[patch, avr] Fix incorrect argument register for push dummy call

[Pitchumani Sivanupandi <pitchumani.s@atmel.com>]

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There are few failures for avr-gdb in callfuncs.exp. These failures are
due to the incorrect argument passing in case of push dummy call. The
last argument register is calculated incorrectly that caused this issue.

Attached patch fixes the calculation so that the push dummy call aligns
with the code generated by avr-gcc.

No new regressions found when tested with internal simulator.

If ok, could someone commit please?

Regards,
Pitchumani

gdb/ChangeLog

2016-02-25  Pitchumani Sivanupandi  <pitchumani.s@atmel.com>

	* avr-tdep.c (AVR_LAST_ARG_REGNUM): Define.
	(avr_push_dummy_call): Correct last needed argument register.

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diff --git a/gdb/avr-tdep.c b/gdb/avr-tdep.c
index 597cfb4..18ecfe4 100644
--- a/gdb/avr-tdep.c
+++ b/gdb/avr-tdep.c
@@ -111,6 +111,7 @@ enum
 
   AVR_ARG1_REGNUM = 24,         /* Single byte argument */
   AVR_ARGN_REGNUM = 25,         /* Multi byte argments */
+  AVR_LAST_ARG_REGNUM = 8,      /* Last argument register */
 
   AVR_RET1_REGNUM = 24,         /* Single byte return value */
   AVR_RETN_REGNUM = 25,         /* Multi byte return value */
@@ -1298,12 +1299,14 @@ avr_push_dummy_call (struct gdbarch *gdbarch, struct value *function,
       const bfd_byte *contents = value_contents (arg);
       int len = TYPE_LENGTH (type);
 
-      /* Calculate the potential last register needed.  */
-      last_regnum = regnum - (len + (len & 1));
+      /* Calculate the potential last register needed.
+         E.g. For length 2, registers regnum and regnum-1 (say 25 and 24)
+         shall be used. So, last needed register will be regnum-1(24).  */
+      last_regnum = regnum - (len + (len & 1)) + 1;
 
       /* If there are registers available, use them.  Once we start putting
          stuff on the stack, all subsequent args go on stack.  */
-      if ((si == NULL) && (last_regnum >= 8))
+      if ((si == NULL) && (last_regnum >= AVR_LAST_ARG_REGNUM))
         {
           ULONGEST val;