Question

Which one is faster –

val = val*10;

or

val = (val<<3) + (val<<2);

How many clock cycles does imul take when compared to shift instruction?

Answer 1

In this case they probably take the same amount of cycles, though your manual “optimization” needs one more register (which can slow down the surrounding code):

val = val * 10;
lea    (%eax,%eax,4),%eax
add    %eax,%eax

vs

val = (val<<3) + (val<<1);
lea    (%eax,%eax,1),%edx
lea    (%edx,%eax,8),%eax

The compiler knows how to do strength reduction, and probably much better than you. Also, when you port your code to other platform (say, ARM), the compiler knows how to do strenght reduction on that platform too (x86’s LEA provides different optimization opportunities than ARM’s ADD and RSB).