since I stronlgly
> you didn't go to the trouble of writng a C wrapper for anything that
> wasn't needed by an LLVM user. Anything internal you'd have left in
> So the API for a C++ *user* could be described as "those parts of the
> internals API that happen to be used in implementing llvm-c.
That's a rather poor definition. Only bindings for such features as
have been required are authored. Still, if this helps you make sense
of the framework, then that's fantastic; but remember that it is an
Using the C bindings, it's still very important to understand the
underlying C++ object model; otherwise, the type rules for the
bindings will appear to be rather capricious.
> Putting this together with the tutorial, http://llvm.org/docs/tutorial/
> which uses CAML instead of C, I think I may be able to get a clue.
If you're not using ocaml, the C++ tutorial (the first one on that
page) is probably more pertinent, even if you do intend to use the C
bindings. Searching the implementation of the bindings (lib/VMCore/
Core.cpp, etc.) is helpful for "going backwards" from C++ to C once
you begin to understand the object model.
>>> where are their API(s) documented?
>> etc etc etc.
>> — Gordon
> The doxygen page describes the complete internal structure of LLVM.
> explicitly says,
> ; This documentation describes the internal software that makes up
> ; not the external use of LLVM. There are no instructions here on
> how to
> ; use LLVM, only the APIs that make up the software. For usage
> ; instructions, please see the programmer's guide or reference manual.
> I haven't yet found a "programmer's guide".
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