llvm dev - Aug 2018 - Adding minimal target support to build clang

Hi!

In Debian, we have recently run into the situation that the package
qttools-opensource-src has added LLVM's clang parser as a build dependency
with the effect that the package can no longer be built for a couple of
architectures like alpha or ia64 [1].
>From my current understanding, qttools-opensource-src is merely using the
parser part in clang to parse C/C++ code for code analysis which is not
necessarily an architecture-specific task.

Now, does anyone know whether it would be possible to enable this feature
on any given architecture without having to add a full target backend to
LLVM? Maybe we could just build an LLVM variant on unsupported targets
with the x86 backend enabled (in case it's mandatory to have at least
one backend enabled).

Thanks,
Adrian
> [1]
https://buildd.debian.org/status/package.php?p=qttools-opensource-src&suite=sid
-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz at debian.org
`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913

Backends are necessary for parsing because clang parses inline assembly
using the llvm integrated assembles, if I recall correctly

On Sun., 19 Aug. 2018, 7:30 am John Paul Adrian Glaubitz via llvm-dev, <
llvm-dev at lists.llvm.org> wrote:
> Hi!
>
> In Debian, we have recently run into the situation that the package
> qttools-opensource-src has added LLVM's clang parser as a build
dependency
> with the effect that the package can no longer be built for a couple of
> architectures like alpha or ia64 [1].
>
> From my current understanding, qttools-opensource-src is merely using the
> parser part in clang to parse C/C++ code for code analysis which is not
> necessarily an architecture-specific task.
>
> Now, does anyone know whether it would be possible to enable this feature
> on any given architecture without having to add a full target backend to
> LLVM? Maybe we could just build an LLVM variant on unsupported targets
> with the x86 backend enabled (in case it's mandatory to have at least
> one backend enabled).
>
> Thanks,
> Adrian
>
> > [1]
>
https://buildd.debian.org/status/package.php?p=qttools-opensource-src&suite=sid
>
> --
>  .''`.  John Paul Adrian Glaubitz
> : :' :  Debian Developer - glaubitz at debian.org
> `. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
>   `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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John Paul Adrian Glaubitz via llvm-dev <llvm-dev at lists.llvm.org>
writes:
> Hi!
>
> In Debian, we have recently run into the situation that the package
> qttools-opensource-src has added LLVM's clang parser as a build
dependency
> with the effect that the package can no longer be built for a couple of
> architectures like alpha or ia64 [1].
>
> From my current understanding, qttools-opensource-src is merely using the
> parser part in clang to parse C/C++ code for code analysis which is not
> necessarily an architecture-specific task.
Building llvm without any backends is supposed to work (and has in the
past!), but it appears to fail quite a few tests currently. You can try
this by calling cmake with -DLLVM_TARGETS_TO_BUILD=''. I don't know
if
clang works in that configuration, especially given David's point about
inline assembly.
> Now, does anyone know whether it would be possible to enable this feature
> on any given architecture without having to add a full target backend to
> LLVM? Maybe we could just build an LLVM variant on unsupported targets
> with the x86 backend enabled (in case it's mandatory to have at least
> one backend enabled).
>
> Thanks,
> Adrian
>
>> [1]
>
https://buildd.debian.org/status/package.php?p=qttools-opensource-src&suite=sid

I've been regularly building llvm+clang with
-DLLVM_TARGETS_TO_BUILD='' but
a non-empty LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

On Sun, Aug 19, 2018 at 11:08 PM, Justin Bogner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> John Paul Adrian Glaubitz via llvm-dev <llvm-dev at lists.llvm.org>
writes:
> > Hi!
> >
> > In Debian, we have recently run into the situation that the package
> > qttools-opensource-src has added LLVM's clang parser as a build
> dependency
> > with the effect that the package can no longer be built for a couple
of
> > architectures like alpha or ia64 [1].
> >
> > From my current understanding, qttools-opensource-src is merely using
the
> > parser part in clang to parse C/C++ code for code analysis which is
not
> > necessarily an architecture-specific task.
>
> Building llvm without any backends is supposed to work (and has in the
> past!), but it appears to fail quite a few tests currently. You can try
> this by calling cmake with -DLLVM_TARGETS_TO_BUILD=''. I don't
know if
> clang works in that configuration, especially given David's point about
> inline assembly.
>
> > Now, does anyone know whether it would be possible to enable this
feature
> > on any given architecture without having to add a full target backend
to
> > LLVM? Maybe we could just build an LLVM variant on unsupported targets
> > with the x86 backend enabled (in case it's mandatory to have at
least
> > one backend enabled).
> >
> > Thanks,
> > Adrian
> >
> >> [1]
> > https://buildd.debian.org/status/package.php?p=qttools-
> opensource-src&suite=sid
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On 8/19/2018 7:29 AM, John Paul Adrian Glaubitz via llvm-dev
wrote:
> Hi!
>
> In Debian, we have recently run into the situation that the package
> qttools-opensource-src has added LLVM's clang parser as a build
dependency
> with the effect that the package can no longer be built for a couple of
> architectures like alpha or ia64 [1].
>
>  From my current understanding, qttools-opensource-src is merely using the
> parser part in clang to parse C/C++ code for code analysis which is not
> necessarily an architecture-specific task.
clang needs to support a target to some extent to parse C code for that 
target.  Among other things, it needs to define target-specific macros, 
it needs to know the correct sizes for a bunch of different types, and 
it needs to parse target-specific intrinsics.  See clang/lib/Basic/Targets/.

And depending on what you're doing, you might need support for parsing 
target-specific options and finding the correct include paths; this code 
is part of clang/lib/Driver/.

If there's some specific use-case for adding this support for a target 
which doesn't have a corresponding LLVM backend, we could consider it; 
the maintenance burden is relatively small.
> Now, does anyone know whether it would be possible to enable this feature
> on any given architecture without having to add a full target backend to
> LLVM? Maybe we could just build an LLVM variant on unsupported targets
> with the x86 backend enabled (in case it's mandatory to have at least
> one backend enabled).
LLVM is always a cross-compiler: you can build it any number of targets 
enabled, and it should build on any machine with a supported C++ 
compiler.  See https://llvm.org/docs/GettingStarted.html .

-Eli

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