Re: [RFC PATCH 1/2] Shrink compat_ioctl.c

[Matt Mackall <mpm@selenic.com>]

On Tue, 2008-09-30 at 01:38 +0200, Andi Kleen wrote:
> On Mon, Sep 29, 2008 at 06:27:10PM -0500, Matt Mackall wrote:
> > I'm throwing this out untested as I don't have a mixed 64/32 system
> > handy at the moment.
> 
> Makes sense (assuming it works, haven't tested)
> 
> > compat_ioctl: shrink structures
> > 
> > Initially, compat_ioctl.c has a ~18k table of translated ioctls.
> > Each table entry is 24 bytes but we can shrink this to 16:
> > 
> > - use short table indexes rather than a pointer for .next values
> > - use unsigned ints for cmd numbers (they're 32-bit ioctls, after all)
> > 
> > In addition, there's a 2k hash table that we can do away with simply
> > by hashifying the main table in place at init time.
> 
> You mean by using a closed hash?

The original hash table was 256 pointers into the main table. I simply
rearrange the main table so the first 256 entries have an appropriate
hash. Basically:

for i in len(table):
  h = hash(table[i])
  swap(table[i], table[hash])

At the end of this loop, table[0:256] will contain an appropriate table
entry, if it exists. So no secondary table is needed.

-- 
Mathematics is the supreme nostalgia of our time.