Re: [avr-gcc-list] [AVR] RTL prologue/epilogue ver.3

[Boyapati, Anitha]

Hi All,

I have couple of comments on this patch, which I think are important
discussing with the avr-gcc community.

Comment - 1: 


Has any one verified prologue for functions using global variables? I
think this patch generates incorrect prologue for such cases.

int global;
void func(int p)
{
 p=global++;
}

int main()
{
   int local=42;
   func(local);
}


Code Generated for func() -

00000228 <func>:
 228:   cf 93           push    r28
 22a:   df 93           push    r29

>>>>
 22c:   00 d0           rcall   .+0             ; 0x22e <func+0x6>
 22e:   0f 92           push    r0
<<<<

 230:   cd b7           in      r28, 0x3d       ; FP = SP
 232:   de b7           in      r29, 0x3e       
 234:   8b 83           std     Y+3, r24        ; store 'local' to 'p'
 236:   9c 83           std     Y+4, r25         
 ...

Read the code between ">>>>  <<<<" and notice that 3/4 bytes ( 3 bytes
for Megas, 4 for Xmegas) have been allocated on stack instead of 2.


Comment -2:

I think this patch severely affects the step in/step out feature of
debugger like AVR Studio (here after referred to as AS). AS depends on
'rcall'/'ret' instruction sequences to recognize a function call and
step into. If a matching 'ret' is not found, the debugger gets confused.
Currently, the problem is worked around but has much lower performance.

Also it is misleading to use 'rcall' for purposes it is not meant for
(like stack allocation). In my opinion, the hack is not desirable by
default. I would therefore recommend making it optional using a switch
like 'mrcall'.

Thoughts?


Anitha









=======================================================================

From:   Anatoly Sokolov
Subject:        Re: [avr-gcc-list] [AVR] RTL prologue/epilogue ver.3
Date:   Sun, 20 Aug 2006 14:29:46 +0400


Hello.
 
 Attached is modifed Andy Hutchinson patch that changes AVR GCC to use
RTL 
prologue/epilogue generation. 
 Original patch: http://gcc.gnu.org/ml/gcc/2005-03/msg00923.html
 
 This pach is for GCC 4.2.

Main difference: 
The order of instructions in a prologue (and epilogue) of interrupt
handler 
functions is changed from:
   push __tmp_reg__
   in __tmp_reg__,__SREG__  
   push __tmp_reg__  
   push __zero_reg__  
   clr __zero_reg__  
....
to old order:
   push    __zero_reg__
   push    __tmp_reg__
   in      __tmp_reg__, __SREG__
   push    __tmp_reg__
   clr     __zero_reg__
....
 
The GDB uses the second variant as a pattern, for a finding of body of 
function. 

 Anatoly.