Re: bug in ceillog2 macro (Lua-5.1.4)

[Leo Razoumov <slonik.az@...>]

On Fri, Jan 7, 2011 at 10:44, Roberto Ierusalimschy
<roberto@inf.puc-rio.br> wrote:
>> Hi List,
>> In Lua-5.1.4 macro
>>
>> #define ceillog2(x)   (luaO_log2((x)-1) + 1)
>>
>> gives wrong answer when x is 0.
>> ceillog2(0) => 32 (should be 1).
>>
>> I attached below a standalone code snippet to demonstrate the problem.
>> I use Lua-5.1.4 on Ubuntu-9.04 x86 (32 bits).
>> There are several  ways to fix it and I like _none_ of them.
>>
>> (1) Ignore the bug. Just make sure that x>0 always.
>>
>> [...]
>
> This is an internal function; it is not for general use. It cannot be
> called with a zero argument (see the calling points). It should not be
> called with a zero argument, because math.ceil(math.log(0, 2)) is -inf
> anyway.
>
> Where is the bug?
>
> -- Roberto
>

Actually, luaO_log2(unsigned int x) is well defined when x is zero

 luaO_log2(0) => -1

Yes, all the existing  calling points carefully ensure that x is never
zero. Therefore, word "bug" is not appropriate here, indeed.

But calling ceillog2(0) will lead to surprises.

Lua implementation is very clean and provides a good example of
general algorithms which are usable outside Lua. It was my primary
interest in luaO_log2 implementation which lead to the ceillog2 issues
(unsigned x in luaO_log2 and signed x in ceillog2 calls).

--Leo--