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

[Tomas Vanek <vanekt@fbl.cz>]

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);
 
-- 
1.9.1