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

Stephen Cross scross at scross.co.uk
Tue Jun 30 17:19:10 PDT 2015 

Hi everyone,

I was wondering if anyone could answer the questions in my first
email. These were:

* Why does Clang generate 8 byte alignment for 16+ byte arrays on
x86-64, even though the AMD64 ABI seems to require 16 byte alignment?
* Clang has some logic in lib/Basic/Targets.cpp to determine CPU
features, but LLVM also does the same task in its backend. Would it be
possible to modify LLVM to expose this logic and modify Clang to use
that?
* (Slightly modified) Clang has some logic in lib/Driver/Tools.cpp
('getCPUName') for selecting a generic CPU. Would it be sensible to
expose this in a header in include/clang/Driver/?

> 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).

Yes, this sounds like a good approach. While I'm not entirely
convinced this is where we want to end up (it doesn't seem right to
split the ABI implementation between the frontend and the backend), I
definitely agree this is a sensible starting point. Unless anyone is
against this I'll start working out the changes needed under the
assumption this is how I should proceed.

My approach would be to just refactor the code to separate classes
within CodeGen and add the necessary headers to
include/clang/CodeGen/. It might make sense to create a separate
library directory (e.g. lib/ABI) but I expect there will be circular
dependency issues to overcome. The very first thing is probably to
split each target's code out of TargetInfo.cpp; I'd suggest that the
structure of CodeGen could mirror <llvm>/lib/Target/ so we'd have
directories for ARM, PowerPC, X86 etc. What do you think?

Note that there's also functionality in CGCall.cpp (and a few other
files) which would need to be separated out to be usable for other
frontends.

> 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.

This will be how it ends up, but while the modifications to Clang are
ongoing llvm-abi is and will continue to be an entirely separate
ABI-implementing codebase. This is just because the Loci frontend
needs this functionality right now. I'm also expecting there will be
functionality going into llvm-abi that for good reason won't be found
in Clang. There's some duplication of effort here but it's
unavoidable.

I like the idea of frontends using the library as a convenient API and
the library's implementation using code from Clang. I know that some
frontends (including the Loci frontend) are designed to be able to be
built on systems that could have slightly older versions of LLVM/Clang
(for Loci, it's LLVM 3.3+), so this factor will have to be considered
when integrating the code from Clang.

> At the moment we have support for x86-64 only: http://llvm.org/klaus/llgo/blob/master/irgen/cabi.go.

I'll add other architectures to llvm-abi soon (it's largely a fairly
trivial process of moving code from Clang and replacing its in-tree
dependencies) so hopefully this will be useful. I can see from other
non-Clang frontends that non-X86 support does tend to be at least
quite incomplete.

> It would be nice to have a simplified API to TargetInfo, ideally with a C API that other languages to can easily bind to.

Absolutely, I agree. I appear to be in the rare situation of having a
frontend written in C++ (i.e. not self-hosting) but I know most
frontends rely on having a C API.

It seems like the approach to this should be to have a C API in
llvm-abi that wraps around its C++ API. Unfortunately my time is
currently fairly thin-spread; would you be able to contribute to this
effort?

Thanks,
Stephen

On Tue, Jun 30, 2015 at 12:47 AM, Andrew Wilkins <axwalk at gmail.com> wrote:
> On Tue, 30 Jun 2015 at 06:02 Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> ----- 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.
>
>
> This is what I was thinking of doing for llgo in the long term. At the
> moment we have support for x86-64 only:
> http://llvm.org/klaus/llgo/blob/master/irgen/cabi.go. It would be nice to
> have a simplified API to TargetInfo, ideally with a C API that other
> languages to can easily bind to. Stephen, if you do go down that route, I'd
> be keen to hear about it.
>
> Cheers,
> Andrew
>
>>  -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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>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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