PowerPC prologue analysis
Aleksandar Ristovski
aristovski@qnx.com
Tue Jul 29 15:42:00 GMT 2008
Aleksandar Ristovski wrote:
> Daniel Jacobowitz wrote:
>> On Mon, Jul 28, 2008 at 04:29:18PM -0400, Aleksandar Ristovski wrote:
>>> Thanks for the link! I briefly looked at the patch and it seems to
>>> address some of the things I am talking about (r30-r31 issue) but the
>>> comment still reads:
>>>
>>> + All gpr's from saved_gpr to gpr31 are saved (except during the
>>> + prologue). */
>>>
>>> Is that in the ABI? I would think that if it is, then the code I am
>>> looking at is not according to it (gcc issue or just me not
>>> understanding powerpc assembly?).
>>
>> Sorry, I don't know which.
>>
>
> There seems to be no such statement in the PowerPC ABI "all gpr's from
> saved_gpr to gpr31 are saved" - non-volatile registers do not need to be
> stored in this manner. For example, a function may save r29 but not r30
> and r31. However PPC prologue analysis in gdb will assume there is r31
> saved as well which will make unwind_register(r31) fail (fetch bogus
> value). (Note: I am using gcc 4.2.3).
>
> Another assumption made in gdb code is that if multiple registers are
> saved by the prologue, they will be saved in the ascending index order -
> I am not sure this is a requirement stated in the ABI either. (Is it?) I
> believe there could be cases where registers are saved in different
> order, e.g. r30, r28, r29, etc... Hopefully this doesn't happen in
> practise.
>
> I think your patch with gpr_mask covers the first case (not all
> registers saved) but the second issue (if real issue) is still not handled.
And for the reference: Daniel's patch ( http://sourceware.org/ml/gdb-patches/2007-12/msg00111.html ) does solve the problem with not all registers saved.
Why is the patch not in HEAD gdb?
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