Hello Nikola, Kevin, Thank you for providing these patches. I tested both patches (Nicola's second patch and Kevin's patch) on ppc64le, and there were no regressions, except for some additional expected failures in gdb.threads/attach-many-short-lived-threads.exp. Some comments: - Nikola's patch moves "if(lr_reg == 0)/r0_contains_arg = 0;" to within the first if. This is useful for the case when a second mflr Rx with Rx != 0 is detected. Previously this would cause r0_contains_arg to be reset, despite Rx not being 0. However, if the second mflr has Rx == 0, it would make sense to reset r0_contains_arg (and this would work accidentally before the patch, assuming the first mflr also had Rx == 0). Maybe the best solution here is to always check the Rx contained in the opcode and clear r0_contains_arg if it is 0, regardless of lr_reg, or leave it as before since it's a separate issue. - Kevin's patch assigns lr_reg = op & 0x03e00000, but lr_reg is an int, and op is an unsigned long. Will the unshifted reg always fit in a int? - I think it's more clear to only set lr_register when needed (pc reaches the limit), as opposed to resetting it to -1 if pc didn't reach the limit. - I wasn't able to directly apply Nikola's patch, I did so manually. I've also attached a reproducer (prologue.c/foo.S) to check if the patches fixed the issue. I had to alter a generated assembly file so that mflr would use a register other than R0 (in which case the old code does work). gcc -g0 -O0 -o prologue prologue.c foo.S (gdb) file prologue Reading symbols from prologue...done. (gdb) break bar Breakpoint 1 at 0x100005b8 (gdb) run Starting program: /home/pedromfc/prologue Before the patch: Breakpoint 1, 0x00000000100005b8 in bar () (gdb) bt #0  0x00000000100005b8 in bar () #1  0x0000000010000644 in foo () Backtrace stopped: frame did not save the PC After (both patches): Breakpoint 1, 0x00000000100005b8 in bar () (gdb) bt #0  0x00000000100005b8 in bar () #1  0x0000000010000644 in foo () #2  0x00000000100005f8 in main () (gdb) info f 1 Stack frame at 0x7fffffffeee0:  pc = 0x10000644 in foo; saved pc = 0x100005f8 Thanks! Pedro On 11/09/2017 04:15 PM, Nikola Prica wrote: > Hi Kevin, > > lr_reg could be also set to -2 in part of code which is reachable > after shifting removal. > >       /* Invalidate lr_reg, but don't set it to -1. >          That would mean that it had never been set.  */ >       lr_reg = -2; > > This part of the code which depends of non shifted lr_reg, and the > part where shifting is removed are only two places where lr_reg is > changed. As so, I've added last condition to set fdata->lr_register on > -1 if lim_pc is not reached. > > If it seems fine now could you pleas commit it because I don't have > rights to do it. > > Thanks, > > Nikola Prica > > > From: Prica > Date: Thu, 9 Nov 2017 13:10:48 +0100 > Subject: Fix for prologue processing on PowerPC > > One of conditions in skip_prologue() is never visited because it > expects non shifted `lr_reg`.  That condition is supposed to set PC > offset.  When body of this condition is visited PC offset is set and > there will be no need to look for it in next frames nor to use frame > unwind directives. > > gdb/ChangeLog: > >     *rs6000-tdep.c (skip_prologue): Remove shifting for lr_reg >       and assign shifted lr_reg to fdata->lr_register when lr_reg is >       set. If iteration do not hit lim_pc lr_register is set as -1. > --- >  gdb/rs6000-tdep.c | 13 ++++++++----- >  1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/gdb/rs6000-tdep.c b/gdb/rs6000-tdep.c > index 6c44995..6f05ef5 100644 > --- a/gdb/rs6000-tdep.c > +++ b/gdb/rs6000-tdep.c > @@ -1655,9 +1655,12 @@ skip_prologue (struct gdbarch *gdbarch, > CORE_ADDR pc, CORE_ADDR lim_pc, >           remember just the first one, but skip over additional >           ones.  */ >        if (lr_reg == -1) > -        lr_reg = (op & 0x03e00000) >> 21; > -          if (lr_reg == 0) > -            r0_contains_arg = 0; > +      { > +        lr_reg = (op & 0x03e00000); > +        fdata->lr_register = lr_reg >> 21; > +        if (lr_reg == 0) > +          r0_contains_arg = 0; > +      } >        continue; >      } >        else if ((op & 0xfc1fffff) == 0x7c000026) > @@ -2180,8 +2183,8 @@ skip_prologue (struct gdbarch *gdbarch, > CORE_ADDR pc, CORE_ADDR lim_pc, >      } >  #endif /* 0 */ > > -  if (pc == lim_pc && lr_reg >= 0) > -    fdata->lr_register = lr_reg; > +  if (pc != lim_pc) > +    fdata->lr_register = -1; > >    fdata->offset = -fdata->offset; >    return last_prologue_pc;