[WinAVR-users] [ winavr-Bugs-2490164 ] XMega (128A1): subroutine calling may corrupt stack pointer

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Bugs item #2490164, was opened at 2009-01-06 17:43
Message generated for change (Comment added) made by aesok
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Category: GCC
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: atxmfan (atxmfan)
Assigned to: Nobody/Anonymous (nobody)
Summary: XMega (128A1): subroutine calling may corrupt stack pointer

Initial Comment:
When calling a subroutine (esp. when using varargs) arguments are put on stack. To achieve this the stack pointer is decremented to make enough space on stack before calling. After returning from subroutine the stack pointer is adjusted back to its old value. The XMega needs special preparing before being able to change the stack pointer (protected I/O-registers) - register CCP must be set to 0xDA. Within the next 4 cycles changes to the SP registers are allowed.

GCC generates the following code in the "cleaning up" code:

   mov __tmp_reg__,r24
   ldi r24,0xD8
   out __CCP__,r24
   out __SP_H__,r25
   out __SP_L__,r24
   mov r24,__tmp_reg__

in which case r24 (SP_L) is destroyed. The next return lets the CPU crash. This happens only, when GCC chooses r24, r25. If different registers are used everything is OK.

The correct XMega patch should look like this:

   mov __tmp_reg__,r24
   ldi r24,0xD8
   out __CCP__,r24
   mov r24,__tmp_reg__
   out __SP_H__,r25
   out __SP_L__,r24

The libc library is also affected and must be recompiled.

atxmfan


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Comment By: Anatoly (aesok)
Date: 2009-01-06 18:35

Message:
Hi Eric.

Try change 52-gcc-4.3.2-xmega-v9.patch from:

  180 +	        {
  181 +		  *l = 6;
  182 +		  return (AS2 (mov,__tmp_reg__,r24)  CR_TAB
  183 +			  AS2 (ldi,r24,0xD8)         CR_TAB
  184 +			  AS2 (out,__CCP__,r24) CR_TAB
  185 +			  AS2 (out,__SP_H__,%B1)         CR_TAB
  186 +			  AS2 (out,__SP_L__,%A1)         CR_TAB
  187 +			  AS2 (mov,r24,__tmp_reg__));
  188 +		}

to:
  	        {
  		  *l = 6;
  		  return (AS2 (mov,__tmp_reg__,r24)  CR_TAB
  			  AS2 (ldi,r24,0xD8)         CR_TAB
  			  AS2 (out,__CCP__,r24) CR_TAB
			  AS2 (mov,r24,__tmp_reg__)  CR_TAB
  			  AS2 (out,__SP_H__,%B1)         CR_TAB
  			  AS2 (out,__SP_L__,%A1));
  		}

Anatoliy.

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Comment By: Eric Weddington (arcanum)
Date: 2009-01-06 17:58

Message:
What versions are you using?

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