[boost] About the static_log2 discussions

From: Daryle Walker (dwalker07_at_[hidden])
Date: 2003-02-04 02:20:14

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