[PING]Fix for prologue processing on PowerPC

[Nikola Prica <nikola.prica@rt-rk.com>]

Can someone please take a look into this?

Thanks,

Nikola Prica

On 22.09.2017. 14:11, Nikola Prica wrote:
> After analyzing dump of ppc program, whose crash occurred after 
> watchdog_force_here () function, GDB couldn't print full back trace 
> because GDB couldn't unwind PC from the watchdog fucntion.
> 
> The problem is introduced with the following patch:
> 
> https://sourceware.org/ml/gdb-patches/2008-08/msg00245.html
> 
> In function skip_prologue(), shifted lr_reg makes below condition always 
> false because non-shifted lr_reg value is expected to be checked.
> 
>     else if (lr_reg >= 0 &&
>          /* std Rx, NUM(r1) || stdu Rx, NUM(r1) */
>          (((op & 0xffff0000) == (lr_reg | 0xf8010000)) ||
>          /* stw Rx, NUM(r1) */
>          ((op & 0xffff0000) == (lr_reg | 0x90010000)) ||
>          /* stwu Rx, NUM(r1) */
>          ((op & 0xffff0000) == (lr_reg | 0x94010000))))
> 
> Before this fix unwinding was able to work because it relied on unwind
> directives or on some of the next frames to find PC. Problem came with
> watchdog_force_here() function which didn't contain unwind directives.
> 
> I wasn't able to produce test case that would show improvements for end
> user. I suppose that changes would be visible if watchdog event was 
> called, but I don't have valid ppc board to try this. I have tried this 
> code on simple test case with few functions in back trace. The back 
> trace is printed correctly with and without this fix, but the difference 
> between those two runs is that the body of the upper condition was 
> visited with this patch. After visiting the body there was no need to 
> look for PC counter in next frames nor to use unwind directives.