[OpenBIOS] [PATCH 2/4] ppc: add new context handler

[Segher Boessenkool]

On Wed, May 04, 2016 at 07:30:03PM +0100, Mark Cave-Ayland wrote:
> On 04/05/16 16:31, Segher Boessenkool wrote:
> 
> >> Can you explain further? The v2 patch uses a single register marked as
> >> clobber for mflr/mtlr and the volatile modifier for __context ensures
> >> that the old value being written to the stack doesn't get optimised away
> >> by the compiler. What else am I missing?
> > 
> > A normal (ABI-following) function can clobber all of r9..r12, so that
> > "bl" instruction can, too.  If the function it calls is special (so at
> > least written as assembler), things are different of course.

[ r3..r12, I can't type ]

> Right, we're talking about this section of code in the caller:
> 
>     asm __volatile__ ("mflr %%r9\n\t"
>                       "stw %%r9, %0\n\t"
>                       "bl __switch_context\n\t"
>                       "lwz %%r9, %0\n\t"
>                       "mtlr %%r9\n\t" : "=m" (lr) : "m" (lr) : "%r9" );
> 
> 
> The mflr/mtlr are being used to preserve the original link register
> before branching to the assembler routine at __switch_context which does
> all the hard work.

And the bl clobbers at least r4,r5.

Just write this in actual assembler...


Segher