If I had my way

[Jacob Carlborg]

On 2011-12-13 15:15, Martin Nowak wrote:
> TLS is not too difficult. We can either use bracketed sections again or
> let the compiler emit small thunks that fetch the complete TLS section for
> a executable/DSO. The important point is that fetching the TLS addresses
> must
> be done for each thread and from a local function within each DSO.
> people.redhat.com/drepper/tls.pdf
> My current approach is to register callbacks, and let the threads update
> their ranges before GC.
> Not sure, but it will probably causes eagerly allocation of the TLS blocks.

Mac OS X doesn't support TLS (it does now in 10.7) so Walter rolled his 
own implementation. I don't know how close that is to the EFL 
implementation specified in the above link.

> Some similar issues happen with the other section brackets (.deh, .minfo).
> We need local symbols as brackets not global ones as they would collide.

There is no need for brackets, they just cause problems. The correct way 
to get section data on Mac OS X is to use the "getsectbynamefromheader" 
function. You can have a look at my fork to see how its used.

https://github.com/jacob-carlborg/druntime/blob/dylib/src/rt/memory.d#L318

> I've tried several approaches.

They way I've done it is to register a callback for the 
"_dyld_register_func_for_add_image" function. The documentation 
describes the function as:

"When you call _dyld_register_func_for_add_image, the dynamic linker 
runtime calls the specified callback (func) once for each of the images 
that are currently loaded into the program. When a new image is added to 
the program, your callback is called again with the mach_header for the 
new image, and the virtual memory slide amount of the new image."

In this callback I extract module infos, exception handling tables and 
TLS data from the mach_header. This is now where the problem comes. I 
need an associative array which maps mach_headers to a struct (or 
similar) containing module infos, exception handling tables and TLS 
data. But I don't have an associative array that works this earily in 
the initialization phase of the runtime.

The next problem is with accessing TLS. When a TLS variable is accessed 
the compiler calls the ___tls_get_addr function, implemented in 
druntime. This function receives the address and then converts it to TLS 
using the TLS data bracketed in the object file. In this function I need 
to somehow access the current object file/mach_header, then use this 
mach_header to access the TLS data in the associative array mentioned above.

You can see my current implementation in my fork, everything below this 
line:

https://github.com/jacob-carlborg/druntime/blob/dylib/src/rt/memory.d#L267

As you can see I don't have an associative array so the above currently 
doesn't work.

-- 
/Jacob Carlborg