algorithm - Previous power of 2

Question

There is a lot of information on how to find the next power of 2 of a given value (see refs) but I cannot find any to get the previous power of two.

The only way I find so far is to keep a table with all power of two up to 2^64 and make a simple lookup.

• Acius' Snippets
• gamedev
• Bit Twiddling Hacks
• Stack Overflow

Answer 1

From Hacker's Delight, a nice branchless solution:

uint32_t flp2 (uint32_t x)
{
    x = x | (x >> 1);
    x = x | (x >> 2);
    x = x | (x >> 4);
    x = x | (x >> 8);
    x = x | (x >> 16);
    return x - (x >> 1);
}

This typically takes 12 instructions. You can do it in fewer if your CPU has a "count leading zeroes" instruction.