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

Tue Jun 27 03:47:24 PDT 2017

> 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 <mailto: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? <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 <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 <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 <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 <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 <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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