Christopher Li pisze:
That is an optimization from Linus. It basically find out the simple variable
case comparing variable and turn it into binary operations and avoiding the
branch. It is cheaper to use "setne" than "cmp; jne; mov;".
It is safe because all the unsafe operations, e.g. dereferencing memory,
should have set the cost high enough to avoid this optimization.
e.g. all local variable dereferencing should be safe, because the address
is in the stack.
Deferencing a pointer is not, so sparse will not optimize it.
Please, check my case. The condition is:
if (st && st->other && st->value > i && i > 0)...
Obviously, if st is NULL, then the execution should be transferred
immediately to the else branch. But it does not. It skips the second
test and goes directly to the third one: st->value > i. If a compiler
was built with sparse as a frontend, execution of the generated code
would end up with a segmentation fault. And this code is perfectly valid.
So either it is an issue with the costs or it is an issue with the
linearization.
Anyway, I believe that this case is worth fixing.
Jacek
--
To unsubscribe from this list: send the line "unsubscribe linux-sparse" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
