Re: [PATCH v3 1/3] lib: find_*_bit reimplementation

[Rasmus Villemoes]

[for some reason google decided to put this in my spam folder, hrmpf]

On Mon, Feb 09 2015, "George Spelvin" <linux@xxxxxxxxxxx> wrote:

> Sorry, I screwed up the bit-twiddling while messing with various options.
> I was trying to get size == 32 to work; that should have been:
>
>> 	tmp &= (2UL << ((size-1) % BITS_PER_LONG)) - 1;	/* Mask last word */
>
> And you're right that LAST_WORD_MASK is a good wrapper.
>

Well, it's not my invention, I just misremembered the
name. linux/bitmap.h already exposes BITMAP_LAST_WORD_MASK.

> Vasrious working solutions include:
> #define LAST_WORD_MASK(bits) 	((2UL << (bits-1) % BITS_PER_LONG) - 1)
> #define LAST_WORD_MASK(bits) 	~(~0UL << bits % BITS_PER_LONG)
> #define LAST_WORD_MASK(bits) 	(~0UL >> -bits % BITS_PER_LONG)

Incidentally, I had a patch lying around for replacing BITMAP_LAST_WORD_MASK by
something like the last of these (it is currently using a ?:). But to allow bits to
have signed type it is safer to spell it

#define BITMAP_LAST_WORD_MASK(bits) (~0UL >> ((-(bits)) & (BITS_PER_LONG-1)))

[also adding lots of parentheses so I don't have to worry about precedence].

> I'm not sure which generates the nicest code.  It's 4 instructions
> each way, with the last being 1 byte smaller:

I think one would have to look at effects on real code; when just compiling a
function doing nothing but this gcc has to use specific registers for in
and out. 

>> Also, I think it is best to handle size==0 appropriately, meaning that
>> one cannot dereference addr in any way (and certainly not addr[-1]).
>
> Ah, okay; l I figured that was a safe case to omit.  But your solution is nicer
> than mine overall.
>
> It may be that omitting the mask *is* safe, but it's a lot of wading through
> callers to prove it.

I think generic library code like this should provide both safety
checks, and only if some true performance bottleneck is found can one
start looking at implementing __shortcuts which have further constraints
on the caller.

Rasmus
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