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

From: Al Viro <[email-masked]>
Date: Tue, 12 Dec 2017 19:45:32 +0000

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?