------- Comment #7 from eweddington at cso dot atmel dot com 2007-08-23 20:10
-------
Confirmed for AVR. GCC 4.2.1 for avr generates this:
foo:
/* prologue: frame size=0 */
push r14
push r15
push r16
push r17
/* prologue end (size=4) */
movw r14,r22
movw r16,r24
sub r14,r18
sbc r15,r19
sbc r16,r20
sbc r17,r21
cp r22,r18
cpc r23,r19
cpc r24,r20
cpc r25,r21
brsh .L2
ldi r24,lo8(100)
ldi r25,hi8(100)
ldi r26,hlo8(100)
ldi r27,hhi8(100)
add r14,r24
adc r15,r25
adc r16,r26
adc r17,r27
.L2:
movw r24,r16
movw r22,r14
/* epilogue: frame size=0 */
pop r17
pop r16
pop r15
pop r14
ret
Ideally it should be something like:
foo:
/* prologue: frame size=0 */
sub r22,r18
sbc r23,r19
sbc r24,r20
sbc r25,r21
brcc .L2
ldi r18,lo8(100)
ldi r19,hi8(100)
ldi r20,hlo8(100)
ldi r21,hhi8(100)
add r22,r18
adc r23,r19
adc r24,r20
adc r25,r21
.L2:
/* epilogue: frame size=0 */
ret
Which is less than half the number of instructions.
Changing summary and target fields
--
eweddington at cso dot atmel dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
GCC target triplet|i386-linux, powerpc-*-* |i386-linux, powerpc-*-*,
| |avr-*-*
Last reconfirmed|2006-03-05 03:13:08 |2007-08-23 20:10:25
date| |
Summary|appalling optimisation with |appalling optimisation with
|sub/cmp on i386 |sub/cmp on multiple targets
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3507
