[llvm] r197272 - [block-freq] Add the method APInt::nearestLogBase2().

[Michael Gottesman]

On Dec 15, 2013, at 2:44 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:

> On 2013 Dec 15, at 13:58, Michael Gottesman <mgottesman at apple.com> wrote:
> 
>> The comment could be made clearer clearing up those issues. I am going to fix up those comments in a bit.
> 
> Thanks!  It might be enough to list the input and expected output of the
> degenerate cases and let the reader confirm the logic.
> 
>> I was avoiding the unnecessary branch. I think with a fixed up comment this will be clear.
> 
> Ah -- I’d missed that the branch in std::min() gets optimized away.
> 
>> I have no problem adding these test cases.
> 
> I thought of another degenerate case, and unfortunately I think the
> current code gets the wrong answer:
> 
> - APInt(1, 1).nearestLogBase2() == 0 // lg(1) should give 0, not 1.

This is incorrect since log2 is not a well defined operation in Z/Z_1. If one defines log2(x) as the largest N such that 2^N < x then since in Z/Z_1, 2^N = 0^N, N could be any number. In reality we should probably add in an assertion to make sure that the routines are only called if the BitWidth is > 1.

Michael

> 
> Duncan