9.1.3 Tip #3 Loop Index

Loops are widely used in 8-bit AVR code. There are "while ( ) { }" loop, "for ( )" loop, and "do { } while ( )" loop. If the -Os optimization option is enabled, the compiler will optimize the loops automatically to have the same code size.

However, we can still reduce the code size slightly. If we use a "do { } while ( )" loop, an increment or a decrement loop index generates different code size. Usually we write our loops counting from 0 to the maximum value (increment), but it is more efficient to count the loop from the maximum value to 0 (decrement).

That is because in an increment loop, a comparison instruction is needed to compare the loop index with the maximum value in every loop to check if the loop index reaches the maximum value.

When we use a decrement loop, this comparison is not needed any more because the decremented result of the loop index will set the Z (zero) flag in SREG if it reaches zero.

Table 9-4 shows the effect of "do { } while ( )" loop with increment and decrement loop index.

Table 9-4. Example of do { } while ( ) Loops with Increment and Decrement Loop Index
Do{ }While( ) with increment loop indexDo{ }While( ) with decrement loop index
C source code 
#include <avr/io.h>
int main(void)
{
uint8_t local_1 = 0;
do {
PORTB ^= 0x01;
local_1++;
 } while (local_1<100);
}
 
#include <avr/io.h>
int main(void)
{
uint8_t local_1 = 100;
 do {
PORTB ^= 0x01;
local_1--;(1)
} while (local_1);
}
AVR Memory Usage

Program: 96 bytes (1.2% Full)

(.text + .data + .bootloader)

Data: 0 bytes (0.0% Full)

(.data + .bss + .noinit)

Program: 94 bytes (1.1% Full)

(.text + .data + .bootloader)

Data: 0 bytes (0.0% Full)

(.data + .bss + .noinit)

Compiler optimization level-Os (Optimize for size)-Os (Optimize for size)
Note:
  1. To have a clear comparison in C code lines, this example is written like "do {count-- ;} while (count);" and not like "do {} while (--count);" usually used in C books. The two styles generate the same code.