On Fri, Sep 02, 2022 at 10:57:27AM -0500, Peter Bergner wrote:

On 8/31/22 5:45 PM, Segher Boessenkool wrote:

quoted

Yes, this is guaranteed.

Agree with Segher here.  That said, there was a gcc bug a looooong time
ago where gcc copied r13 into a temporary register and used it from there.

r13 is a fixed register on most of our ABIs (everything that is not AIX
or Darwin, even), so this can never happen.  Except if there are bugs,
of course ;-)

That's ok (correctness wise, but not ideal) from user land standpoint,
but we took a context switch after the reg copy and it was restarted on
a different cpu, so differnt local_paca and r13 value.  We went boom
because the copy wasn't pointing to the correct local_paca anymore.
So it is very important the compiler always use r13 when accessing
the local_paca.

Yes.  So we either whould use -ffixed-r13, or just not use unsupported
compilers.  powerpc*-linux and powerpc*-elf work fine for example :-)


Segher