TLS for Android (and iOS)

[Dan Olson]

"Joakim" <joakim at airpost.net> writes:

> Have you experimented with seeing which of that TLV stuff from OS X
> that iOS actually supports?  The iOS dyld could be pretty different.
> We don't know since they don't release the source for the iOS core
> like they do for OS X, ie is tlv_get_addr even available in the iOS
> dyld and does it execute other possible TLS relocations?  Only way to
> find out is to try it, or somehow inspect their iOS binaries. ;) Their
> source does show an ARM assembly implementation of tlv_get_address but
> it's commented out:
> http://www.opensource.apple.com/source/dyld/dyld-210.2.3/src/threadLocalHelpers.s

I did try it in an iOS app.  The function _tlv_bootstrap is unresolved
when I link in Xcode using the current iPhoneSDK.  That is why I had to
provide a stub.   Pretty sure tlv functions are not available.

> I wonder if it'd be easier to pack your own Mach-O sections rather
> than figuring out how to access all their sections and reimplementing
> their TLV functions, assuming they're not available.  You might even
> be able to do it as an llvm patch since the relevant lib/MC/ files
> where llvm packs the TLS data into Mach-O sections seem pretty
> straightforward.

I think we can use their sections and it did not take long to figure
out.  Here is what an example link map has for one of my test apps:

0x0004E22C	0x00000084	__DATA	__thread_vars
0x0004E2B0	0x0000000C	__DATA	__thread_data
0x0004E2BC	0x00000024	__DATA	__thread_bss

The _thread_vars section has a TVLDescriptors for each thread local.  It
is used for caching the pthread_get/set key and has the variable offset
into the thread local chunk of memory that can be initialized by copying
_thread_data and _thread_bss (or just zerofill it).

> I've never submitted anything to llvm, so not really based on anything
> than speculation, but I doubt they would accept such a patch, doesn't
> mean we can't use it though. ;)

Another thing, Apple might consider the tlv functions and thread local
sections a reserved API.

A long way off from submitting anything to App Store.  With the way
things change, tlv may show up in a near future sdk, then this just
becomes a bridge.
-- 
Dan