Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE_SIZE

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Nov 21, 2011 at 3:47 PM, David Daney <ddaney.cavm@gmail.com> wrote:
>
> These symbols are on dead code paths, so they are eliminated by the
> compiler's Dead Code Elimination (DCE) optimizations, and the BUG() code
> never gets emitted to the final executable.

If you are so damn sure of that, then DON'T MAKE IT A BUG_ON! If you
are 100% syre, then you might as well leave out the BUG_ON() entirely.

Seriously. What's so hard to understand?

Either you are 100% sure, or you are not. If you are 100% sure, then
the BUG_ON() is pointless. And if you are not, then the BUG_ON() is
*wrong*.

Notice? The BUG_ON() is never *ever* valid. You cannot have it both
ways. So stop pushing crap, already!

So what are non-crap solutions?

 - the current one: error out at compile time (early) if somebody uses
them in invalid contexts.

   This seems to be a good case, especially since apparently no actual
current code wants to use them outside of the existing #ifdef's. And
there is no reason to think that some random MIPS-only future code is
a good enough reason to re-introduce these things

 - if you really want to use them, but expect the compiler to always
compile them away as dead code, use a non-existing function linkage,
so that you at least get a static failure at link-time for incorrect
code, rather than some random BUG_ON() at run-time that may be
impossible to find.

See? There are real solutions. BUG_ON() is not one of them.

                  Linus