[boost] About the static_log2 discussions

[Daryle Walker]

I haven't been following Boost that much lately. Spent the past few days catching up.

Some people have been discussing changes to the algorithm used by static_log2 lately. Actually, I've forgotten why I used the implementation I did. I sometimes tend to be verbose in my code; hopefully compilers can optimize some of it out. Also, when you see an algorithm looking more complex than you think it should, be mindful that the code may be bigger to workaround some gotchas that the simpler alternative doesn't consider.

The newer code I've seen on the list uses the same ideas as the current code, but there are enough differences that I have questions about.

1. The current static_log2_helper_t uses three template arguments, the new code only uses two. I guess there's some merging of the Place and Index uses; can you explain further.

2. The base case of the new code uses fully specialized static_log2_impl<1,0>, while the current code uses a partially specialized static_log2_helper_t<Val, Place, 1> (which requires workarounds for some compilers). One of the posts implied that the "1" in the first template argument is guaranteed in the usual circumstances. Can you explain further, including how to ensure that the usual circumstances occur.

3. One reason for the timing differences could be that the current code has an extra masking step that the new code doesn't have. Is the masking I'm doing really needed? Would commenting-out/changing that part of the code give the speed boost you have?

Daryle

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