llvm dev - Jul 2015 - [LLVMdev] [cfe-dev] llvm-abi: A library for generating ABI-compliant LLVM IR

On Wed, 1 Jul 2015 at 08:19 Stephen Cross <scross at scross.co.uk> wrote:
> 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?
>
Sounds good. My time is likewise thinly spread, but I'll be happy to
contribute to the effort when I can.

Cheers,
Andrew

> 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
> >> > >
> >> > >
> >> > _______________________________________________
> >> > cfe-dev mailing list
> >> > cfe-dev at cs.uiuc.edu
> >> > 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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Chiming in from the LLILC project:


Overall, the idea of an ABI support library sounds great. It'd certainly
save us a fair amount of duplicated effort in our frontend, as the CoreCLR ABI
(mostly) corresonds to the native C ABI of its host platform. What we have
today[1, 2] is largely inspired by Clang, and so looks pretty similar to
Stephen's existing work in llvm-abi.
> I like the idea of frontends using the library as a convenient API and
> the library's implementation using code from Clang.
Although I understand the motivations behind wrapping the Clang implementation,
I'm a bit leery of this as a long-term direction. Admittedly, this is
selfishly motivated: LLILC currently has no Clang dependency, and I think
we'd prefer to avoid taking one if possible. A reasonable compromise would
be the refactoring Stephen suggested below (i.e. moving ABI-related code into
its own library within Clang):
> 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.
Although the refactoring could be tricky, this would at least let external users
limit their dependency to the new library rather than pulling in libCodeGen.

Thanks,
- Pat

[1] https://github.com/dotnet/llilc/blob/master/include/Reader/abi.h
[2] https://github.com/dotnet/llilc/blob/master/lib/Reader/abi.cpp

________________________________
From: llvmdev-bounces at cs.uiuc.edu <llvmdev-bounces at cs.uiuc.edu> on
behalf of Andrew Wilkins <axwalk at gmail.com>
Sent: Tuesday, June 30, 2015 6:24 PM
To: Stephen Cross; Hal Finkel
Cc: Clang Developers List; LLVM Developers Mailing List
Subject: Re: [LLVMdev] [cfe-dev] llvm-abi: A library for generating
ABI-compliant LLVM IR

On Wed, 1 Jul 2015 at 08:19 Stephen Cross <scross at
scross.co.uk<mailto:scross at scross.co.uk>> wrote:
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<https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_klaus_llgo_blob_master_irgen_cabi.go&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-Iirv3ocYjaPGb9zIT82vHWpazIJhUFv6POGu334YXo&s=-HKcBselyiXmmBPOX9tN1pnpOIt82rdZWUAuus7TW4w&e=>.
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?

Sounds good. My time is likewise thinly spread, but I'll be happy to
contribute to the effort when I can.

Cheers,
Andrew

Thanks,
Stephen

On Tue, Jun 30, 2015 at 12:47 AM, Andrew Wilkins <axwalk at
gmail.com<mailto:axwalk at gmail.com>> wrote:
> On Tue, 30 Jun 2015 at 06:02 Hal Finkel <hfinkel at
anl.gov<mailto:hfinkel at anl.gov>> wrote:
>>
>> ----- Original Message -----
>> > From: "Stephen Cross" <scross at
scross.co.uk<mailto:scross at scross.co.uk>>
>> > To: "Reid Kleckner" <rnk at google.com<mailto:rnk
at google.com>>
>> > Cc: "Clang Developers List" <cfe-dev at
cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>>, "LLVM Developers
>> > Mailing List" <llvmdev at cs.uiuc.edu<mailto: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<https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_klaus_llgo_blob_master_irgen_cabi.go&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-Iirv3ocYjaPGb9zIT82vHWpazIJhUFv6POGu334YXo&s=-HKcBselyiXmmBPOX9tN1pnpOIt82rdZWUAuus7TW4w&e=>.
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<mailto: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<mailto: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<https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_scross99_llvm-2Dabi&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-Iirv3ocYjaPGb9zIT82vHWpazIJhUFv6POGu334YXo&s=hV2-aaQYIMI-S0TwecPz9lEQSKeHNzTLe0H6uDJ7rHU&e=>
>> > >>
>> > >> 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<https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_scross99_locic&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-Iirv3ocYjaPGb9zIT82vHWpazIJhUFv6POGu334YXo&s=AYIqYKlv0AVTlDWgGXreFzgK6cUAfCWlBmV24CfMtsA&e=>
>> > >> _______________________________________________
>> > >> cfe-dev mailing list
>> > >> cfe-dev at cs.uiuc.edu<mailto:cfe-dev at
cs.uiuc.edu>
>> > >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> > >
>> > >
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
>> > 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<mailto:LLVMdev at cs.uiuc.edu>        
http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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> Overall, the idea of an ABI support library sounds great. It'd
certainly save us a fair amount of duplicated effort in our frontend, as the
CoreCLR ABI (mostly) corresonds to the native C ABI of its host platform.
Yes indeed. I'm starting to wonder just how many frontends exist that
implement this logic.

Of course I'm naturally going to ask whether you'd be able to help
with any aspects of llvm-abi? :-) In particular it'd be great to have
people contributing the parts that they specifically need (e.g. it
sounds like WIndows support would be important for you).
> Although I understand the motivations behind wrapping the Clang
implementation, I'm a bit leery of this as a long-term direction.
Admittedly, this is selfishly motivated: LLILC currently has no Clang
dependency, and I think we'd prefer to avoid taking one if possible. A
reasonable compromise would be the refactoring Stephen suggested below (i.e.
moving ABI-related code into its own library within Clang):
Yes, I know. It's worth pointing out that llvm-abi doesn't need to
depend on a built version of Clang, just the same source code, so we
can do fun things like Git submodules or subtrees (presumably using
the Clang Git Mirror). So we'd stay in-sync with the upstream code
from Clang but llvm-abi would actually build the relevant parts of
Clang as part of its own build. I'm not necessarily advocating this
direction but just raising it as a potential solution. It really
depends on exactly what problems you would encounter with a dependency
on Clang.
> Although the refactoring could be tricky, this would at least let external
users limit their dependency to the new library rather than pulling in
libCodeGen.
Yes, I'd like to do this but the current Clang ABI code depends
heavily on other code in libCodeGen. This is something that I'll
ultimately try to do but it's not currently clear exactly what
restructuring is appropriate.

On Wed, Jul 1, 2015 at 8:33 PM, Pat Gavlin <pagavlin at microsoft.com>
wrote:
> Chiming in from the LLILC project:
>
>
> Overall, the idea of an ABI support library sounds great. It'd
certainly
> save us a fair amount of duplicated effort in our frontend, as the CoreCLR
> ABI (mostly) corresonds to the native C ABI of its host platform. What we
> have today[1, 2] is largely inspired by Clang, and so looks pretty similar
> to Stephen's existing work in llvm-abi.
>
>
>> I like the idea of frontends using the library as a convenient API and
>> the library's implementation using code from Clang.
>
> Although I understand the motivations behind wrapping the Clang
> implementation, I'm a bit leery of this as a long-term direction.
> Admittedly, this is selfishly motivated: LLILC currently has no Clang
> dependency, and I think we'd prefer to avoid taking one if possible. A
> reasonable compromise would be the refactoring Stephen suggested below
(i.e.
> moving ABI-related code into its own library within Clang):
>
>> 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.
>
> Although the refactoring could be tricky, this would at least let external
> users limit their dependency to the new library rather than pulling in
> libCodeGen.
>
> Thanks,
> - Pat
>
> [1] https://github.com/dotnet/llilc/blob/master/include/Reader/abi.h
> [2] https://github.com/dotnet/llilc/blob/master/lib/Reader/abi.cpp
>
> ________________________________
> From: llvmdev-bounces at cs.uiuc.edu <llvmdev-bounces at cs.uiuc.edu>
on behalf of
> Andrew Wilkins <axwalk at gmail.com>
> Sent: Tuesday, June 30, 2015 6:24 PM
> To: Stephen Cross; Hal Finkel
> Cc: Clang Developers List; LLVM Developers Mailing List
> Subject: Re: [LLVMdev] [cfe-dev] llvm-abi: A library for generating
> ABI-compliant LLVM IR
>
> On Wed, 1 Jul 2015 at 08:19 Stephen Cross <scross at scross.co.uk>
wrote:
>>
>> 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?
>
>
> Sounds good. My time is likewise thinly spread, but I'll be happy to
> contribute to the effort when I can.
>
> Cheers,
> Andrew
>
>>
>> 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
>> >> > >
>> >> > >
>> >> > _______________________________________________
>> >> > cfe-dev mailing list
>> >> > cfe-dev at cs.uiuc.edu
>> >> > 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