[WinAVR-users] [ winavr-Bugs-2905895 ] use -ffixed-reg registers

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Bugs item #2905895, was opened at 2009-11-30 13:23
Message generated for change (Tracker Item Submitted) made by i501-q
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Category: GCC
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Ilya Potrepalov (i501-q)
Assigned to: Nobody/Anonymous (nobody)
Summary: use -ffixed-reg registers

Initial Comment:
In GCC manual "3.18 Options for Code Generation Conventions":

-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). 
reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. 



Code:

main.c
-----------------
long bug( long a, long b, long c );

register unsigned char reg16 asm( "r16" );
register unsigned char reg17 asm( "r17" );


int main( void )
{
	long t = bug( 1, 2, 3 );
}
-----------------

bug.c:
-----------------
long bug( long a, long b, long c )
{
	return a + b + c;
}
-----------------

make.bat:
-----------------
avr-gcc -ffixed-r16 -ffixed-r17 -mmcu=atmega64 main.c bug.c
avr-objdump -S a.out > dis.lst
-----------------

dis.lst (only intresting part):
-----------------
000000a4 <main>:
  a4:	ef 92       	push	r14
  a6:	ff 92       	push	r15
  a8:	df 93       	push	r29
  aa:	cf 93       	push	r28
.... 
  c6:	0f 2e       	mov	r0, r31
  c8:	f3 e0       	ldi	r31, 0x03	; 3
  ca:	ef 2e       	mov	r14, r31
  cc:	f0 e0       	ldi	r31, 0x00	; 0
  ce:	ff 2e       	mov	r15, r31
  d0:	f0 e0       	ldi	r31, 0x00	; 0
  d2:	0f 2f       	mov	r16, r31
  d4:	f0 e0       	ldi	r31, 0x00	; 0
  d6:	1f 2f       	mov	r17, r31
  d8:	f0 2d       	mov	r31, r0
  da:	0e 94 80 00 	call	0x100	; 0x100 <bug>
...
  fe:	08 95       	ret
-----------------

GCC use registers r16, r17 (and it did not save them).

GCC must not use registers that list in -ffixed-reg options.

GCC use registers r18, r19 in bultin functions when r18, r19 list in -ffixed-reg option.

----------------------------------------------------------------------

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