Re: [PATCH v2 1/4] powerpc64: Use medium model toc accesses throughout

[Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>]

Alan Modra via Libc-alpha <libc-alpha@sourceware.org> writes:

> The PowerPC64 linker edits medium model toc-indirect code to toc-pointer
> relative:
> 	addis r9,r2,tc_entry_for_var@toc@ha
> 	ld r9,tc_entry_for_var@toc@l(r9)
> becomes
> 	addis r9,r2,(var-.TOC.)@ha
> 	addi r9,r9,(var-.TOC.)@l
> when "var" is known to be local to the binary.  This isn't done for
> small-model toc-indirect code, because "var" is almost guaranteed to
> be too far away from .TOC. for a 16-bit signed offset.  And, because
> the analysis of which .toc entry can be removed becomes much more
> complicated in objects that mix code models, they aren't removed if
> any small-model toc sequence appears in an object file.
>
> Unfortunately, glibc's build of ld.so smashes the needed objects
> together in a ld -r linking stage.  This means the GOT/TOC is left
> with a whole lot of relative relocations which is untidy, but in
> itself is not a serious problem.  However, static-pie on powerpc64
> bombs due to a segfault caused by one of the small-model accesses
> before _dl_relocate_static_pie.  (The very first one in rcrt1.o
> passing start_addresses in r8 to __libc_start_main.)
>
> So this patch makes all the toc/got accesses in assembly medium code
> model, and a couple of functions hidden.  By itself this is not
> enough to give us working static-pie, but it is useful in isolation to
> enable better linker optimisation.
>
> There's a serious problem in libgcc too.  libgcc ifuncs access the
> AT_HWCAP words stored in the tcb with an offset from the thread
> pointer (r13), but r13 isn't set at the time _dl_relocate_static_pie.
> A followup patch will fix that.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>

-- 
Tulio Magno