Porting to DragonFly BSD

[Tyson Whitehead]

On November 15, 2009 07:42:46 Goetz Isenmann wrote:
> I am still not sure, but the generated code might be ok
>
> 001e5565 <__hscore_get_errno>:
>   1e5565:       55                      push   %ebp
>   1e5566:       89 e5                   mov    %esp,%ebp
>   1e5568:       65 a1 00 00 00 00       mov    %gs:0x0,%eax
>   1e556e:       8b 15 00 00 00 00       mov    0x0,%edx
>   1e5574:       8b 04 10                mov    (%eax,%edx,1),%eax
>   1e5577:       c9                      leave
>   1e5578:       c3                      ret
>
> The corresponding relocation info is
>
> 001e5570 R_386_TLS_IE      errno

Not sure about DragonFly, but under Linux that would be the TLS initial-
executable model .  It is assuming that the TLS memory has already been 
allocated and is at the location pointed to by the word at %gs:0x0.

The default when working with position-independent code is the dynamic model.  
It has to go through ___tls_get_addr in order to give the dynamic loader a 
chance to allocate the thread local storage memory if this is the first access

The GNU variant of the code for this model is

0x00 leal x at tlsgd(,%ebx,1),%eax
0x07 call ___tls_get_addr at plt

It returns the address in eax.  The two relocation are R_386_TLS_GD to load 
eax to point to the GOT entry for the desired symbol and R_386_PLT32 for the 
call ___tls_get_addr.

For more details on the four different modes (each one is a successive 
optimization of the general dynamic model for cases where the symbol is in the 
some shared object and/or the TLS memory is know to be allocated)

http://people.redhat.com/drepper/tls.pdf

(pages 19, 27, 36, and 44 give the code for the fully generic and each 
combination of the optimizations respectively).

Under gcc you can specify the model via the "-ftls-model=<model>" option or on 
a per-identifier basis with __attribute__((tls_model("<model>"))).  Besides 
changing what code is emitted, it sets the STATIC_TLS flag in the dynamic 
section to tell force allocation at thread creation if required.

Cheers!  -Tyson
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