On RISC cpus with many registers, it is often helpful to keep constants (such
as addresses) in registers for later use, since loading a register with a
constant takes longer than moving data between registers, and since
register-constant operations are seldom available. On the AVR, however,
register-constant operations are just as small and fast as register-register
operations (excluding the movw operation), so it is often better to use the
constants directly - it saves a few instructions (time and code space), and
avoids tying up a register pair unnecessarily.
Example:
extern uint16_t data[64];
extern uint16_t foo(uint16_t x);
extern uint16_t a;
uint16_t bar(void) {
uint16_t x;
x = foo(data[a]);
return foo(data[x]);
}
In this case, the compiler caches the address of "data" in r16:r17 :
59 bar:
60 /* prologue: frame size=0 */
61 001a 0F93 push r16
62 001c 1F93 push r17
63 /* prologue end (size=2) */
64 001e E091 0000 lds r30,a ; a, a
65 0022 F091 0000 lds r31,(a)+1 ; a, a
66 0026 00E0 ldi r16,lo8(data) ; tmp45,
67 0028 10E0 ldi r17,hi8(data) ; tmp45,
68 002a EE0F lsl r30 ; a
69 002c FF1F rol r31 ; a
70 002e E00F add r30,r16 ; a, tmp45
71 0030 F11F adc r31,r17 ; a, tmp45
72 0032 8081 ld r24,Z ; , data
73 0034 9181 ldd r25,Z+1 ; , data
74 0036 0E94 0000 call foo ;
75 003a 880F lsl r24 ; tmp50
76 003c 991F rol r25 ; tmp50
77 003e 080F add r16,r24 ; tmp45, tmp50
78 0040 191F adc r17,r25 ; tmp45, tmp50
79 0042 F801 movw r30,r16 ; , tmp45
80 0044 8081 ld r24,Z ; , data
81 0046 9181 ldd r25,Z+1 ; , data
82 0048 0E94 0000 call foo ;
83 /* epilogue: frame size=0 */
84 004c 1F91 pop r17
85 004e 0F91 pop r16
86 0050 0895 ret
Better code could be generated by using "data" directly:
; Prologue avoided - saved 2 words code
lds r30, a
lds r31, (a) + 1
; Load of r16:r17 avoided - saved 2 words code
lsl r30 ; a
rol r31 ; a
subi r30, lo8(-(data))
sbci r31, hi8(-(data))
ld r24, z
ldd r25, Z+1
call foo
lsl r24 ; x
rol r25 ; x
; Using constant for data is just as small as register
subi r24, lo8(-(data))
sbci r25, hi8(-(data))
movw r30, r16
ld r24, z
ldd r25, Z+1
call foo
; Epilogue avoided - saved 2 words code
ret
This saves 6 code words, and corresponding time.
--
Summary: Missed optimisation on avr - sometimes the compiler
keeps addresses in registers unnecessarily
Product: gcc
Version: 4.2.2
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: david at westcontrol dot com
GCC target triplet: avrgcc
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34789
