Re: [avr-libc-dev] Missed optimizations

[Georg-Johann Lay]

On 16.04.2017 19:30, Raul Sanchez wrote:

Your email is badly broken, something it terribly wrong with your line 
breaks.

> Just wanted to report the following observations in generated code.
> Target: __AVR_ATtiny85__
> Version: AVR gcc 4.6.4

Quite old a version, dates 6 years back which is 6 major GCC versions.

> Optimization level: -Os
> -------- C source ----------
>   uint16_t sum_adc;
>   uint16_t analog;
>   uint16_t adc_buffer[64];
>   uint8_t adc_index;
> ...

This is not valid C. "..." may only occur as varargs function parameter

>   sum_adc += ADC;

This is not valid C: Assignments may only occur inside functions.

>   sum_adc -= adc_buffer[adc_index];
>   analog = (uint16_t) (sum_adc >> 3);
> ...
> ------- end C source ------------
> ------- assembly code (with corresponding C statements) --------
> //  sum_adc += ADC;
>         lds r18,sum_adc
>         lds r19,sum_adc+1
>         in r24,36-0x20
>         in r25,36+1-0x20
>         add r24,r18
>         adc r25,r19
>         sts sum_adc+1,r25
>         sts sum_adc,r24
> // sum_adc -= adc_buffer[adc_index];
>         lds r18,sum_adc              ; why not "mov r18,r24" "mov r19,r25" ?

Presumably, because you are using an outdated version of avr-gcc. The 
oldest version I have handy is avr-gcc 4.7, and with that, the generated 
code is different.  But very likely, your "missed optimization" roots 
somewhere in the extra obfuscation "...".

>         lds r19,sum_adc+1
>         lds r30,adc_index
>         ldi r31,lo8(0)                    ;why not "clr r31"?

Because LDI *,0 performs as well as CLR * but with the advantage that 
the former does not clobber SREG.

>         ldi r24,lo8(adc_buffer)
>         ldi r25,hi8(adc_buffer)
>         lsl r30
>         rol r31
>         add r30,r24
>         adc r31,r25
>         ld r20,Z
>         ldd r21,Z+1
>         sub r18,r20
>         sbc r19,r21
>         sts sum_adc+1,r19
>         sts sum_adc,r18
> // analog = (uint16_t) (sum_adc >> 3);
>         lds r18,sum_adc               ; loading into r18, r19 the same values 
> they alredy hold!
>         lds r19,sum_adc+1
>         lsr r19
>         ror r18
>         lsr r19
>         ror r18
>         lsr r19
>         ror r18

This shift appears to be invoked by -O2 or -O3, not by -Os as stated above.

>         sts analog+1,r19
>         sts analog,r18


Just for the recored, I compiled the following C code

#include <avr/io.h>

uint16_t sum_adc;
uint16_t analog;
uint16_t adc_buffer[64];
uint8_t adc_index;

void func (void)
{
    sum_adc += ADC;
    sum_adc -= adc_buffer[adc_index];
    analog = (uint16_t) (sum_adc >> 3);
}

with

$ avr-gcc-4.7 code.c -mmcu=attiny85 -S -O2

and the resulting code.s reads:

func:
        in r24,0x4
        in r25,0x4+1
        lds r18,sum_adc
        lds r19,sum_adc+1
        add r24,r18
        adc r25,r19
        lds r30,adc_index
        ldi r31,0
        lsl r30
        rol r31
        subi r30,lo8(-(adc_buffer))
        sbci r31,hi8(-(adc_buffer))
        ld r18,Z
        ldd r19,Z+1
        sub r24,r18
        sbc r25,r19
        sts sum_adc+1,r25
        sts sum_adc,r24
        lsr r25
        ror r24
        lsr r25
        ror r24
        lsr r25
        ror r24
        sts analog+1,r25
        sts analog,r24
        ret


Johann