[cfe-dev] Clang ABI library

Keno Fischer kfischer at college.harvard.edu
Thu Aug 14 15:30:23 PDT 2014

Yes, that's right. We (Julia) have an implementation partly based on
the one used by LDC, but it would great to have a library that allows
you to give to callbacks to classify your types according to the
distinction that the C ABI makes (the rest hopefully being possible to
infer from the llvm struct types themselves).

We also have a package that pulls in Clang and uses it's code
generation to interoperate with C++, so we'd love to see those
interfaces made public, since we'll then be able to use a shared
library rather than having to link our own with those interfaces made
forcibly public.

On Thu, Aug 14, 2014 at 2:01 PM, Reid Kleckner <rnk at google.com> wrote:
> Usually when people talk about wanting an "ABI library" for LLVM, they are
> writing their own language frontend and merely want logic for C ABI lowering
> so they can use libraries with C interfaces. They probably don't want to
> link in Clang's entire AST and CodeGen libraries, for example. They want
> something lightweight that only understands C types: structs, unions,
> complex, floating point types, vectors, arrays, and integers. I think Julia
> wants something like this, for example.
> What you're talking about doesn't sound like an ABI library so much as
> improving Clang's library interfaces to the point that it can be used as a
> library to emit IR that interoperates with C++. This sounds like a good
> general goal to me. You could use it to build something superior to SWIG,
> for example.
> On Thu, Aug 14, 2014 at 3:10 AM, DeadMG <wolfeinstein at gmail.com> wrote:
>> Hello all,
>> There has apparently been discussion for some time now about creating an
>> ABI support library in Clang to be used for external developers. For some
>> time now I've been wondering about the status of the proposed ABI support
>> library. But I realized that I've effectively already implemented a good
>> part of it myself. As an external developer I've been building a project on
>> top of Clang for some time now with the aim of implementing the following
>> features:
>> Call into C and C++ functions declared in a header analyzed by Clang,
>> including special members (including implicitly generated ones), operator
>> overloads, those with defaulted arguments, and such,
>> Implement C and C++ functions declared in a header analyzed by Clang,
>> Access members of C and C++ types, including the virtual pointer and
>> vtables,
>> Call C++ virtual functions, and implement types that effectively inherit
>> from C++ types,
>> Code-generate C++ translation units into an LLVM module for use by
>> external code generation mechanisms,
>> Create thunks with an arbitrary body for a given signature,
>> Perform C++ semantic analysis from external source; e.g. name lookup,
>> template instantiation, overload resolution, and such,
>> Evaluate C++ macros and expressions within limited contexts; e.g. "Valid
>> expression at global namespace scope in given TU". I am aware of one other
>> who would be looking for "Valid statements at function scope; for given
>> imaginary function X with local variables Y and Z of type W.",
>> Implement exception handling and RTTI with binary compatibility with
>> Clang,
>> Some of this stuff has been easily accessible via existing APIs; but some
>> has not. So far I've had success implementing most of these features most of
>> the time, but I've reached the inescapable conclusion that some of them will
>> require patches for Clang. Since I know that I'm not the only one who would
>> like these features, then I may as well just upstream them.
>> For semantic analysis, parsing and macro evaluation, I've generally
>> determined that the existing APIs offer all the features required, but the
>> user-friendliness is often a bit lacking. For example, when considering
>> SourceLocations, some functions in the Sema randomly don't accept invalid
>> ones, which hinders their use when invoking them externally, and I had ...
>> fun ... trying to create my own SourceLocations that point to external
>> locations.
>> The existing CodeGen modules were never designed, as far as I'm aware, to
>> interface with external code generation, but in my experience, they can be
>> made to do so relatively easily in most cases.
>> Currently, I've been operating on Itanium systems only (Ubuntu x64 and
>> Windows x86 with MinGW) but I figure that it's a good start since Clang's
>> own support for MSVC ABI is limited. I don't have any other targets
>> available to test for. As for tests themselves, as far as I'm aware Clang
>> uses a totally different testing strategy to me, so whilst I have a couple
>> of basic existing tests, I'm not sure how they can be adapted.
>> Officially, IR-level interoperation with Clang isn't supported at all
>> right now, and the CodeGen interfaces are private. I figure that before I
>> start working on patches and providing interfaces, I want to find out if
>> this is something that the Clang devs in general want to move forward with.
>> My machine is very slow and building LLVM/Clang takes some time, so I'd
>> really rather not get cracking only to find out that my patches are
>> unwanted.
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