5.4 Volatile ...?
If you are familiar with kernel sources or some beautiful code like that, you must have seen many functions declared as volatile or __volatile__ which follows an asm or __asm__. I mentioned earlier about the keywords asm and __asm__. So what is this volatile?
If our assembly statement must execute where we put it, (i.e. must not be moved out of a loop as an optimization), put the keyword volatile after asm and before the ()’s. So to keep it from moving, deleting and all, we declare it as
asm volatile ( ... : ... : ... : ...);
Use __volatile__ when we have to be verymuch careful.
If our assembly is just for doing some calculations and doesn’t have any side effects, it’s better not to use the keyword volatile. Avoiding it helps gcc in optimizing the code and making it more beautiful.
In the section Some Useful Recipes, I have provided many examples for inline asm functions. There we can see the clobber-list in detail.