Re: a question about rs6k's prologue analysis

[Ramana Radhakrishnan <ramana.radhakrishnan@codito.com>]

I am guessing a bit but you could use it for "milli" calls to functions
to save callee save registers ?  Was this done on the PowerPC at any
time ? 

If it is in the first 3 instructions then maybe its a milli-call and you
might have other instructions in the prologue after that. 

My 10 paise. 

cheers
-Ramana


On Wed, 2006-10-11 at 18:12 +0800, Wu Zhou wrote:
> In function skip_prologue of rs6000-tdep.c, when it finds an instruction like "bl foo", and that it 
> is not within the first three instructions of the prologue, the prologue analysis will stop.
> 
>        else if ((op & 0xfc000001) == 0x48000001)
>          {                       /* bl foo,
>                                     to save fprs??? */
> 
>            fdata->frameless = 0;
>            /* Don't skip over the subroutine call if it is not within
>               the first three instructions of the prologue.  */
>            if ((pc - fdata->func_start) > 8)
>              break;
>            ....
>           }
> 
> Any specific reason for this?
> 
> I encountered some prologue code like this:
> 
>      61a0:       7c 08 02 a6     mflr    r0
>      61a4:       94 21 ff f0     stwu    r1,-16(r1)
>      61a8:       93 e1 00 0c     stw     r31,12(r1)
>      61ac:       48 01 8a 81     bl      1ec2c <__JCR_END__+0x4>
>      61b0:       7f e8 02 a6     mflr    r31
>      61b4:       90 01 00 14     stw     r0,20(r1)
> 
> In the first instruction, lr is saved in r0, its content is saved on stack at the last instruction. 
>   But between these two instruction, a "bl foo" instruction exists, which make skip_prologue return. 
> In this scenario, gdb won't have any way to know where lr is saved on the stack. This might make the 
> backtrace loops at this point.
> 
> Could it be possible that we delete this conditional statement?  I guess this will only make 
> prologue analysis get a little longer.  Any other impact?
> 
> Or that some changes in the compiler's side is needed?  But now that these code are already there, 
> and it also seems that they work ok, I guess it is not very easy for compiler to change that.
> 
> Any insight?  Thanks a lot!
> 
> Regards
> - Wu Zhou
-- 
Ramana Radhakrishnan <ramana.radhakrishnan@codito.com>
Codito Technologies Pvt. Ltd.