[llvm-dev] Building llvm with clang and lld on arm and the llvm arm backend relocation on position independent code

[Peter Smith via llvm-dev]

Hello Alessandro,

Despite the statement in the HowToCrossCompileLLVM guide "If you’re
using Clang as the cross-compiler, there is a problem in the LLVM ARM
back-end that is producing absolute relocations on
position-independent code (R_ARM_THM_MOVW_ABS_NC), so for now, you
should disable PIC:" I can't find an existing upstream PR or any
record that this has been fixed. If the ARM backend is still producing
an R_ARM_THM_MOVW_ABS_NC relocation when -fpic is given as an option
then it would be great to get a small reproducible example in a new
PR, or if there is one already to update it.

The last time I ran into this error when trying to cross compile was
that a dependency not compiled by clang (libtinfo.a) library contained
an R_ARM_THM_MOVW_ABS_NC relocation. I don't think that this is the
fault of clang/llvm though, I think it was my fault in installing a
non pic compiled version of libtinfo.a (I'm still trying to learn how
Ubuntu multilibs work).

Would it be possible for you to share the details of which objects
contain the R_ARM_THM_MOVW_ABS_NC relocations when compiled -fpic?

Peter

On 27 June 2017 at 11:47, Alessandro Pistocchi via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> On 26 Jun 2017, at 16:25, Rui Ueyama <ruiu at google.com> wrote:
>
> On Sun, Jun 25, 2017 at 6:40 AM, Alessandro Pistocchi via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>>
>> Hi,
>>
>> I am trying to build a completely GNU free linux toolchain for the
>> raspberry pi.
>>
>> I successfully managed to compile llvm and clang for armv7 hard float (
>> both as a cross compiler and as a native compiler ) together with the
>> following:
>>
>> Llvm with clang and lld
>> Clang builtins
>> Musl libc
>> libc++, libc++abi, libunwind
>>
>> All works well with the only thing to notice being the need to use -fPIC
>> in order to access some library functions when my own c/c++ programs access
>> those functions.
>>
>> The fact seems to be that musl libc exports some symbols as protected (
>> probably correctly ) and lld ( probably correctly ) says it cannot preempt
>> those symbols.
>> For this reason I seem to have to use -fPIC in the C and CXX flags but
>> everything seems to work ok.
>
>
> Is this the same problem mentioned in
> https://bugs.llvm.org/show_bug.cgi?id=32425?
>
>
> My aim is to build llvm+clang+lld using llvm+clang+lld on arm ( using musl,
> libc++, libc++abi, libunwind and the clang builtins, no gcc runtime at all
> ).
>
> I think the fact that lld cannot preempt some symbols without using -fPIC is
> similar to the problem mentioned in
> https://bugs.llvm.org/show_bug.cgi?id=32425 .
>
> However I am not particularly bothered at this stage about having to use
> -fPIC. That would be fine.
>
> What I am struggling with is that having to use -fPIC is in conflict with
> the arm backend issue that creates bad relocations when building clang with
> -fPIC as mentioned in http://llvm.org/docs/HowToCrossCompileLLVM.html and so
> the build with -fPIC fails when building the clang binary.
>
> Regarding https://bugs.llvm.org/show_bug.cgi?id=32425 , please notice that
> contrary to what is reported there I definitely can build and execute a C
> hello world program without having to use -fPIC.
>
> It is only some symbols from musl that cannot be preempted by lld, not all
> of them.
>
> Unfortunately, when building llvm+clang+lld quite a few of those symbols are
> explicitly looked for by cmake and not found unless I use -fPIC.
>
> If I go ahead and build without using -fPIC then the build fails because it
> cannot preempt those symbols.
>
> It looks like a paradox to me and I think the solution would be to fix the
> fact that the arm backend does not like -fPIC as mentioned in
> http://llvm.org/docs/HowToCrossCompileLLVM.html . While it is probably
> correct that lld says it cannot preempt protected symbols, the arm backend
> issue is a known issue.
>
>
>>
>> Then I tried to use this compiler ( both the cross compiler and the native
>> compiler ) to compile llvm + clang + lld ( I want to have the toolchain
>> built with itself, again without any GNU software involved ) but when
>> building the clang executable I ran into the arm relocation problems
>> mentioned here in the “Hacks” section when using -fPIC:
>> http://llvm.org/docs/HowToCrossCompileLLVM.html .
>> On the other hand, I seem to need -fPIC otherwise cmake fails to find some
>> libc functions such as futimes/futimens and many others. If I use -fPIC for
>> CFLAGS but not for CXXFLAGS then cmake finds those symbols but then
>> obviously fails at a later stage with lld unable to preempt those symbols.
>>
>> This seems to be a conflict I cannot solve without someone within the llvm
>> team fixing the arm relocation problem. Is there any estimate of when this
>> could happen? I am happy to spend the time testing any solutions by building
>> my own toolchain until it succeeds. Or is there any other solution?
>>
>> Thank you,
>> Alex
>>
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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>
>
>
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