Re: [mips-tls] Revised versions of MIPS TLS ABI specification

[Nigel Stephens <nigel@xxxxxxxx>]

Daniel Jacobowitz wrote:

> > The following issue does need resolution:
> >
> > 1. Should we extend the 32K Local Dynamic Model to 64K by using a biased 
> > offset?
> > Daniel and I thought this would be more trouble than it's worth; Thiemo 
> > thinks otherwise.  Are there any other opinions?
> >    
> I have since learned that PowerPC already does this:
>
> | The PowerPC32 TLS ABI is similar to the PowerPC64 model.  The
> | thread-local storage data structures follow variant I.  The TCB is 8
> | bytes, with the first 4 bytes containing the pointer to the dynamic
> | thread vector. tlsoffset calculations and definition of __tls_get_addr
> | are identical to PowerPC64.  r2 is the thread pointer, and points
> | 0x7000 past the end of the thread control block.  Dynamic thread vector
> | pointers point 0x8000 past the start of each TLS block.  (*) This
> | allows the first 64K of each block to be addressed from a dtv pointer
> | using fewer machine instructions.  The tp offset allows for efficient
> | addressing of the TCB and up to 4K-8 of other thread library
> | information.
> | 
> | (*) For implementation reasons the actual value stored in dtv may point
> | to the start of a block, however values returned by accessor functions
> | will be offset by 0x8000.
>  
Hi Daniel

I'd be interested to know where that quoted passage comes from - is it a 
public document, I can't find it with Google?
> Shall we use this model? 

It does sounds like we could use the same trick. But I'd like to 
understand it better. I thought that the problem was that in variant I 
the size of the TCB was indeterminate, so the static linker couldn't 
insert optimised references to initial-exec TLS data as fixed offsets 
from the thread pointer. But the implication of the above is that the 
TCB does have a known, fixed size, so the a fixed offset can now  be 
inserted by the linker, as for variant II. Have I got that right?
Nigel