[avr-gcc-list] Bad optimization AVR-GCC 4.1.1 compared to AVR-GCC 3.4.6

[Peter Dannegger]

Dear GCC experts,

following an example code, which was about 25% bigger on using the 
AVR-GCC 4.1.1. instead AVR-GCC 3.4.6.

The code was bigger on 3 points:

1.
From a 16 bit variable, where only the high byte was used, also the low 
byte was read from SRAM into a register (3 words more).

2.
A function was inlined nevertheless, the optimization for size -Os was 
forced and no inline or static attribute was given. It seems, that the 
-Os switch was ignored.

3.
A fully crazy behaviour was seen on the attempt to clear a variable 
conditional.
Instead clearing the register (1 word instruction ) like on the AVR-GCC 
3.4.6, a big sequence of code (23 words) was duplicated.


Following the code, it was compiled with this command line:

avr-gcc.exe -xc -Os -mmcu=attiny26 -Wall -g disptime.c

int main(void)
{
 for(;;){

 }
}


unsigned int sec_timer;

unsigned char minute, hour;

unsigned char digit[4];

unsigned char numbers[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };


unsigned char get7s( unsigned char val )
{
 return numbers[val];
}


void display_time( unsigned char set )
{
 unsigned char t = sec_timer >> 8;     // high byte only

 if( t < 10 )                          // 1/2 second
   set = 0;

 if( set == 1 ){
   digit[3] = 0;
   digit[2] = 0;
 }else{
   digit[3] = get7s( hour / 10);
   digit[2] = get7s( hour % 10);
 }
 if( set == 2 ){
   digit[1] = 0;
   digit[0] = 0;
 }else{
   digit[1] = get7s( minute / 10);
   digit[0] = get7s( minute % 10);
 }
}

--
        mit freundlichen Grüssen

        Peter Dannegger

	SPECS - COMPETENCE IN SURFACE SCIENCE 
	-------------------------------------
        Peter Dannegger
        Specs GmbH
        Voltastr. 5
        13355 Berlin
        Germany
        phone +49 (30) 467 824-21
        fax   +49 (30) 464 20 83
        address@hidden
        www.specs.de