log(~0)/log(2)
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Hmm, I tried a similar idea (get the index of the highest bit), but it failed. Now I remember:

helmut@w3:~$ perl -e 'print log(-1)/log(2),"\n";'
Can't take log of -1 at -e line 1.
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Thx, this is extremely fast and IMHO reliable, because it tests the capabilty directly.

Helmut "wollmers" Wollmersdorfer