Re: [Boost-users] run-time integer log2

Subject: Re: [Boost-users] run-time integer log2
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-07-13 16:23:44

What is "NT²"

I've used an inline asm instruction for that.  On the x86, there is a "Bit Scan Reverse" (BSR) which stores the index of the most-significant set bit.

> -----Original Message-----
> In the good'ol Bits Twiddling Hacks page:
> 
> unsigned int v;	         // 32-bit value to find the log2 of
> unsigned int r; // result of log2(v) will go here
> unsigned int shift;
> 
> r =     (v > 0xFFFF) << 4; v >>= r;
> shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
> shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
> shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
> 

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