Messages in this thread | | | Date | Mon, 9 Oct 2006 10:17:09 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [PATCH 1/4] LOG2: Implement a general integer log2 facility in the kernel [try #4] |
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On Mon, 9 Oct 2006, David Howells wrote:
> On FRV, for example, I don't want to wrap fls() because the code for ilog2() > can be shorter and simpler. > > If I did fls() as a wrapper around ilog2() then it would have to involve a > conditional jump because the compiler can't alter the inline asm of ilog2() to > turn the SCAN instruction into CSCAN (which is a conditionally executed > version of SCAN).
Hmmm.. Why not? If you can define fls on a per arch basis then that should be possible? You can tell the compiler to produce the correct version of the scan instructions. We do that frequently on IA64.
> (I have defined ilog2(n) as returning an undefined value if n < 1).
Undefined values are bad. Could you produce a runtime error instead?
> > > (5) fls() and fls64() can't be used to initialise a variable at compile > > > time, ilog2() can. > > > > Well that is the same issue as (4). > > Not quite. I think (4) might be sufficiently achievable with an inline > function, but (5) is definitely not.
With the appropriate per arch modification of fls() this should also be possible within the existing framework.
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