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SubjectRe: [PATCH] Fix get_order()

On Tue, 6 Mar 2007, David Howells wrote:
> /**
> + * ilog2_up - rounded up log of base 2 of 32-bit or a 64-bit unsigned value
> + * @n - parameter
> + *
> + * constant-capable log of base 2 calculation
> + * - this can be used to initialise global variables from constant data, hence
> + * the massive ternary operator construction
> + * - the result is rounded up
> + * - the result is undefined when n < 1
> + *
> + * selects the appropriately-sized optimised version depending on sizeof(n)
> + */
> +#define ilog2_up(n) ((n) == 1 ? 0 : ilog2((n) - 1) + 1)

This is wrong. It uses "n" twice, which makes it unsafe as a macro.

It would need to be an inline function, but then the global initializer
comment is wrong.

Or it could use a "__builtin_constant_p()" (which gcc defines to not have
side effects) to allow the multiple use for constant data.

Or we could require that "ilog2(0)" returns -1, and then we could just say

#define ilog2_up(n) (ilog2((n)-1)+1)

Or.. ?

The whole "get_order()" macro also has some serious lack of parenthesis.
In general, commit 39d61db0edb34d60b83c5e0d62d0e906578cc707 just was
pretty damn bad!

I'm becoming a bit disgruntled about this whole thing, I have to admit.
I'm just not sure the bugs here are worth it. Especially considering that
__get_order() has apparently never even tested these things to begin with,
since nobody but FRV has ever #defined the ARCH_HAS_ILOG2_U?? macros.

The whole *reason* for that mess seems to be bogus too, since at least
ia64 still has its own inline "get_order()", which means that nobody can
use get_order() for constant initializers *anyway*, quite unlike the
comments say.

So the whole thing is:
- buggy
- untested
- has untrue comments
- makes no real sense

and I'm inclined to just revert 39d61db0 instead of adding more and more
breakage to it, since it's simply not going to help with the fundamental

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