[avr-libc-dev] Missed optimizations

[Raul Sanchez]

Just wanted to report the following observations in generated code.
Target: __AVR_ATtiny85__Version: AVR gcc 4.6.4Optimization level: -Os
-------- C source ----------  uint16_t sum_adc;  uint16_t analog;  uint16_t 
adc_buffer[64];  uint8_t adc_index;...  sum_adc += ADC;  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" ?        lds 
r19,sum_adc+1        lds r30,adc_index        ldi r31,lo8(0)                    
;why not "clr r31"?        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  
      sts analog+1,r19        sts analog,r18