[avr-gcc-list] multiplication order effects machine cycles?

[intiha Ho gai]

Hello everyone,
In one of the programs that I am trying to optimize there is a long
multiplication (I am using Atmega128L). Using GPIO toggling across the
lines of code I get the following result


double skew_; 
int32_t offset_;//can also be negative
uint32_t full;

//assume full to be initialize dto something
GPIO(on)//my own MACRO
full  = (uint32_t)(skew_*(double)full + offset_); //this gives me 348usec on the scope 
GPIO(off)

REPLACEMENT CODE FOR ABOVE LINE
GPIO(on)
   full+=offset_; 
   full  =(uint32_t) skew_ *(double)full; //AFFAN: by
observing the scope results, for some reason this results in less time
spent (180 us as opposed to 348 usec above)
GPIO(off)

Can anyone explain why this significantly large change has happened. I
know I cna lok at the disassembly, but is there any intuitive
reason/principal that I can lear from this about how to code
multiplication. 

Thank you 

Affan