On 13/08/16 11:40, Eric Tang wrote:
Hi avr-gcc mailing list,
I have a question about the asm generated for C code that sequentially checks
the bits of an integer in a loop. From what I can gather, during each iteration
of the loop, the integer is shifted right until the bit to be checked becomes
the least significant bit of its least significant byte, at which point that bit
is checked. This seems to be the case both when the loop index counts up and
when it counts down. I am wondering why the shifting starts over every time, and
if it would be better to retain the shifted result and shift it one bit more
every iteration of the loop or to maintain a mask which is used in an AND
operation with the integer and shifted one bit more during every iteration of
the loop?
#include <stdint.h>
#include <avr/io.h>
int main(void)
{
uint8_t temp;
temp = 0xA5;
DDRB |= 0xFF;
for (uint8_t i = 0; i < 8; ++i)
if (temp & 1 << i)
PORTB ^= 0xFF;
for (uint8_t i = 8; i--;)
if (temp & 1 << i)
PORTB ^= 0xFF;
return 0;
}
.file"avr_asm_src.c"
...
You could do something like:
int main(void)
{
uint8_t temp = 0xA5;
uint8_t temp_orig = temp;
DDRB |= 0xFF;
for (uint8_t i = 0; i < 8; ++i) {
if (temp & 1)
PORTB ^= 0xFF;
temp >>= 1;
}
temp = temp_orig;
for (uint8_t i = 8; i--;) {
if (temp & 128)
PORTB ^= 0xFF;
temp <<= 1;
}
return 0;
}
_______________________________________________
AVR-GCC-list mailing list
AVR-GCC-list@nongnu.org
https://lists.nongnu.org/mailman/listinfo/avr-gcc-list
