Re: [PATCH 1/12] generic *_bit()

[Grant Grundler <iod00d@hp.com>]

On Wed, Feb 01, 2006 at 01:41:03PM -0800, Chen, Kenneth W wrote:
> > Well, if it doesn't matter, why is unsigned int better?
> 
> I was coming from the angle of having bitop operate on unsigned
> int *, so people don't have to type cast or change bit flag variable
> to unsigned long for various structures.  With unsigned int type for
> bit flag, some of them are not even close to fully utilized. for example:
> 
> thread_info->flags uses 18 bits
> thread_struct->flags uses 7 bits
> 
> It's a waste of memory to define a variable that kernel will *never*
> touch the 4 MSB in that field.

Agreed. Good point. But this can be mitigated if the code using "unsigned int"
(or unsigned byte) first loads the value into a local unsigned long variable.
That typically translates into a tmp register anyway. Compiler will help
you find places where that needs to happen.

Counter point is bit arrays (e.g. bit maps) like cpumask_t are
typically much larger than 32-bits (typically distro's ship with
NR_CPUS set to 256 or so).  File system code also likes bit arrays
for block allocation tables. Searching a bit array using unsigned
long is 2x faster on 64-bit architectures. I don't want to give
that up and I'm pretty sure Tony Luck, Paul Mckerras and a few
others would object unless you can give a better reason.

Obviously neither memory footprint nor speed of walking memory is an
the issue for 32-bit arches (where unsigned long = unsigned int).


> > unsigned long is typically the native register size, right?
> > I'd expect that to be more efficient on most arches.
> 
> The only difference that I can think of on Itanium processor is the
> memory operation, you either load/store 4 or 8 bytes. Once the data
> is in the CPU register, it doesn't make any difference whether it is
> operating on 32bit or entire 64 bit. I don't know about others RISC
> arch though whether it is more efficient with native register size.

agreed. I was thinking mostly of the bit map search - not searching
within a single unsigned int.

grant