[PATCH] handle fouled-bitwise

[PATCH] handle fouled-bitwise

This stuff comes from handling smaller-than-int bitwise types (e.g. __le16).
The problem is in handling things like
	__be16 x, y;
	...
	if (x == (x & ~y))
The code is bitwise-clean, but current sparse can't deduce that.  Operations
allowed on bitwise types have the following property: (type)(x <op> y) can
be substituted for x <op> y in any expression other than sizeof.  That allows
us to ignore usual arithmetical conversions for those types and treat e.g.
| as __be16 x __be16 -> __be16, despite the promotion rules; resulting
semantics will be the same.  However, ~ on smaller-than-int does not have
such property; indeed, ~y is guaranteed to _not_ fit into range of __be16
in the example above.

That causes a lot of unpleasant problems when dealing with e.g. networking
code - IP checksums are 16bit and ~ is often used in their (re)calculations.
The way to deal with that is based on the observation that even though we do
get junk in upper bits, it normally ends up being discarded and sparse can
be taught to prove that.  To do that we need "fouled" conterparts for short
bitwise types.  They will be assigned to (sub)expressions that might carry
junk in upper bits, but trimming those bits would result in the value we'd
get if all operations had been done within the bitwise type.  E.g. in the
example above y would be __be16, ~y - fouled __be16, x & ~y - __be16 again
and x == (x & ~y) - boolean.

Basically, we delay reporting an error on ~<short bitwise> for as long as
possible in hope that taint will be cleansed later.  Exact rules follow:

        * ~short_bitwise => corresponding fouled
        * any arithmetics that would be banned for bitwise => same warning
as if we would have bitwise
        * if t1 is bitwise type and t2 - its fouled analog, then
t1 & t2 => t1, t1 | t2 => t2, t1 ^ t2 => t2.
        * conversion of t2 to t1 is silent (be it passing as argument
or assignment).  Other conversions are banned.
        * x ? t1 : t2 => t2
        * ~t2 => t2 (_not_ t1; something like ~(x ? y : ~y) is still fouled)
        * x ? t2 : t2 => t2, t2 {&,|,^} t2 => t2 (yes, even ^ - same as before).
        * x ? t2 : constant_valid_for_t1 => t2
        * !t2 => warning, ditto for comparisons involving t2 in any way.
        * wrt casts t2 acts exactly as t1 would.
        * for sizeof, typeof and alignof t2 acts as promoted t1.  Note that
fouled can never be an lvalue or have types derived from it - can't happen.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>