Supporting emulated tls

[Rainer Schuetze]

On 3/18/2012 12:32 PM, 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.
>
> * 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)
>
> * 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)

Check the implementation of ranges in gcx.d: it's rather fast to add a 
range (vector like appending to exponentially growing data), and a 
simple loop over the ranges is done in the collection that would not 
change performance a lot when being executed in one memory chunk: it's 
the marking of references in the scanned data that is expensive.

I would be more concerned about removal of ranges, though. It scans 
existing ranges linearly to find the one to remove and moves the 
remaining entries in memory. Some optimizations might be helpful here.

>
>
>
> * 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.
>