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

[Mark Cave-Ayland]

On 02/05/16 18:35, Segher Boessenkool wrote:

>> It seems the problem John was seeing is related to the compile flags in
>> that the asm above don't work correctly when the binary is compiled with
>> -Os. For comparison here is what I see with the different optimisation
>> settings:
>>
>> -O0 (works):
>>
>>    0xfff08350 <+68>:    mflr    r9
>>    0xfff08354 <+72>:    stw     r9,8(r31)
>>    0xfff08358 <+76>:    bl      0xfff08060 <__switch_context>
>>    0xfff0835c <+80>:    lwz     r9,8(r31)
>>    0xfff08360 <+84>:    mtlr    r9
>>
>> -Os (broken):
>>
>>    0xfff082a8 <+32>:    mflr    r9
>>    0xfff082ac <+36>:    bl      0xfff08060 <__switch_context>
>>    0xfff082b0 <+40>:    mtlr    r9
>>
>> So the optimiser has decided to remove the storage to/from the lr stack
>> variable which of course means that the lr value isn't being preserved.
>> Do I somehow need to mark the "r" (lr) as clobbered or something else?
> 
> Oh, yeah, that's another problem: a call can clobber all volatile
> (caller-save) registers.  The best solution is to write this whole
> thing in actual assembler, instead of fighting the compiler to get
> just the assembler you want ;-)

Heh actually the bug was a missing volatile qualifier from the __context
pointer which is interesting since this code is used almost boilerplate
for other architectures - I'll spin up a patch to update the other archs
once this has gone in. The v2 version of the patchset contains this
correction though.


ATB,

Mark.