Supporting emulated tls

[Alex Rønne Petersen]

On 18-03-2012 12:32, Johannes Pfau wrote:
> I thought about supporting emulated tls a little. The GCC emutls.c
> implementation currently can't work with the gc, as every TLS variable
> is allocated individually and therefore we don't have a contiguous
> memory region for the gc. I think these are the possible solutions:
>
> * Try to fix GCCs emutls to allocate all tls memory for a module
>    (application/shared object) at once. That's the best solution
>    and native TLS works this way, but I'm not sure if we can extract
>    enough information from the runtime linker to make this work (we
>    need at least the combined size of all tls variables).
>
> * Provide a callback in GCC's emutls which is called after every
>    allocation. This could call GC.addRange for every variable, but I
>    guess adding huge amounts of ranges is slow.

We should avoid this if possible, yes. A small root set is desirable.

>
> * Make it possible to register a custom allocator for GCC's emutls (not
>    sure if possible, as this would have to be set up very early in
>    application startup). Then allocate the memory directly from the GC
>    (but this memory should only be scanned, not collected)

Such an allocator would probably just allocate a decently-sized memory 
block from libc and add it as a root range (rather than individual 
word-sized roots). The memory doesn't necessarily have to be allocated 
with the GC.

>
> * Replace the calls to mallloc in emutls.c with a custom, region based
>    memory allocator. (This is not a perfect solution though, it can
>    always happen that we'll need more memory)
>
>
>
> * Do not use GCC's emutls at all, roll a custom solution. This could be
>    compatible with / based on dmd's tls emulation for OSX. Most of the
>    implementation is in core.thread, all that's necessary is to group
>    the tls data into a _tls_data_array and call ___tls_get_addr for
>    every tls access. I'm not sure if this can be done in the
>    'middle-end' though and it doesn't support shared libraries yet.
>


-- 
- Alex