I have been trying new-ra for improved register allocation on avr target (8 bit registers, small memory etc)
The improvements are very good - 10% or more on code size. In particular it gets rid of redundant reg-reg moves between back to back library/function calls.
However, the gain is lost because the frame size is wrong. Or more precisely the frame size reflects the inclusion of stack slots that have been optimised away.
Code below show a simple case where frame size is 2 - presumably reflecting the "i" local variable that is in a register.
Without -fnew-ra, the epilog/prolog is empty (ie frame pointer can be eliminated)
I have managed to determine that the AVR target is being told that the frame size is 2.
Is this a bug? or am I missing somthing?
compiler options (GCC 3.3.1 target AVR). Host win2k and XP
OPT = s
CC = avr-gcc
CFLAGS = -W -Wall -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -O$(OPT)
.c.s:
$(CC) $(CFLAGS) -c -da -S $<
all: testcase.s
clean:
rm -f *.o
---------------------
.file "testcase.c"
.arch avr2
__SREG__ = 0x3f
__SP_H__ = 0x3e
__SP_L__ = 0x3d
__tmp_reg__ = 0
__zero_reg__ = 1
.global __do_copy_data
.global __do_clear_bss
.text
.global Timer0_Init
.type Timer0_Init, @function
Timer0_Init:
/* prologue: frame size=0 */
/* prologue end (size=0) */
ldi r24,lo8(0)
ldi r30,lo8(CallbackFunc)
ldi r31,hi8(CallbackFunc)
.L6:
st Z+,__zero_reg__
st Z+,__zero_reg__
subi r24,lo8(-(1))
cpi r24,lo8(10)
brlo .L6
/* epilogue: frame size=0 */
ret
/* epilogue end (size=1) */
/* function Timer0_Init size 9 (8) */
.size Timer0_Init, .-Timer0_Init
.comm CallbackFunc,20,1
/* File "testcase.c": code 9 = 0x0009 ( 8), prologues 0, epilogues 1 */
NOW WITH -new-ra we get:
========================
.file "testcase.c"
.arch avr2
__SREG__ = 0x3f
__SP_H__ = 0x3e
__SP_L__ = 0x3d
__tmp_reg__ = 0
__zero_reg__ = 1
.global __do_copy_data
.global __do_clear_bss
.text
.global Timer0_Init
.type Timer0_Init, @function
Timer0_Init:
/* prologue: frame size=2 */<============================
push r28<========================================
push r29<========================================
in r28,__SP_L__
in r29,__SP_H__
sbiw r28,2
in __tmp_reg__,__SREG__
cli
out __SP_H__,r29
out __SREG__,__tmp_reg__
out __SP_L__,r28
/* prologue end (size=10) */
ldi r24,lo8(0)
ldi r30,lo8(CallbackFunc)
ldi r31,hi8(CallbackFunc)
.L6:
st Z+,__zero_reg__
st Z+,__zero_reg__
subi r24,lo8(-(1))
cpi r24,lo8(10)
brlo .L6
/* epilogue: frame size=2 */
adiw r28,2
in __tmp_reg__,__SREG__
cli
out __SP_H__,r29
out __SREG__,__tmp_reg__
out __SP_L__,r28
pop r29
pop r28
ret
/* epilogue end (size=9) */
/* function Timer0_Init size 27 (8) */
.size Timer0_Init, .-Timer0_Init
.comm CallbackFunc,20,1
/* File "testcase.c": code 27 = 0x001b ( 8), prologues 10, epilogues 9 */
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