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

[Tim Northover via llvm-dev]

On 8 February 2017 at 04:57, Gaetano Checinski
<gaetano.checinski at gmail.com> wrote:
> What exactly do the compiler flags`-femulated-tls` and `tls-model` do ?
> Why does tls-emulation not solve the problem ?

It requires runtime support, specifically the __emultls_get_address
function by the looks of it. That's not available on all platforms
(it's not supplied by macOS for example) and is likely to be slower
than native TLS if it is.

> Looking at the generated IR, it seems not to remove thread_local variable declarations.
> What is the reasoning behind that ?

Using "-emit-llvm" doesn't actually print out the very final form of
the LLVM IR. There are a few passes that run on the IR but are
considered part of the final "CodeGen" phase. As a rule of thumb those
are passes that are required for correctness reasons, in this case
because without the LowerEmuTLS.cpp pass affected backends couldn't
handle TLS constructs.

Unfortunately it doesn't look like lli has support for emulated TLS
either, though that would be pretty simple to add.

Cheers.

Tim.