[Patch,AVR]: Use EIND consistently

[Georg-Johann Lay]

This patch adds support to consistently use EIND.

The compiler never sets this SFR but uses it in table jumps and EIJMP/EICALL
instructions.

Custom startup code could set EIND to an other value than 0 and the compiler
should use EIND consistently given that EIND might not be zero.

EIND != 0 will need support of custom linker script to locate jump pads in an
other segment, but that's a different story.

The patch undoes some changes from r179760 and introduces using EIND the other
way round: Not trying to avoid EIND altogether and assume code is supposed to
work in the lower segment only, but instead use EIND and not zero-reg when
simulating indirect jump by means of RET.

With this patch, the application may set EIND to a custom value and invent own
linker script to place jump pads.  The assertion is that EIND never changes
throughout the application and therefore ISR prologue/epilogue need not care.

With the gs() magic, code using indirect jumps works fine with that, e.g.
- Indirect calls
- Computed goto
- Jumping to 1: in prologue_saves

What does not work as expected is to jump to const_int addresses like

int main()
{
   ((void(*)(void))0)();
   return 0;
}

Instead, code must read

extern void my_address (void);

int main()
{
    my_address();
    return 0;
}

and compiled with, say -Wl,--defsym,my_address=0x20000, so that a jump pad is
generated.

Patch ok for trunk?

Johann

        * config/avr/libgcc.S (__EIND__): New define to 0x3C.
        (__tablejump__): Consistently use EIND for indirect jump/call.
        (__tablejump_elpm__): Ditto.

Index: config/avr/libgcc.S
===================================================================
--- config/avr/libgcc.S (revision 180262)
+++ config/avr/libgcc.S (working copy)
@@ -28,6 +28,7 @@ see the files COPYING3 and COPYING.RUNTI
 #define __SP_H__ 0x3e
 #define __SP_L__ 0x3d
 #define __RAMPZ__ 0x3B
+#define __EIND__  0x3C

 /* Most of the functions here are called directly from avr.md
    patterns, instead of using the standard libcall mechanisms.
@@ -821,17 +822,12 @@ ENDF __tablejump2__

 DEFUN __tablejump__
 #if defined (__AVR_HAVE_LPMX__)
-#if defined (__AVR_HAVE_EIJMP_EICALL__)
-       lpm  __tmp_reg__, Z+            
-       push __tmp_reg__
-       lpm  __tmp_reg__, Z             
-       push __tmp_reg__
-       push __zero_reg__
-       ret
-#else
        lpm __tmp_reg__, Z+
        lpm r31, Z
        mov r30, __tmp_reg__
+#if defined (__AVR_HAVE_EIJMP_EICALL__)
+       eijmp
+#else
        ijmp
 #endif

@@ -842,7 +838,8 @@ DEFUN __tablejump__
        lpm
        push r0
 #if defined (__AVR_HAVE_EIJMP_EICALL__)
-       push __zero_reg__
+       in   __tmp_reg__, __EIND__
+       push __tmp_reg__
 #endif
        ret
 #endif /* !HAVE_LPMX */
@@ -1034,7 +1031,8 @@ DEFUN __tablejump_elpm__
        elpm
        push    r0
 #if defined (__AVR_HAVE_EIJMP_EICALL__)
-        push    __zero_reg__
+       in      __tmp_reg__, __EIND__
+       push    __tmp_reg__
 #endif
        ret
 #endif