Re: BUGed to death

[Martin J. Bligh]

>> >> Seems all these bug checks are fairly expensive. I can get 1%
>> >> back on system time for kernel compiles by changing BUG to
>> >> "do {} while (0)" to make them all compile away. Profiles aren't
>> >> very revealing though ... seems to be within experimental error ;-(
>> > 
>> > What happens if you just turn BUG_ON into "do {} while (0)"?
>> 
>> I believe I already did that by turning BUG() into a null expression.
>> 
>> # define BUG_ON(condition)
>> 	do { if (unlikely((condition)!=0)) BUG(); } while(0)
>> 
>> The compiler should be smart enough to optimise that away, methinks.
> 
> No, I meant what happens if BUG() is non-trivial and BUG_ON is a no-op.
> I thought it might give an indication of whether time was being lost
> evaluating the condition (occurs with BUG and BUG_ON), or mispredicting
> the branch (only occurs with BUG).

Mmmm. wouldn't that just optimise away the BUG_ONs, but not the BUGs ?
Which will tell you something, but I'm not sure anything interesting.
My impression is that if I do:

if (foo)
	do {} while (0);

the compiler will just ditch the whole thing, and not evaluate foo.
I'll admit to not having checked that, but still ....

M.