[PING] Fix for prologue processing on PowerPC

  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 <nprica@rt-rk.com>
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(-)

        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;
  

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 <nprica@rt-rk.com>
> 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;
.file	"foo.c"
	.abiversion 2
	.section	".text"
	.align 2
	.globl foo
	.type	foo, @function
foo:
.LCF0:
0:	addis 2,12,.TOC.-.LCF0@ha
	addi 2,2,.TOC.-.LCF0@l
	.localentry	foo,.-foo
	mflr 3
	std 3,16(1)
	std 31,-8(1)
	stdu 1,-112(1)
	mr 31,1
	bl bar
	nop
	mr 9,3
	mr 3,9
	addi 1,31,112
	ld 0,16(1)
	mtlr 0
	ld 31,-8(1)
	blr
	.long 0
	.byte 0,0,0,1,128,1,0,1
	.size	foo,.-foo
	.ident	"GCC: (SUSE Linux) 7.2.1 20170901 [gcc-7-branch revision 251580]"
	.section	.note.GNU-stack,"",@progbits