[cfe-dev] lli: LLVM ERROR: Cannot select: X86ISD::WrapperRIP TargetGlobalTLSAddress:i64

[Tim Northover via cfe-dev]

On 6 February 2017 at 03:49, Gaetano Checinski via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> LLVM ERROR: Cannot select: 0xd012e0:
>      i64 = X86ISD::WrapperRIP TargetGlobalTLSAddress:i64<i8** @_ZSt15__once_callable> 0 [TF=10]
>
>  0xd020c0: i64 = TargetGlobalTLSAddress<i8** @_ZSt15__once_callable> 0 [TF=10]
> In function: _ZSt9call_onceIMNSt13__future_base13_State_baseV2EFvPSt8functionIFSt10unique_ptrINS0_12_Result_baseENS4_8_DeleterEEvEEPbEJPS1_S9_SA_EEvRSt9once_flagOT_DpOT0_
>
> Questions:
>
> What does it mean?

It's a missing feature/bug in the x86 backend. The specific problem is
that it seems we don't support thread-local variables with what Clang
& GCC would call "-mcmodel=large".

That's a compilation mode where the assembly is emitted that can run
from anywhere in memory. During normal compilation "small" is used,
the compiler assumes code will end up in the low 2GB of memory, and
the linker makes sure this is true.

Unfortunately when JITing, you can't usually guarantee that the OS
will give you memory in the low 2GB so the default is "large", which
is obviously less robust since it's less commonly used.

> Are there any compiler-flags that fix this problem?

Unfortunately it doesn't seem so. Using "lli -relocation-model=pic"
gets around the immediate problem but then the JIT dynamic loader
can't cope with the relocations that get generated.
> what specific features is libstdc++ using that cause this issue ?

Any thread-local storage would do it.

> How does my problem relate to Bug 21431?

Looks like exactly the same issue.

Cheers.

Tim.