Supporting emulated tls

[Iain Buclaw]

On 19 March 2012 15:25, Johannes Pfau <nospam at example.com> wrote:
> Am Mon, 19 Mar 2012 09:22:01 +0000
> schrieb Iain Buclaw <ibuclaw at ubuntu.com>:
>
>> On 19 March 2012 08:15, Johannes Pfau <nospam at example.com> wrote:
>> >
>> > * Our own, emulated TLS support is implemented in GCC. This means
>> > it's also used in C, which is great. Also GCC's emulated tls needs
>> >  absolutely no special features in the runtime linker, compile time
>> >  linker or language frontends. It's very portable and works with all
>> >  weird combinations of dynamic libraries, dlopen, etc.
>> >  But it has one quirk: It doesn't allocate TLS memory in a
>> > contiguous way, every tls variable is allocated using malloc. This
>> > means we can't pass a range to the GC for the tls variables. So we
>> > can't support this emutls in the GC.
>> >
>>
>> As far as my thought process goes, the only (implementable in the GDC
>> frontend) way to force contiguous layout of all TLS symbols is to pack
>> them up ourselves into a struct that is accessible via a single global
>> module-level variable.  And in the .ctor section, the module adds this
>> range to the GC.  This should be enough so it also works for shared
>> libraries too, however I'm sure there is quite a few details I am
>> missing out on here that would block this from working. :)
>>
>
> Good idea, I should have thought about that. I can't think of a
> reason why it wouldn't work and it should be quite fast as well.
>

Initial things to think about on the top of my head:

* Speed to access symbols.
* Accessing thread local symbols across modules.


> Just to clarify: 'module-level' as in D module(/object file) or as in
> one variable per shared library/application? If we can support one
> variable per shared library/application that'd be great, as we will
> then only have a few tls ranges for the gc.
>

Per module - see the code that initialises _Dmodule_ref.  We're really
just adding two extra fields to that which includes starting address
and size.


-- 
Iain Buclaw

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