[cfe-dev] Clang ABI library

DeadMG wolfeinstein at gmail.com
Thu Aug 14 03:10:09 PDT 2014

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

   - 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
   - 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

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

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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