Hi,

here is a tentative patch to implement a new attribute nonzero,
which is similar to nonnull, but is not a function attribute
but a type attribute.

One reason is that nonnull is awkward to use. For this reason,
clang allows the use of nonnull in function parameters, but this
is incompatible with old and current use of this attribute in gcc
(though in a rather obscure use case).
See: https://gcc.gnu.org/ml/gcc/2015-05/msg00077.html

The other reason is that a nonzero type attribute is conceptually
much simpler and at the same time more general than the existing
nonnull and nonnull_return function attributes (and could replace
both), e.g. we can define non-zero types and diagnose all stores
of known constant 0 to variables of this type, use this for 
computing better value ranges, etc.

This patch implements the attribute and adds a warning if a 
zero constant is passed as argument with nonzero type attribute.
Also infer_nonnull_range in gcc/gimple.c is extended to make use 
of the attribute (which in turn is used by ubsan).

In addition, in C the attribute is automatically added to
pointers declared as array parameters with static, e.g:

void foo(int a[static 5]);


Martin

(tested on x86_64-unknown-linux-gnu)