------- Comment #16 from rask at gcc dot gnu dot org 2007-09-16 13:38 -------
Created an attachment (id=14211)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14211&action=view)
quick and dirty patch to reduce code size
A fundamental problem with the AVR back end is that it sabotages the RTL
optimizer's attempts to optimize away redundant move insns. With this patch, I
get better code (-O2):
foo:
push r29 ; 73 *pushhi/1 [length = 2]
push r28
rcall . ; 79 *addhi3_sp_R_pc2 [length = 3]
rcall .
rcall .
in r28,__SP_L__ ; 90 *movqi/6 [length = 1]
in r29,__SP_H__ ; 91 *movqi/6 [length = 1]
/* prologue: function */
/* frame size = 6 */
ldi r24,lo8(2) ; 68 *movqi/2 [length = 1]
std Y+5,r24 ; 70 *movqi/3 [length = 1]
std Y+6,__zero_reg__ ; 71 *movqi/3 [length = 1]
ldi r22,lo8(2) ; 21 *movqi/2 [length = 1]
ldi r23,lo8(0) ; 23 *movqi/2 [length = 1]
ldi r18,lo8(1) ; 42 *movqi/2 [length = 1]
ldi r19,lo8(0) ; 43 *movqi/2 [length = 1]
ldi r20,lo8(0) ; 44 *movqi/2 [length = 1]
ldi r21,lo8(0) ; 45 *movqi/2 [length = 1]
ldi r24,lo8(0) ; 48 *movqi/2 [length = 1]
ldi r25,lo8(0) ; 49 *movqi/2 [length = 1]
/* epilogue start */
adiw r28,6 ; 85 *addhi3/2 [length = 1]
out __SP_L__,r28 ; 92 *movqi/5 [length = 1]
out __SP_H__,r29 ; 93 *movqi/5 [length = 1]
pop r28 ; 87 pophi [length = 2]
pop r29
ret ; 88 return_from_epilogue [length = 1]
.size foo, .-foo
/* File "/home/rask/pr11180.c": code 0 = 0x0000 ( 0), prologues 0,
epilogues 0 */
Perhaps a peephole optimization could replace insns 48 and 49 with "movw
r25:r24, r21:r20".
The only big further improvement I see would be to get rid of the stack var.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11180
