Re: [mpm@selenic.com: Re: buggy ia64_fls() ? (was Re: /dev/random problem on 2.6.12-rc1)]

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Fri, 8 Apr 2005 15:49:13 -0700, Matt Mackall <mpm@selenic.com> said:

  >> Yes, that'll give the right result for fls(-1).  But what'll it
  >> give for fls(-2)?

  Matt> There's no such thing, it takes an unsigned long. There's two
  Matt> problems:

  Matt> input generic_fls ia64_fls exponent (with bias +65535) 0000000
  Matt> 0 -65535 0 0000001 1 0 65535 1000000 32 31 65566

  Matt> So there's the off-by-one problem. And then there's the huge
  Matt> discontinuity at 0. Trouble is the bias is 65535 rather than
  Matt> 65536 so there's no masking trick that works. We could instead
  Matt> to do exp((x*2)+1).

ia64_fls() returns an undefined result for 0 and, as you observed,
returns bit numbers starting from 0.  Also, ia64_fls() works on full
64-bit values, not just 32 bits.

Fixing fls() is trivial:

static inline int
fls (int x)
{
	if (!x)
		return 0;
	return ia64_fls((unsigned int) x) + 1;
}

However, as mentioned in the earlier mails, I want to revisit this
anyhow (which I should have done after McKinley came out, but never
got around to it).

	--david