[RFC,2/5] gdb/arm: PR 29716 Fix FNC_RETURN stack selection in exception unwinder

  Unwinding of FNC_RETURN selected the process stack whenever zero IPSR
indicated thread mode.

This does not comply
Arm v8-M Architecture Reference Manual
B3.8 Stack pointer
IDMLS "In Thread mode, CONTROL.SPSEL determines whether the PE uses
the main or process stack"

Check SPSEL bit of CONTROL_S register.

For simplicity the CONTROL_S is not tracked for changes
in the inner frames, the CONTROL_S value is passed unchanged from
the innermost frame.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
---
 gdb/arm-tdep.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)
  

Hi Tomas,

I'd like to address the performance regression first before we get to these additional improvements.

I've been discussing PR 29738 with Torbjörn and have provided some feedback.

On 11/5/22 09:44, Tomas Vanek wrote:
> Unwinding of FNC_RETURN selected the process stack whenever zero IPSR
> indicated thread mode.
> 
> This does not comply
> Arm v8-M Architecture Reference Manual
> B3.8 Stack pointer
> IDMLS "In Thread mode, CONTROL.SPSEL determines whether the PE uses
> the main or process stack"
> 
> Check SPSEL bit of CONTROL_S register.
> 
> For simplicity the CONTROL_S is not tracked for changes
> in the inner frames, the CONTROL_S value is passed unchanged from
> the innermost frame.
> 
> Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
> ---
>   gdb/arm-tdep.c | 28 +++++++++++++++++++++-------
>   1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
> index 564ee43..4180277 100644
> --- a/gdb/arm-tdep.c
> +++ b/gdb/arm-tdep.c
> @@ -3485,13 +3485,27 @@ struct frame_unwind arm_stub_unwind = {
>   	  return cache;
>   	}
>   
> -      ULONGEST xpsr = get_frame_register_unsigned (this_frame, ARM_PS_REGNUM);
> -      if ((xpsr & 0x1ff) != 0)
> -	/* Handler mode: This is the mode that exceptions are handled in.  */
> -	arm_cache_switch_prev_sp (cache, tdep, tdep->m_profile_msp_s_regnum);
> -      else
> -	/* Thread mode: This is the normal mode that programs run in.  */
> -	arm_cache_switch_prev_sp (cache, tdep, tdep->m_profile_psp_s_regnum);
> +      bool spsel = true;
> +      if (tdep->m_profile_control_s_regnum >= 0)
> +	{
> +	  ULONGEST control_s
> +	    = get_frame_register_unsigned (this_frame,
> +					   tdep->m_profile_control_s_regnum);
> +	  spsel = (control_s & (1 << 1)) != 0;
> +	}
> +
> +      bool s_process_stack = false;
> +      if (spsel)
> +	{
> +	  ULONGEST xpsr = get_frame_register_unsigned (this_frame,
> +						       ARM_PS_REGNUM);
> +	  s_process_stack = (xpsr & 0x1ff) == 0;
> +	}
> +
> +      arm_cache_switch_prev_sp (cache, tdep,
> +				s_process_stack ?
> +				tdep->m_profile_psp_s_regnum :
> +				tdep->m_profile_msp_s_regnum);
>   
>         CORE_ADDR unwound_sp = arm_cache_get_prev_sp_value (cache, tdep);
>