llvm dev - Apr 2012 - [LLVMdev] Adding support for explicitly specified TLS models (PR9788)

On Wed, Apr 25, 2012 at 14:04, Joerg Sonnenberger
<joerg at britannica.bec.de> wrote:
> On Wed, Apr 25, 2012 at 12:39:49PM +0100, Hans Wennborg wrote:
>> Just as it is illegal to specify thread_local for a target that
>> doesn't support it, specifying a TLS model that isn't supported
by the
>> target would be illegal.
>
> For ELF, it doesn't make sense to reject one as it can always be
> relaxed. An application program shouldn't have to worry about that, it
> is just an implementation detail.
I'd be fine with that.

(Clang should still worry about it though, so it can provide a "the
TLS model you selected is not supported by the target, it will fall
back to another one" warning.)

 - Hans

On Wed, Apr 25, 2012 at 03:17:13PM +0100, Hans Wennborg
wrote:
> On Wed, Apr 25, 2012 at 14:04, Joerg Sonnenberger
> <joerg at britannica.bec.de> wrote:
> > On Wed, Apr 25, 2012 at 12:39:49PM +0100, Hans Wennborg wrote:
> >> Just as it is illegal to specify thread_local for a target that
> >> doesn't support it, specifying a TLS model that isn't
supported by the
> >> target would be illegal.
> >
> > For ELF, it doesn't make sense to reject one as it can always be
> > relaxed. An application program shouldn't have to worry about
that, it
> > is just an implementation detail.
> 
> I'd be fine with that.
> 
> (Clang should still worry about it though, so it can provide a "the
> TLS model you selected is not supported by the target, it will fall
> back to another one" warning.)
There is no point in warning about it. Basically, anything but global
dynamic a pure performance optimisation.

Joerg

On Thu, Apr 26, 2012 at 01:10, Joerg Sonnenberger
<joerg at britannica.bec.de> wrote:
>> > For ELF, it doesn't make sense to reject one as it can always
be
>> > relaxed. An application program shouldn't have to worry about
that, it
>> > is just an implementation detail.
>>
>> I'd be fine with that.
>>
>> (Clang should still worry about it though, so it can provide a
"the
>> TLS model you selected is not supported by the target, it will fall
>> back to another one" warning.)
>
> There is no point in warning about it. Basically, anything but global
> dynamic a pure performance optimisation.
It was pointed out in #llvm that programs that mix C and assembly to
access thread-local variables would break if the TLS model isn't what
they specify, so I still think there's a case for warning about it.

 - Hans