I took the patch in
https://gcc.gnu.org/ml/gcc-patches/2014-01/msg01715.html and removed
the Wnull-attribute part, since most of it can be done from the FE as
shown in https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01857.html  and
also to make the patch smaller and easier to review.

I also fixed the comments by Florian here:
https://gcc.gnu.org/ml/gcc-patches/2014-02/msg00149.html and added
more tests from the PR and its duplicates (one xfailed, I'll open a
new PR about it).

Futher cleanups may be possible (infer_nonnull_range_by_attribute
checks flag_delete_null_pointer_checks, which seems weird to me but it
matches the existing behavior of infer_nonnull_range).

I added this to Wall to get as much testing as possible, we can always
move it to Wextra or disable it by default just before the release if
it turns out to be too noisy.

Boostrapped and regression tested on x86_64-linux-gnu.

OK?

gcc/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>
        Jeff Law  <law@redhat.com>

    PR c/16351
    * doc/invoke.texi (Wnull-dereference): New.
    * tree-vrp.c (infer_value_range): Update call to infer_nonnull_range.
    * gimple-ssa-isolate-paths.c (find_implicit_erroneous_behaviour):
    Warn for potential NULL dereferences.
    (find_explicit_erroneous_behaviour): Warn for NULL dereferences.
    * ubsan.c (instrument_nonnull_arg): Call
    infer_nonnull_range_by_attribute.
    (instrument_nonnull_return): Likewise.
    * common.opt (Wnull-dereference); New.
    * gimple.c (infer_nonnull_range): Remove bool arguments.
    (infer_nonnull_range_by_dereference): New.
    (infer_nonnull_range_by_attribute): New.
    * gimple.h: Update declarations.

gcc/testsuite/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>
        Jeff Law  <law@redhat.com>

    PR c/16351
    * gcc.dg/tree-ssa/isolate-2.c: Close comment.
    * gcc.dg/tree-ssa/isolate-4.c: Likewise.
    * gcc.dg/tree-ssa/wnull-dereference.c: New test.
    * gcc.dg/tree-ssa/isolate-1.c: Test warnings with -Wnull-dereference.
    * gcc.dg/tree-ssa/isolate-3.c: Likewise.
    * gcc.dg/tree-ssa/isolate-5.c: Likewise.
    * c-c++-common/wnonnull-1.c: New test.