Re: [PATCH] powerpc: Define USE_PPC64_NOTOC iff compiler and linker also supports it

["Fāng-ruì Sòng" <maskray@google.com>]

On Tue, Nov 16, 2021 at 12:49 PM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> * Tulio Magno Quites Machado Filho via Libc-alpha:
>
> > Tulio Magno Quites Machado Filho via Libc-alpha <libc-alpha@sourceware.org> writes:
> >
> >> Alan Modra via Libc-alpha <libc-alpha@sourceware.org> writes:
> >>
> >>> I have a question about powerpc64/ppc-mcount.S.  Why is the assembly
> >>> wrapper using ENTRY and a NOTOC call?  By using ENTRY you are saying
> >>> that calls to _mcount must have r2 valid.  Given that r2 is valid, why
> >>> then use NOTOC?
> >>
> >> I wouldn't be surprised if that was a misinterpretation of the ABI on my part.
> >>
> >> Looking at table 2.20 "Protocols for Local Function Calls", I think this is a
> >> scenario that falls to the last row of this table, where we have:
> >
> > OK, I should be looking at table 2.19 because this is not a call within the
> > same compilation unit.
>
> I'm not entirely sure about that.  I think “local” in this context means
> “the caller has complete information about the callee”, and this is true
> for most direct calls within libc.so.6.  It's also necessary to make
> LTO-based calling convention optimizations effective.
>
> Thanks,
> Florian
>

I think "external" in the context means the symbol may be preemptible
at link time (and therefore at run time).
"local" in the context means it must be non-preemptible.

STV_DEFAULT STB_GLOBAL (-shared or non-definition) symbols may be
preemptible (the compiler needs to be conservative and assumes that
the linker option --version-script, --dynamic-list,
-Bsymbolic-functions may not be specified) and use the external call
protocol.