On Wednesday, 9 January 2013 at 07:57:12 UTC, Jacob Carlborg
wrote:
On 2013-01-07 09:04, Walter Bright wrote:
Please nail down what is necessary first. (BTW, I don't know
how the
compiler can tell what image an address comes from. Remember,
shared
libraries are loaded at runtime, not compile time.)
I've done some investigation. Currently DMD inserts a call to
the __tls_get_addr function when accessing a TLS variable. This
is implemented in druntime.
In Mac OS X 10.7 it works similar but instead of inserting a
call to __tls_get_addr there's a struct looking like this
(written in D) :
struct TLVDescriptor
{
void* function (TLVDescriptor*) thunk;
size_t key;
size_t offset;
}
The dynamic linker will iterate all loaded images and extract
the section containing the TLS data. I guess this section
consists of a list of TLVDescriptor*. The dynamic linker will
set the "thunk" to a function "tlv_get_addrs", implemented in
assembly in the dynamic linker. It will set the key to a key
created by "pthread_key_create". It will also map the image
with this key. This key is same for all TLVDescriptor of a
given image.
Instead of calling "__tls_get_addr" I think that the compiler
will need to call the thunk passing in the TLVDescriptor itself
to the thunk.
The "tlv_get_addrs" function will then extract the key and from
that key it can get the image the address belongs to.
Does that make any sens? Is that something the DMD could do,
that is call the thunk instead of "__tls_get_addr".?
Isn't it horrible performancewise ?
