Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

Jakub Kicinski
2017-12-12T20:04:18Z

On Tue, 12 Dec 2017 19:45:32 +0000, Al Viro wrote:
> On Tue, Dec 12, 2017 at 06:20:02AM +0000, Al Viro wrote:
>
> > Umm... What's wrong with
> >
> > #define FIELD_FOO 0,4
> > #define FIELD_BAR 6,12
> > #define FIELD_BAZ 18,14
> >
> > A macro can bloody well expand to any sequence of tokens - le32_get_bits(v, FIELD_BAZ)
> > will become le32_get_bits(v, 18, 14) just fine. What's the problem with that?
>
> FWIW, if you want to use the mask, __builtin_ffsll() is not the only way to do
> it - you don't need the shift. Multiplier would do just as well, and that can
> be had easier. If mask = (2*a + 1)<<n = ((2*a)<<n) ^ (1<<n), then
> mask - 1 = ((2*a) << n) + ((1<<n) - 1) = ((2*n) << n) ^ ((1<<n) - 1)
> mask ^ (mask - 1) = (1<<n) + ((1<<n) - 1)
> and
> mask & (mask ^ (mask - 1)) = 1<<n.
>
> IOW, with
>
> static __always_inline u64 mask_to_multiplier(u64 mask)
> {
> return mask & (mask ^ (mask - 1));
> }
>
> we could do
>
> static __always_inline __le64 le64_replace_bits(__le64 old, u64 v, u64 mask)
> {
> __le64 m = cpu_to_le64(mask);
> return (old & ~m) | (cpu_to_le64(v * mask_to_multiplier(mask)) & m);
> }
>
> static __always_inline u64 le64_get_bits(__le64 v, u64 mask)
> {
> return (le64_to_cpu(v) & mask) / mask_to_multiplier(mask);
> }
>
> etc. Compiler will turn those into shifts... I can live with either calling
> conventions.
>
> Comments?

Very nice! The compilation-time check if the value can fit in a field
covered by the mask (if they're both known) did help me catch bugs
early a few times over the years, so if it could be preserved we can
maybe even drop the FIELD_* macros and just use this approach?