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

Al Viro
2017-12-12T23:49:03Z

On Tue, Dec 12, 2017 at 12:04:09PM -0800, Jakub Kicinski wrote:

> > static __always_inline u64 mask_to_multiplier(u64 mask)
> > {
> > return mask & (mask ^ (mask - 1));
> > }

D'oh. Even simpler than that, of course -

static __always_inline u64 mask_to_multiplier(u64 mask)
{
return mask & -mask;
}

> 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?

Umm... Something like this, perhaps? Same bunch, plus u{16,32,64}_...
variants for host-endian. Adding sanity check on mask is also not
hard, but I don't know how useful it actually is...

diff --git a/include/linux/byteorder/generic.h b/include/linux/byteorder/generic.h
index 451aaa0786ae..a032de9aa03d 100644
--- a/include/linux/byteorder/generic.h
+++ b/include/linux/byteorder/generic.h
@@ -187,4 +187,36 @@ static inline void be32_to_cpu_array(u32 *dst, const __be32 *src, size_t len)
dst[i] = be32_to_cpu(src[i]);
}

+extern void __compiletime_error("value doesn't fit into mask")
+__field_overflow(void);
+static __always_inline u64 mask_to_multiplier(u64 mask)
+{
+ return mask & -mask;
+}
+
+#define ____MAKE_OP(type,base,to,from) \
+static __always_inline __##type type##_replace_bits(__##type old, \
+ base val, base mask) \
+{ \
+ __##type m = to(mask); \
+ if (__builtin_constant_p(val) && \
+ (val & ~(mask/mask_to_multiplier(mask)))) \
+ __field_overflow(); \
+ return (old & ~m) | \
+ (to(val * mask_to_multiplier(mask)) & m); \
+} \
+static __always_inline base type##_get_bits(__##type v, base mask) \
+{ \
+ return (from(v) & mask)/mask_to_multiplier(mask); \
+}
+#define __MAKE_OP(size) \
+ ____MAKE_OP(le##size,u##size,cpu_to_le##size,le##size##_to_cpu) \
+ ____MAKE_OP(be##size,u##size,cpu_to_be##size,be##size##_to_cpu) \
+ ____MAKE_OP(u##size,u##size,,)
+__MAKE_OP(16)
+__MAKE_OP(32)
+__MAKE_OP(64)
+#undef __MAKE_OP
+#undef ____MAKE_OP
+
#endif /* _LINUX_BYTEORDER_GENERIC_H */