[commit] Add overlay return stub comment

[Ulrich Weigand]

Hello,

this adds a comment about the LR usage with overlay return stubs.

Bye,
Ulrich

ChangeLog:

	* spu-tdep.c (spu_frame_unwind_cache): Add comment.
  
Index: gdb/spu-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/spu-tdep.c,v
retrieving revision 1.13
diff -c -p -r1.13 spu-tdep.c
*** gdb/spu-tdep.c	11 May 2007 19:58:38 -0000	1.13
--- gdb/spu-tdep.c	11 May 2007 20:08:19 -0000
*************** spu_frame_unwind_cache (struct frame_inf
*** 743,755 ****
    /* The previous SP is equal to the CFA.  */
    trad_frame_set_value (info->saved_regs, SPU_SP_REGNUM, info->frame_base);
  
!   /* The previous PC comes from the link register.  In the case
!      of overlay return stubs, we unwind to the real return address.  */
    if (trad_frame_addr_p (info->saved_regs, SPU_LR_REGNUM))
      target_read_memory (info->saved_regs[SPU_LR_REGNUM].addr, buf, 16);
    else
      frame_unwind_register (next_frame, SPU_LR_REGNUM, buf);
  
    if (extract_unsigned_integer (buf + 8, 4) != 0)
      trad_frame_set_value (info->saved_regs, SPU_PC_REGNUM,
  			  extract_unsigned_integer (buf + 8, 4));
--- 743,761 ----
    /* The previous SP is equal to the CFA.  */
    trad_frame_set_value (info->saved_regs, SPU_SP_REGNUM, info->frame_base);
  
!   /* Read full contents of the unwound link register in order to
!      be able to determine the return address.  */
    if (trad_frame_addr_p (info->saved_regs, SPU_LR_REGNUM))
      target_read_memory (info->saved_regs[SPU_LR_REGNUM].addr, buf, 16);
    else
      frame_unwind_register (next_frame, SPU_LR_REGNUM, buf);
  
+   /* Normally, the return address is contained in the slot 0 of the
+      link register, and slots 1-3 are zero.  For an overlay return,
+      slot 0 contains the address of the overlay manager return stub,
+      slot 1 contains the partition number of the overlay section to
+      be returned to, and slot 2 contains the return address within
+      that section.  Return the latter address in that case.  */
    if (extract_unsigned_integer (buf + 8, 4) != 0)
      trad_frame_set_value (info->saved_regs, SPU_PC_REGNUM,
  			  extract_unsigned_integer (buf + 8, 4));
-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com