[mips-tls] A couple of potential changes to the MIPS TLS ABI

[Dominic Sweetman]

Daniel Jacobowitz (dan at codesourcery.com) writes:

> > > Why not re-write the spec to say "unless you generate this sequence
> > > exactly like this, you'll probably prevent any future linker
> > > optimization from working" - and then leave it to the compiler
> > > toolchain. 
> > > 
> > > My opinion is that linker optimizations could be very valuable, given
> > > a cheap way of accessing a thread pointer.  But MIPS Technologies are
> > > very unlikely to do heroic work on the linker for the o32 ABI - but we
> > > do intend to at least try that out for our NUBI ABI.
> > 
> > Fine by me; if no one else responds, I'll do this.
> > 
> > Note that when I make this change, I'm also going to let the compiler
> > schedule the sequences; whoever implements the linker optimizations can
> > go back and undo that.
> 
> Unfortunately, I've realized that we can't sit on this fence.  Here's a
> somewhat contrived example...

A very devilishly cunning example, too.

> The only way to make this work is either to mandate that an
> ABI-conforming compiler can not optimize the TLS access sequences, or
> to define additional relaxation marker relocations to mark valid
> sequences.  Some platforms do one, some do the other.
> 
> My preference is to do the latter, which we can postpone until someone
> is ready to implement them.

I agree.

-- 
Dominic Sweetman
MIPS Technologies
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