wineport: Use a lookup table to make our slow ffs() a bit faster.

Alexandre Julliard julliard at
Fri Mar 11 07:19:07 CST 2011

Henri Verbeet <hverbeet at> writes:

> Something like the following may be faster:
> static const int bit_pos[] =
> {
>     0x00, 0x01, 0x0e, 0x02, 0x0f, 0x1b, 0x12, 0x03,
>     0x1e, 0x10, 0x1c, 0x16, 0x18, 0x13, 0x0a, 0x04,
>     0x1f, 0x0d, 0x1a, 0x11, 0x1d, 0x15, 0x17, 0x09,
>     0x0c, 0x19, 0x14, 0x08, 0x0b, 0x07, 0x06, 0x05,
> };
> const unsigned int c = 0x7dc4d65;
> return bit_pos[((x & -x) * c) >> 27];
> I'd expect that mostly due to eliminating the branches and the
> implicit shift & mask operations in something like "y.c[1]".

Maybe, but there's no point. This is just a fallback implementation for
broken platforms, any platform that we care about should never use this.

Alexandre Julliard
julliard at

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