Re: boost md context switching on macppc

On Tue, Aug 27, 2019 at 01:58:22PM -0400, George Koehler wrote:

> On Tue, 27 Aug 2019 14:30:50 +0200
> Otto Moerbeek <otto@drijf.net> wrote:
>
> > What I'm seeing is (in jump_ppc32_sysv_elf_gas.S/jump_context)
> > register r3 that is supposed to point to the memory that will receive
> > the transfer_t (the return value of jump_fcontext()) is 0.
> >
> > # return transfer_t
> > stw %r6, 0(%r3)
> > stw %r5, 4(%r3)
>
> Ouch. This code is for Linux; there is a secret incompatibility
> between BSD and Linux on 32-bit PowerPC. These systems have almost
> the same System V ABI, but differ when returning a small struct of up
> to 8 bytes. transfer_t from <boost/context/detail/fcontext.hpp> is
> such a struct: its 2 pointers measure 8 bytes. BSD has no return area
> in %r3; the callee should return the transfer_t in %r3 and %r4. The
> params were in %r4 and %r5 in Linux, but are in %r3 and %r4 in BSD.
>
> The fixes might be to
>
> - delete the lines `stw %r3, 228(%r1)` and `lwz %r3, 228(%r1)`,
> because the return area no longer exists.
> - change `mr %r1, %r4` to `mr %r1, %r3`, because first param is %r3.
> - change `stw %r6, 0(%r3)` to `mr %r3, %r6` (move to %r3 from %r6),
> because first word of transfer_t is %r3.
> - delete `stw %r5, 4(%r3)`, because second param is %r4, and second
> word of transfer_t is %r4, so we would move %r4 to itself.
>
> One might guard the changes with #ifdef __Linux__ ... #else ...