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

[Mark Cave-Ayland]

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.

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.


ATB,

Mark.