[Valgrind-developers] TLS support

[Jeremy F. <je...@go...>]

OK, I've had even more of a look into what's happening here.  The quick
answer is that is can probably be done without too much nastiness,
though our libpthread will need to know some of glibc's secret
knowledge.

The key thing is that %gs points to a thing called tcbhead_t, which
contains pointers to the rest of the thread-specific data structures. 
pthread_create doesn't really do much with this except init it;
___tls_get_addr does all the hard work of on-demand copying, which we
don't care about.

Aside from the shape of tcbhead_t (which is simple), libpthread doesn't
need to know much about how it works - it ends up calling _dl_* internal
functions to manipulate it, and to find out other info it needs to know
(like how big the static per-thread info is, which it needs to build
into the thread stack).

Our libpthread can call those same functions, and thereby do the same
thing.

For reference, they appear to be:
_dl_get_tls_static_info(size_t *size, size_t *alignment)
        Get the size and alignment of the per-thread variable space we
        need to stick on the stack under tcbhead_t.

_dl_allocate_tls_init(void *mem)
        (Re)Initialize the TLS info at mem.  mem points to the
        tcbhead_t.

_dl_allocate_tls(void *mem)
	Do the initial initialization of the TCB (ie, allocate the DTV).

_dl_deallocate_tls(void *mem, bool free_tcb)
	Deallocate memory.

tcbhead_t is:
        typedef struct
        {
          void *tcb;		/* Pointer to the TCB.  Not necessary the
        			   thread descriptor used by libpthread.  */
          dtv_t *dtv;
          void *self;		/* Pointer to the thread descriptor.  */
          int multiple_threads;
          uintptr_t sysinfo;
        } tcbhead_t;
We don't care about dtv_t, since the dynamic linker will do all the work
to maintain it.

The other part is implementing the TLS "kernel" support, which is easy. 
So in all, this only looks like a few day's work (maybe less).

	J