llvm dev - Mar 2016 - Building a program with LLVM on Unix

Building LLVM itself involves Cmake, but what's the best way to build a C++
program that needs to link with the LLVM libraries?

If you're trying to optimise for making life as easy as possible for users
and people creating binary packages, in the normal course of events,
autotools is recommended. But the tutorial mentions running llvm-config to
get things like library paths - does autotools know how to do this?

(I mostly use Windows, not as familiar with Unix, so please let me know if
the question I'm asking is not quite the same as the question I should be
asking.)
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There are several m4 macros that let you write autoconf detection for
llvm.  I recently needed to implement one.  Have a look here:

https://github.com/google/autofdo/blob/master/m4/ax_llvm.m4


Diego.

On Wed, Mar 23, 2016 at 12:48 PM, Russell Wallace via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Building LLVM itself involves Cmake, but what's the best way to build a
C++
> program that needs to link with the LLVM libraries?
>
> If you're trying to optimise for making life as easy as possible for
users
> and people creating binary packages, in the normal course of events,
> autotools is recommended. But the tutorial mentions running llvm-config to
> get things like library paths - does autotools know how to do this?
>
> (I mostly use Windows, not as familiar with Unix, so please let me know if
> the question I'm asking is not quite the same as the question I should
be
> asking.)
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

On 23 Mar 2016, at 16:48, Russell Wallace via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Building LLVM itself involves Cmake, but what's the best way to build a
C++ program that needs to link with the LLVM libraries?
> 
> If you're trying to optimise for making life as easy as possible for
users and people creating binary packages, in the normal course of events,
autotools is recommended. But the tutorial mentions running llvm-config to get
things like library paths - does autotools know how to do this?
> 
> (I mostly use Windows, not as familiar with Unix, so please let me know if
the question I'm asking is not quite the same as the question I should be
asking.)
I think that you’re answering your own question, almost.  There are basically
two approaches:

If you’re using CMake, then you can use the CMake LLVM modules to find the
things that you need to link to.  I’ve found this to be fairly fragile.

If you’re using any build system, then you can use llvm-config to find the LLVM
version, CXXFLAGS, and LDFLAGS that are needed for your particular use.  This is
quite easy to do from any build system (from simple Makefiles to complex build
system generators such as auto* and CMake).

David

Given that the LLVM API changes constantly, any reasonable C++ program may
very well compile properly against only one specific version/release of
LLVM. The probability that you can properly build against whatever some
user has installed on some arbitrary system approaches zero.

If you accept this, then you likely work around it by configuring and
installing an appropriate version of LLVM and its libraries yourself. At
that point, you may just as well hard-code the path to what you've
installed into your build system. Perhaps hard-code only the path to
llvm-config, and bootstrap the rest from that.


On Wed, Mar 23, 2016 at 12:48 PM, Russell Wallace via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Building LLVM itself involves Cmake, but what's the best way to build a
> C++ program that needs to link with the LLVM libraries?
>
> If you're trying to optimise for making life as easy as possible for
users
> and people creating binary packages, in the normal course of events,
> autotools is recommended. But the tutorial mentions running llvm-config to
> get things like library paths - does autotools know how to do this?
>
> (I mostly use Windows, not as familiar with Unix, so please let me know if
> the question I'm asking is not quite the same as the question I should
be
> asking.)
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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When you say 'configuring and installing an appropriate version of LLVM and
its libraries yourself' - that is indeed what I did on my own machine
(Ubuntu 14.04 apt-get thinks 3.4 is latest, so I'm currently building 3.8
from source) - but how would you recommend doing this on a user's machine?

On Wed, Mar 23, 2016 at 5:17 PM, David Jones <djones at xtreme-eda.com>
wrote:
> Given that the LLVM API changes constantly, any reasonable C++ program may
> very well compile properly against only one specific version/release of
> LLVM. The probability that you can properly build against whatever some
> user has installed on some arbitrary system approaches zero.
>
> If you accept this, then you likely work around it by configuring and
> installing an appropriate version of LLVM and its libraries yourself. At
> that point, you may just as well hard-code the path to what you've
> installed into your build system. Perhaps hard-code only the path to
> llvm-config, and bootstrap the rest from that.
>
>
> On Wed, Mar 23, 2016 at 12:48 PM, Russell Wallace via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Building LLVM itself involves Cmake, but what's the best way to
build a
>> C++ program that needs to link with the LLVM libraries?
>>
>> If you're trying to optimise for making life as easy as possible
for
>> users and people creating binary packages, in the normal course of
events,
>> autotools is recommended. But the tutorial mentions running llvm-config
to
>> get things like library paths - does autotools know how to do this?
>>
>> (I mostly use Windows, not as familiar with Unix, so please let me know
>> if the question I'm asking is not quite the same as the question I
should
>> be asking.)
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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Thanks!

Bearing in mind that I don't know m4, looking at the code, it seems to work
by allowing an override to specify where to find llvm-config (which is the
right thing to do, of course) but by default looking in the current path.

Based on both my own limited experience in the matter and what David Jones
and David Chisnall are saying, one could argue that the path version
probably will _not_ be the correct one. That the correct default is
actually more likely to be e.g. ~/llvm-3.8. Does that make sense?

On Wed, Mar 23, 2016 at 4:51 PM, Diego Novillo <dnovillo at google.com>
wrote:
> There are several m4 macros that let you write autoconf detection for
> llvm.  I recently needed to implement one.  Have a look here:
>
> https://github.com/google/autofdo/blob/master/m4/ax_llvm.m4
>
>
> Diego.
>
> On Wed, Mar 23, 2016 at 12:48 PM, Russell Wallace via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Building LLVM itself involves Cmake, but what's the best way to
build a
> C++
> > program that needs to link with the LLVM libraries?
> >
> > If you're trying to optimise for making life as easy as possible
for
> users
> > and people creating binary packages, in the normal course of events,
> > autotools is recommended. But the tutorial mentions running
llvm-config
> to
> > get things like library paths - does autotools know how to do this?
> >
> > (I mostly use Windows, not as familiar with Unix, so please let me
know
> if
> > the question I'm asking is not quite the same as the question I
should be
> > asking.)
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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