I discovered why one of the Fiber unittests in core.thread would crash on iOS. TLS address is looked up based on the thread. In LDC optimized code, the address lookup may only happen once in a function if the address can be cached in a register. This is good, because it makes multiple TLS access faster in a single function.

Now, Fibers are permitted to be run on different threads, as long as only one thread has it active at a time. The problem is that the Fiber context may be on a different thread before and after a yeild(). If a TLS address is cached, the previous thread's TLS is accessed after the yield call instead of the current threads TLS.

To make the one Fiber unittest work on multicore, I am switch to using pthread_getspecific intead of TLS for sm_this. But in general this is a problem with other architectures besides arm. I checked assembly of x86_64 and TLS addresses are cached too when optimization is on.

I also looked at DMD asm, and it does not seem to cache TLS addresses on OS X at least.
--
Dan

On Saturday, 28 February 2015 at 18:10:13 UTC, Dan Olson wrote:
> I discovered why one of the Fiber unittests in core.thread would crash
> on iOS. TLS address is looked up based on the thread. In LDC optimized
> code, the address lookup may only happen once in a function if the
> address can be cached in a register. This is good, because it makes
> multiple TLS access faster in a single function.
>
> Now, Fibers are permitted to be run on different threads, as long as
> only one thread has it active at a time. The problem is that the Fiber
> context may be on a different thread before and after a yeild(). If a
> TLS address is cached, the previous thread's TLS is accessed after the
> yield call instead of the current threads TLS.
>
> To make the one Fiber unittest work on multicore, I am switch to using
> pthread_getspecific intead of TLS for sm_this. But in general this is a
> problem with other architectures besides arm. I checked assembly of
> x86_64 and TLS addresses are cached too when optimization is on.
>
> I also looked at DMD asm, and it does not seem to cache TLS addresses on
> OS X at least.
> --
> Dan

Hi Dan,

this is issue 666: https://github.com/ldc-developers/ldc/issues/666

Regards,
Kai

"Kai Nacke" <kai@redstar.de> writes:
> this is issue 666: https://github.com/ldc-developers/ldc/issues/666

Thanks Kai.  I could not imagine I was the first to stumble into this.