Hi!
On Tue, Nov 12, 2019 at 02:35:54PM +0000, Wilco Dijkstra wrote:
> Support common idioms for count trailing zeroes using an array lookup.
> The canonical form is array[((x & -x) * C) >> SHIFT] where C is a magic
> constant which when multiplied by a power of 2 contains a unique value
> in the top 5 or 6 bits. This is then indexed into a table which maps it
> to the number of trailing zeroes. When the table is valid, we emit a
> sequence using the target defined value for ctz (0):
>
> int ctz1 (unsigned x)
> {
> static const char table[32] =
> {
> 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
> 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
> };
>
> return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
> }
Out of interest, what uses this? I have never seen it before.
Segher
