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Re: Questions about powerpc __tls_get_addr optimization
On Fri, Oct 12, 2018 at 10:20:42PM +1030, Alan Modra wrote:
> On Thu, Oct 11, 2018 at 10:00:46PM -0400, Rich Felker wrote:
> > On Fri, Oct 12, 2018 at 11:38:14AM +1030, Alan Modra wrote:
> > > On Thu, Oct 11, 2018 at 05:26:54PM -0400, Rich Felker wrote:
> > > > My concern with doing this would be that there's no relocation on the
> > > > second (offset) slot when the local-dynamic model is used, and I
> > > > thought ld would be doing something special to account for this when
> > > > the optimization is used, but apparently your code in the glibc
> > > > dynamic linker just ignores this and fills in both slots when
> > > > processing the R_PPC64_DTPMOD64 relocation.
> > >
> > > Right, the slots are filled in for local dynamic at that point.
> > Do you think it's safe to overwrite these in the dynamic linker?
> The implementation we have currently for powerpc is safe, because the
> reloc processing is all done at startup. If you're talking about
> modifying the tls_index pair at run time by __tls_get_addr, then that
> should be safe too. You'd be modifying thread-local data.
Oops, wires crossed. tls_index pairs are typically in GOT which isn't
thread-local. So you'd need to ensure the update of the two words is
Australia Development Lab, IBM