True, but costs of comparison to NULL aren't massive. It is one of the fastest things out there.
It was my experience when tuning a linear algebra library (just for internal use) that such comparisons are almost unobservable in total performance tests.
If you're writing a linear algebra library, then the bulk of executing code should be floating point vector operations in tight loops with known bounds. A NULL pointer check in the prolog where you set up the loop is going to be negligible.
This is very different from kernel code, which by its nature isn't very computational but doing resource management all day long and pretty much all it does is stuff that looks like walking linked lists.
That was a very specific linear algebra, we toyed around with huge but sparse matrices. Not anything graphics-related, rather number theory-related. But there was a lot of integer comparisons in there.
This was around 2001-2002 anyway. Things might have changed.
Depends on how often you are doing the comparison. Is it 1 time/second, or 10 million times a second?
There is a difference.
If the code above is the one in charge of putting/removing network packets in a queue, or putting threads ordered by priority for the scheduler, then you should consider the side effects of checking for NULL.
If you are going to implement this function in a library for Jimmy The Programmer[1], then check for NULL.
It was my experience when tuning a linear algebra library (just for internal use) that such comparisons are almost unobservable in total performance tests.