[cfe-dev] llvm-abi: A library for generating ABI-compliant LLVM IR

Mon Jun 29 14:55:55 PDT 2015

----- Original Message -----
> From: "Stephen Cross" <scross at scross.co.uk>
> To: "Reid Kleckner" <rnk at google.com>
> Cc: "Clang Developers List" <cfe-dev at cs.uiuc.edu>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Monday, June 29, 2015 3:03:40 PM
> Subject: Re: [cfe-dev] llvm-abi: A library for generating ABI-compliant LLVM	IR
> 
> Hi Reid,
> 
> Thanks for your response.
> 
> The issue is that every LLVM frontend needing ABI compliance has to
> re-implement the same target-dependent logic, which is a significant
> burden; the ABI compliance code inside Clang isn't really usable for
> other frontends as-is. We haven't got many good options here :-). I
> think a lot of people would've hoped that LLVM would provide the
> means
> for achieving ABI compliance (at least for C), though I'm well aware
> of the complexity involved and understand the decisions taken thus
> far.
> 
> Within this context, llvm-abi is an immediately actionable way of
> sharing code between frontends, that should lead to a higher quality
> codebase for a significantly reduced effort on everyone's part. It
> seems like this would further build on LLVM's success because I think
> many people would like to generate code that can interact with C
> APIs.
> 
> Ultimately it would be great to see this functionality be provided in
> an accessible form inside LLVM and hence for Clang to use that
> functionality. This would move much of the target-dependent logic out
> of Clang while at the same time making this functionality available
> to
> other LLVM-based tools.
> 
> I am sure this is a substantial and complex long term project, but I
> think it is a worthwhile aim. Do you (and the LLVM community as a
> whole) agree? (Acknowledging that some of the details would need to
> be
> worked out.)
> 
> I have a long term and substantial interest in this (as I expect do
> other frontend developers), so I'm willing to contribute significant
> effort to move this forward. Hopefully the llvm-abi library can
> provide a better understanding of what needs to be represented and
> help non-Clang frontends :-).
> 
> Implementation-wise the changes to LLVM could involve encoding ABI
> information into LLVM IR or simply providing C++ APIs for generating
> ABI-compliant code inside LLVM (much like llvm-abi). Each solution
> has
> advantages and disadvantages and I'm planning to make a proposal
> about
> this on the mailing list later (hence I'd suggest leaving that
> discussion until I make the proposal).
> 
> In terms of the mentioned deficiencies of the Type structure, these
> are all problems that I intend to address (it would be great if you
> could provide details on any of the more difficult areas).

First, let me say that I also think this is an incredibly important area in definite need of a solution. However, I also think that we really need to work from Clang's logic here: We don't want to have two different implementations of the ABI rules. They're complicated (and subtle), and it is hard enough to have one correct implementation in our C/C++ compiler. Given that we're talking about the C/C++ ABI, this is a perfectly appropriate place for the logic to live. The issue, as you well know, is that other languages need to interact with C/C++ functions, and so need to understand the C/C++ ABI. My recommended strategy is this:

 1. Refactor the logic in Clang's lib/CodeGen/TargetInfo.cpp so that it can be used by external clients (just as Clang's AST headers are available to external clients).

 2. Write your library as a wrapper around that logic so that simple cases can be handled without burdening other frontends with the subtleties of Clang's AST data structures.

 -Hal

> 
> Thanks again,
> Stephen
> 
> On Mon, Jun 29, 2015 at 7:32 PM, Reid Kleckner <rnk at google.com>
> wrote:
> > I agree it would be really nice to build a library for ABI
> > lowering, but any
> > solution that isn't clang or isn't ultimately picked up by clang
> > will
> > necessarily be incomplete. Perhaps that is OK for your frontend's
> > uses, but
> > I think it's the main reason that we haven't done something like
> > this in
> > LLVM already.
> >
> > Any solution that doesn't involve actual Clang ASTs is unlikely to
> > be able
> > to represent all C-with-extensions types (unions, bitfields,
> > alignment
> > attributes, transparent_union attribute). I took a look at Type.hpp
> > in your
> > project, and it seems to be missing some of these things. Keeping
> > such a
> > library up to date with new extensions is going to be a maintenance
> > burden.
> >
> > That said, I wish you luck, and I hope the project eases some of
> > the
> > difficulties for new frontends. It is very possible that the corner
> > cases
> > that keep me up at night are not the problems that users actually
> > face. :)
> >
> > On Sun, Jun 28, 2015 at 3:22 PM, Stephen Cross
> > <scross at scross.co.uk> wrote:
> >>
> >> Hi everyone,
> >>
> >> (Also CC'ed cfe-dev since this seems relevant to Clang,
> >> particularly
> >> the questions at the end.)
> >>
> >> I've been working on a library to generate LLVM IR that complies
> >> with
> >> platform ABIs (the current focus is on C but I'm also interested
> >> in
> >> ABIs for other languages).
> >>
> >> You can find it here: https://github.com/scross99/llvm-abi
> >>
> >> To explain further (for those who are unfamiliar), LLVM frontends
> >> have
> >> to modify function argument types, attributes etc. in order to
> >> ensure
> >> the backend generates code that satisfies the ABI; this is needed
> >> because LLVM's type system can't encode all the necessary
> >> information.
> >> This is a complex task and involves substantial amounts of
> >> target-dependent logic. Clang performs this encoding and indeed
> >> much
> >> of the current functionality is derived from Clang's source.
> >>
> >> This project originated as a necessary piece of functionality for
> >> the
> >> Loci compiler frontend [1] and is now an external dependency; my
> >> aim
> >> is to make this usable for other LLVM frontends that also need to
> >> generate ABI-compliant IR (I assume this is a fairly large subset
> >> of
> >> the frontends).
> >>
> >> I'd be very interested in any suggestions/queries/comments.
> >>
> >> I made a few interesting discoveries while working on this:
> >>
> >> * Clang generates 8 byte alignment for 16+ byte arrays on x86-64,
> >> even
> >> though the AMD64 ABI seems to require that arrays of 16+ bytes are
> >> aligned to 16 bytes. Is this a bug or am I missing something
> >> obvious?
> >>
> >> * Clang determines the features for a CPU (e.g. whether we have
> >> AVX
> >> support on x86-64 CPUs), even though this functionality is already
> >> available in LLVM (but appears to be very difficult to query).
> >> Would
> >> it be possible to expose the information from LLVM and hence
> >> eliminate
> >> the duplication in Clang?
> >>
> >> * Clang determines a 'generic CPU' if the user doesn't specify a
> >> CPU.
> >> My understanding is that we don't usually generate code for the
> >> native
> >> CPU because it may have features unavailable on other similar
> >> CPUs.
> >> LLVM can provide full details of the native CPU but can't
> >> determine a
> >> generic CPU; could this functionality be added to LLVM?
> >>
> >> (Separately I've also been considering a proposal to add ABI
> >> information directly inside LLVM IR in a language-independent way
> >> and
> >> I'll discuss this in a later email.)
> >>
> >> Thanks,
> >> Stephen
> >>
> >> [1] https://github.com/scross99/locic
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >
> >
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory