I think something strange is being produced by -Os
I reduced a confusing large application to the below code.
Compiling with default optimizaton produced an avr-objdump that follows
the C code as I expected.
> avr-gcc -mmcu=atmega16 -c -o err.o err.c
> avr-gcc -mmcu=atmega16 err.o -o err.elf -lm -lc
> avr-objdump -d err.elf
...
000000fa <use_libc>:
fa: ef 92 push r14
fc: ff 92 push r15
fe: 0f 93 push r16
100: 1f 93 push r17
102: df 93 push r29
104: cf 93 push r28
106: cd b7 in r28, 0x3d ; 61
108: de b7 in r29, 0x3e ; 62
10a: 68 97 sbiw r28, 0x18 ; 24
10c: 0f b6 in r0, 0x3f ; 63
10e: f8 94 cli
110: de bf out 0x3e, r29 ; 62
112: 0f be out 0x3f, r0 ; 63
114: cd bf out 0x3d, r28 ; 61
116: 89 e1 ldi r24, 0x19 ; 25
118: 94 e0 ldi r25, 0x04 ; 4
11a: ae e9 ldi r26, 0x9E ; 158
11c: bf e3 ldi r27, 0x3F ; 63
...
However, using -Os, I see strange code at c8, ca, etc. The poly
calculation and the sin call seem to be below here. The stack is
cleaned correctly by the 4 ea pop r0's. Why the strange rcalls???
> avr-gcc -Os -mmcu=atmega16 -c -o err.o err.c
> avr-gcc -Os -mmcu=atmega16 err.o -o err.elf -lm -lc
> avr-objdump -d err.elf
...
000000b8 <use_libc>:
b8: 8f 92 push r8
ba: 9f 92 push r9
bc: af 92 push r10
be: bf 92 push r11
c0: cf 92 push r12
c2: df 92 push r13
c4: ef 92 push r14
c6: ff 92 push r15
c8: 0f 93 push r16
ca: 1f 93 push r17
cc: df 93 push r29
ce: cf 93 push r28
d0: 00 d0 rcall .+0 ; 0xd2 <use_libc+0x1a>
d2: 00 d0 rcall .+0 ; 0xd4 <use_libc+0x1c>
d4: cd b7 in r28, 0x3d ; 61
d6: de b7 in r29, 0x3e ; 62
d8: 0f 2e mov r0, r31
da: f0 e0 ldi r31, 0x00 ; 0
...
tomdean
// optimization problem?????
//
// 20101228 tomdean - initial version
////////////////////////////////////////////////////
// includes
#include <avr/io.h>
#include <math.h>
////////////////////////////////////////////////////
// prototypes
void use_libc(void);
////////////////////////////////////////////////////
// initialize
void initialize(void) {
DDRB |= (_BV(0) | _BV(1) | _BV(4) | _BV(5) | _BV(7));
}
////////////////////////////////////////////////////
// main
int main() {
initialize();
while (1) {
PORTB |= _BV(0);
use_libc();
PORTB &= ~_BV(0);
}
return 0; // never get here
}
////////////////////////////////////////////////////
// prototypes
void use_libc(void);
////////////////////////////////////////////////////
// use-libc
void use_libc(void) {
float x,y,z;
float a = 1.2345;
float b = 4.98778;
float c = 123.9876554;
// 200 iterations
for (x=-1.0; x<= 1.0; x+=0.01) {
// do a poly
y = a*x*x + b*x + c;
// some trig
z=sin(y);
PORTA = *(unsigned char *)&z; // use it so not optmized away...
}
}
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