Messages in this thread | | | Date | Tue, 12 Dec 2017 19:45:32 +0000 | From | Al Viro <> | Subject | Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits() |
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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?
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